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#41
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
"Scott Dorsey" wrote in message ... dBFS has not got a damn thing to do with sine waves or reference levels or anything in the analogue world. Well actually *dB Full Scale*, by simple definition can be the *full scale dB value* of any digital OR analog system. It's use now is more commonly connected to digital systems of course. MrT. |
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#42
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
"Randy Yates" wrote in message ... You guys have danced around this one all day. It's getting humorous. That you expect a simple answer without providing any real definition is humorous. There is no answer except when defined. The only simple answer has been given many times, dBFS is simply the Full Scale point of any relative power measurement scale as defined. MrT. |
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#43
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
On 11/19/2010 8:32 PM, Randy Yates wrote:
My question is this: What is the definition of dBFS? Decibels relative to full scale. Nonsense. All you've given is the meaning of the acronym, not an engineering definition of the unit. This is similar to stating the definition of RMS is "root mean square." Question asked and answered. You've also had plenty of people explain to you the significance of "full scale." Ferchrissake, what more do you want to know? -- "Today's production equipment is IT based and cannot be operated without a passing knowledge of computing, although it seems that it can be operated without a passing knowledge of audio." - John Watkinson http://mikeriversaudio.wordpress.com - useful and interesting audio stuff |
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#44
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
On 11/19/2010 11:08 PM, Les Cargill wrote:
0dBFs is the upper limit for an instantaneous voltage measure for the output of a system of digital to analog conversion. I'm still having a difficult time understanding what he wants to be defined. We're telling him what 0 dBFS means because that's really the only significant point in a sampling system. He's asking for some arbitrary conversion to a sine wave of arbitrary amplitude, and as far as I know, there's no way to calculate that from anything but an infinite number of samples. I don't have time for that. -- "Today's production equipment is IT based and cannot be operated without a passing knowledge of computing, although it seems that it can be operated without a passing knowledge of audio." - John Watkinson http://mikeriversaudio.wordpress.com - useful and interesting audio stuff |
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#45
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
Mr.T MrT@home wrote:
"Scott Dorsey" wrote in message ... dBFS has not got a damn thing to do with sine waves or reference levels or anything in the analogue world. Well actually *dB Full Scale*, by simple definition can be the *full scale dB value* of any digital OR analog system. It's use now is more commonly connected to digital systems of course. What is the full scale of an analogue system? Is it where it starts to get nonlinear, where it gets really nonlinear, or where it stops and won't go any more at all? dBFS isn't really useful on a system that does not clip abruptly and simultaneously on all stages. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
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#46
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
On Fri, 19 Nov 2010 23:01:58 -0500, Randy Yates
wrote: (Eric Jacobsen) writes: The ratio of the level measured to the Full Scale level provides the argument for the logarithm, and the scaled result is dBFS. "the level measured"? I can immediately think of three different ways to measure levels. And the point remains: If you measure peak, then the peak is the value referenced to Full Scale. If you measure RMS, then the RMS level is the value reference to Full Scale. If you measure furriness, then the furriness is the value referneced to Full Scale. I really don't see the source of the confusion. The odd part is that you're not seeing this after being told correctly what it is several times. You don't define it with any precision yourself, Eric. It's a very simple definition. dB is always pretty simple, scale the log of the ratio of the value measured to the reference level. The only possible complication is keeping the units compatible between the reference and the measured value. In this case Full Scale can be thought of as an amplitude or a power reference (and probably interpreted otherwise as well). You should know all this, and I suspect you do. It puzzles me why this seems difficult. -- Randy Yates % "Rollin' and riding and slippin' and Digital Signal Labs % sliding, it's magic." % http://www.digitalsignallabs.com % 'Living' Thing', *A New World Record*, ELO Eric Jacobsen Minister of Algorithms Abineau Communications http://www.abineau.com |
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#47
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
On Sat, 20 Nov 2010 21:26:21 +1100, "Mr.T" MrT@home wrote:
"rickman" wrote in message ... None of the blind men are really right and none are wrong. In the meantime no coherent picture of the dBFS elephant has emerged and more disjointed statements are made on the topic. It seems to me if you realise the Bell or dB is a RELATIVE LOG term of power (and it's constituents) ratio's with no absolute UNLESS defined as a subset, (eg dBv, dBu, dBm etc) then asking for a SINGLE absolute point of reference, or single definition, is simply asking for the impossible. dBFS is simply the *Full Scale* point of ANY system so defined. IF you want it to mean anything specific, you must define it as such. MrT. Pretty much. As long as the units of the reference and the measured value are compatible it's just an equation to plug into. Eric Jacobsen Minister of Algorithms Abineau Communications http://www.abineau.com |
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#48
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
On Sat, 20 Nov 2010 09:21:25 +0000 (UTC), glen herrmannsfeldt
wrote: In comp.dsp Eric Jacobsen wrote: (snip, someone wrote) Nonsense. All you've given is the meaning of the acronym, not an engineering definition of the unit. This is similar to stating the definition of RMS is "root mean square." (snip) I think you're asking what color the sky is, and people are telling you "blue", but you're expecting a wavelength or something, so you're not accepting the answer. As you know, dB measurements are always relative to some reference level. With dBFS the reference level is Full Scale of the converter or number system or whatever. The ratio of the level measured to the Full Scale level provides the argument for the logarithm, and the scaled result is dBFS. But there is more to it than just the reference. Well, if you just measure one sample then that is all, but for a signal of some duration, it is more complicated. I can, for example, compute RMS for a whole CD track. I could also compute the mean of the absolute value, the geometric mean of the absolute value, or many other mathematical functions of the samples. If I have a sine that reaches peak at exactly a sample point, and reaches full scale at that point, then RMS is 5 log(2), or about 1.5dB lower. For mean absolute value, 10 log(2/pi), or about 1.96dB lower. Yup. If you have a different measure, you'll get a different value. Why is that confusing? People in our area don't normally confuse amplitude and power, or even various ways to measure power. Why does it become so difficult when you plug it into a simple equation? Eric Jacobsen Minister of Algorithms Abineau Communications http://www.abineau.com |
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#49
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
Scott Dorsey wrote:
In article , Randy Yates wrote: If dBFS is defined as dBFS = 20 * log_10(XRMS / (RMS value of full-scale sine wave), where XRMS is the RMS value of the digital data stream, and you're generating a "digital square wave," then you are wrong. The digital square wave can go to +3dBFS as defined above. dBFS has not got a damn thing to do with sine waves or reference levels or anything in the analogue world. When the input sensitivity of the convertor is considered we have a basis upon which to spec the relationship between the analog and digital levels. This holds only for that make and model of converter set to that particular sensitivity (is such is adjustable). It has ONLY to do with how far a digital level is below the point at which the digital value reaches full scale (all bits on). --scott -- shut up and play your guitar * http://hankalrich.com/ http://armadillomusicproductions.com/who'slistening.html http://www.sonicbids.com/HankandShai...withDougHarman |
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#51
Posted to rec.audio.tech,rec.audio.pro
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Questions on Levels
PStamler wrote:
I smell a troll. I think maybe not, based on Randy's previous postings. I think he's a knowledgeable cat in the digital domain, has a basic understanding of analog audio concepts, but isn't getting his head around this particular question, now answered for him many times. I think the answer is one he was not expecting, and like many engineers who are appreciably rational, the irrational aspect of an audio metering system in which the measured level increments need bear no specific relationship to the corresponding analog signal levels is disturbing to him. When somebody asks a question and gets the precise answer from several people at once, and keeps on arguing that nobody has given him the answer, then a troll should be suspected. I do understand that part of it, but unless someone has hijacked his account I think he's just not getting it, and he might think we're screwing around with him. All responses I have seen here have been well intentioned. 0dBFS is the level at which one or the other extremes of a digital waveform is at maximum codeable level. Therefrom, the answer to his question can only be stated in absolute terms when we know how many bits a given converter uses to convert an analog signal of a particular level. There are no established standards relating that to any standards in the analog world, be they dBu, dBV, dBm or any other. There are some informal standards in the movie and broadcast world, but no standards body such as IEC or AES has adopted an official standard. And Randy, before you tell me "I don't want to know what isn't, I want to know what is," what I've written above is what is (a definition of dBFS), and there really ain't no more, and until a standards committee gets together and votes out a standard, there won't be. Peace, Paul -- shut up and play your guitar * http://hankalrich.com/ http://armadillomusicproductions.com/who'slistening.html http://www.sonicbids.com/HankandShai...withDougHarman |
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#52
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
"Mike Rivers" wrote in message
... | On 11/19/2010 11:08 PM, Les Cargill wrote: | | 0dBFs is the upper limit for an instantaneous voltage | measure for | the output of a system of digital to analog conversion. | | I'm still having a difficult time understanding what he | wants to be defined. We're telling him what 0 dBFS means | because that's really the only significant point in a | sampling system. He's asking for some arbitrary conversion | to a sine wave of arbitrary amplitude, and as far as I know, | there's no way to calculate that from anything but an | infinite number of samples. I don't have time for that. Mike, he's just yanking your chain. He gets off on it. It has nothing to do with anything but that. Steve King |
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#53
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
On Fri, 19 Nov 2010 21:46:25 -0500, Randy Yates
wrote: (Scott Dorsey) writes: In article , Randy Yates wrote: If dBFS is defined as dBFS = 20 * log_10(XRMS / (RMS value of full-scale sine wave), where XRMS is the RMS value of the digital data stream, and you're generating a "digital square wave," then you are wrong. The digital square wave can go to +3dBFS as defined above. dBFS has not got a damn thing to do with sine waves or reference levels or anything in the analogue world. Again, I'm not asking how it's not defined, I'm asking how it is defined. You guys have danced around this one all day. It's getting humorous. It has ONLY to do with how far a digital level is below the point at which the digital value reaches full scale (all bits on). If you know what it means, and you're literate, then you should be able to come up with a precise definition. I haven't seen one yet. The problem is that dB is defined as a unit of power, usually applied to signals with some time duration. Obviously, a square wave at full scale of a converter has more power than, say, a sine wave or a 1% duty cycle signal at full scale. So, how can one define dBFS so it represents how the figure is actually used? How about "a signal at 0 dBFS is one whose instantaneous power reaches but never exceeds the instantaneous power associated with full scale of the converter"? Modifying your formula above, dBFS = 20 * log_10(peak signal voltage / converter maximum voltage) -- John |
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#54
Posted to rec.audio.tech,rec.audio.pro
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Questions on Levels
hank alrich wrote:
I think the answer is one he was not expecting, and like many engineers who are appreciably rational, the irrational aspect of an audio metering system in which the measured level increments need bear no specific relationship to the corresponding analog signal levels is disturbing to him. I don't see what's irrational about that, because the way we think about analogue and digital levels are so totally different. They really are different things. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
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#55
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
rickman wrote in news:27189e84-f6c3-4f34-8d1d-
: On Nov 19, 10:42*pm, (Eric Jacobsen) wrote: On Fri, 19 Nov 2010 20:32:08 -0500, Randy Yates wrote: Mike Rivers writes: On 11/19/2010 7:52 PM, Randy Yates wrote: My question is this: What is the definition of dBFS? Decibels relative to full scale. Nonsense. All you've given is the meaning of the acronym, not an engineering definition of the unit. This is similar to stating the definition of RMS is "root mean square." [...] But it's not defined that way. I'm not asking how it's not defined. I'm asking how it is defined (in a sensible way). I think you're asking what color the sky is, and people are telling you "blue", but you're expecting a wavelength or something, so you're not accepting the answer. As you know, dB measurements are always relative to some reference level. * With dBFS the reference level is Full Scale of the converter or number system or whatever. * The ratio of the level measured to the Full Scale level provides the argument for the logarithm, and the scaled result is dBFS. If you can do dBm, or dBW, or dBC, you should be able to do dBFS. The odd part is that you're not seeing this after being told correctly what it is several times. I think you are using an inappropriate metaphor. It is more like Randy is asking what is the elephant like and the blind men are all telling him something different in these two threads. One person says 0 dBFS is a sample of all 1's and all 0's is -96 dBFS (I won't even go into what is wrong with that one)! Another describes how a VU meter works. Yet another tells him 0 dBFS is the peak clipping point (that one alone actually says somethng). I'm not sure I want to jump in again at all, but here are a couple of points: 1. I think I concur with Erik Jacobsen on definitions. 2. 0 dBFS does not mean the level of all 1s as someone suggested. It is the value of the full scale range of the converter which is virtually always expressed as twos complement in the audio world. This is either 0x7FFFFF...etc depending or word length for positive peaks or 0x80000... for negative peaks. If we really wanted to nitpik, I suppose it should be the positive value which is 1 bit less than the absolue value of full scale negative. This distinction is meaningless in dB when the bit depth is large. 3. dBFS does not by itself refer to rms levels at all. In a practical system, there will be a relationship to the rms level of a sine wave to dBFS. This is because the crest factor of a sine wave is fixed at 3dB. A square wave has a crest factor of 0dB, music and voice has a crest factor 3dB in almost all cases. We can relate a the rms level of a +4dBu sinusoid to an equivalent dBFS value only when we know the conversion. This may be 18 or 20 dB (or something else) and simply establishes the balance between headroom and low level noise. 4. RMS measurements will also vary. In most cases, we will be using exponential averaging with some arbitrary time constant. It really doesn't matter whether we a considering signals in either the digital domain or analog domain. With exponential averaging, the most recent signals (samples) have more weight than earlier signals (samples). A long time constant will yield a measurement that reflects the overall long term level. A shorter time constant will accent more current events. A sinusoid would measure the same assuming that the averaging filter has settled to a steady state value. VU meters are similar where averaging is at least partially the result of meter balistics. Like a typical low cost multimeter, they may not be TRMS either. Al Clark www.danvillesignal.com None of the blind men are really right and none are wrong. In the meantime no coherent picture of the dBFS elephant has emerged and more disjointed statements are made on the topic. Another metaphor is that this is a can of worms! Rick |
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#56
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
John O'Flaherty writes:
On Fri, 19 Nov 2010 21:46:25 -0500, Randy Yates wrote: (Scott Dorsey) writes: In article , Randy Yates wrote: If dBFS is defined as dBFS = 20 * log_10(XRMS / (RMS value of full-scale sine wave), where XRMS is the RMS value of the digital data stream, and you're generating a "digital square wave," then you are wrong. The digital square wave can go to +3dBFS as defined above. dBFS has not got a damn thing to do with sine waves or reference levels or anything in the analogue world. Again, I'm not asking how it's not defined, I'm asking how it is defined. You guys have danced around this one all day. It's getting humorous. It has ONLY to do with how far a digital level is below the point at which the digital value reaches full scale (all bits on). If you know what it means, and you're literate, then you should be able to come up with a precise definition. I haven't seen one yet. The problem is that dB is defined as a unit of power, usually applied to signals with some time duration. YES!!! Thank you, John! Obviously, a square wave at full scale of a converter has more power than, say, a sine wave or a 1% duty cycle signal at full scale. So, how can one define dBFS so it represents how the figure is actually used? Not a bad question, but I was hoping there was _THE_ definition. Apparently there is not. And this is really the crux of the issue (for dBFS). Some people say it's a peak (instantaneous) measurement, yet I see meters that use it for RMS measurements. I'm afraid the truth is that there is no universal meaning for it like there is for dBm, dBV, and several other dB units. How about "a signal at 0 dBFS is one whose instantaneous power I'm not comfortable with the concept of "instantaneous power." Rather, I think we have to just concede that the "dB" sometimes breaks tradition and works with instantanous quantities rather than power. reaches but never exceeds the instantaneous power associated with full scale of the converter"? Modifying your formula above, dBFS = 20 * log_10(peak signal voltage / converter maximum voltage) That is essentially what I wrote last night. Thanks for your input, John. -- Randy Yates % "Maybe one day I'll feel her cold embrace, Digital Signal Labs % and kiss her interface, % til then, I'll leave her alone." http://www.digitalsignallabs.com % 'Yours Truly, 2095', *Time*, ELO |
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#57
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
On Sat, 20 Nov 2010 15:09:19 -0500, Randy Yates
wrote: John O'Flaherty writes: On Fri, 19 Nov 2010 21:46:25 -0500, Randy Yates wrote: (Scott Dorsey) writes: In article , Randy Yates wrote: If dBFS is defined as dBFS = 20 * log_10(XRMS / (RMS value of full-scale sine wave), where XRMS is the RMS value of the digital data stream, and you're generating a "digital square wave," then you are wrong. The digital square wave can go to +3dBFS as defined above. dBFS has not got a damn thing to do with sine waves or reference levels or anything in the analogue world. Again, I'm not asking how it's not defined, I'm asking how it is defined. You guys have danced around this one all day. It's getting humorous. It has ONLY to do with how far a digital level is below the point at which the digital value reaches full scale (all bits on). If you know what it means, and you're literate, then you should be able to come up with a precise definition. I haven't seen one yet. The problem is that dB is defined as a unit of power, usually applied to signals with some time duration. YES!!! Thank you, John! FFS. The dB is NOT a unit of anything. It is a ratio expressed in log form for convenience (to avoid huge numbers). Nothing more and nothing less. Obviously, a square wave at full scale of a converter has more power than, say, a sine wave or a 1% duty cycle signal at full scale. So, how can one define dBFS so it represents how the figure is actually used? Not a bad question, but I was hoping there was _THE_ definition. Apparently there is not. And this is really the crux of the issue (for dBFS). Some people say it's a peak (instantaneous) measurement, yet I see meters that use it for RMS measurements. I'm afraid the truth is that there is no universal meaning for it like there is for dBm, dBV, and several other dB units. This is getting increasingly ridiculous. FS is simply the point at which you hit the ceiling. There is no more. You have limited. dBFS is the ratio of the present signal to that ceiling. The relevant measurement is instantaneous - this very next sample is the one you have to care about. Normal good practice would suggest that you keep between 10 and 20dB below to allow for the unexpected. If you do that, you won't be troubled by either overload or noise. How about "a signal at 0 dBFS is one whose instantaneous power I'm not comfortable with the concept of "instantaneous power." Rather, I think we have to just concede that the "dB" sometimes breaks tradition and works with instantanous quantities rather than power. There is no problem with instantaneous power. Instantaneous energy is the one you can't be doing with. That final sentence of yours is simply gibberish. reaches but never exceeds the instantaneous power associated with full scale of the converter"? Modifying your formula above, dBFS = 20 * log_10(peak signal voltage / converter maximum voltage) That is essentially what I wrote last night. Thanks for your input, John. How many people have slightest idea what power their converters will handle? Everybody thinks in terms of voltage, which is the fixed term in non-matched systems such as audio gear. d |
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#58
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
On Sat, 20 Nov 2010 15:09:19 -0500, Randy Yates
wrote: John O'Flaherty writes: On Fri, 19 Nov 2010 21:46:25 -0500, Randy Yates wrote: (Scott Dorsey) writes: In article , Randy Yates wrote: If dBFS is defined as dBFS = 20 * log_10(XRMS / (RMS value of full-scale sine wave), where XRMS is the RMS value of the digital data stream, and you're generating a "digital square wave," then you are wrong. The digital square wave can go to +3dBFS as defined above. dBFS has not got a damn thing to do with sine waves or reference levels or anything in the analogue world. Again, I'm not asking how it's not defined, I'm asking how it is defined. You guys have danced around this one all day. It's getting humorous. It has ONLY to do with how far a digital level is below the point at which the digital value reaches full scale (all bits on). If you know what it means, and you're literate, then you should be able to come up with a precise definition. I haven't seen one yet. The problem is that dB is defined as a unit of power, usually applied to signals with some time duration. YES!!! Thank you, John! Except he's wrong. As others have said, dB is a way of rescaling and is independent of the units involved or the characteristics of the measurement. It is simply the scaled log of a ratio, where one of the terms in the ratio is a reference level. If the reference level and the measurement have units of power, then the resulting dB value will have units of power, and will usually reflect that, e.g., dBm, dBW, etc. If the reference level and measurement have units of amplitude, then the output will generally reflect that as well, e.g., dBV. Perhaps the confusion is that FS is unitless and can be anything; power, amplitude, time, price, whatever. Since it is just a reference to a number within a number system, the output will then have the units of whatever that number represents. Meanwhile, since it is just a number within a particular dynamic range indicated by FS, dBFS is still a useful expression for evaluating a system. But it is not inherently power or amplitude or anything. It takes on the units (or unitlessness) of whatever the number system represents. Obviously, a square wave at full scale of a converter has more power than, say, a sine wave or a 1% duty cycle signal at full scale. So, how can one define dBFS so it represents how the figure is actually used? Not a bad question, but I was hoping there was _THE_ definition. Apparently there is not. And this is really the crux of the issue (for dBFS). Some people say it's a peak (instantaneous) measurement, yet I see meters that use it for RMS measurements. I'm afraid the truth is that there is no universal meaning for it like there is for dBm, dBV, and several other dB units. dBx always takes on the units of the input values. The reference and the measurement have to have the same units for the result to be meaningful. How about "a signal at 0 dBFS is one whose instantaneous power I'm not comfortable with the concept of "instantaneous power." Rather, I think we have to just concede that the "dB" sometimes breaks tradition and works with instantanous quantities rather than power. It can be anything. Instantaneous, averaged, glacial, whatever. Time may not be involved at all, or it might be. reaches but never exceeds the instantaneous power associated with full scale of the converter"? Modifying your formula above, dBFS = 20 * log_10(peak signal voltage / converter maximum voltage) That is essentially what I wrote last night. Thanks for your input, John. -- Randy Yates % "Maybe one day I'll feel her cold embrace, Digital Signal Labs % and kiss her interface, % til then, I'll leave her alone." http://www.digitalsignallabs.com % 'Yours Truly, 2095', *Time*, ELO Eric Jacobsen Minister of Algorithms Abineau Communications http://www.abineau.com |
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#59
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
(Don Pearce) writes:
[...] FFS. The dB is NOT a unit of anything. It is a ratio expressed in log form for convenience (to avoid huge numbers). Nothing more and nothing less. "dB" is a ratio of powers. http://www.digitalsignallabs.com/db.pdf "dBm", "dBV", etc., are units. -- Randy Yates % "...the answer lies within your soul Digital Signal Labs % 'cause no one knows which side % the coin will fall." http://www.digitalsignallabs.com % 'Big Wheels', *Out of the Blue*, ELO |
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#60
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
(Eric Jacobsen) writes:
[...] It can be anything. Thanks for your input, Eric. I realize this is honestly what you believe, but I'm not sure I agree with it. -- Randy Yates % "How's life on earth? Digital Signal Labs % ... What is it worth?" % 'Mission (A World Record)', http://www.digitalsignallabs.com % *A New World Record*, ELO |
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#61
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
On Sat, 20 Nov 2010 15:57:15 -0500, Randy Yates
wrote: (Don Pearce) writes: [...] FFS. The dB is NOT a unit of anything. It is a ratio expressed in log form for convenience (to avoid huge numbers). Nothing more and nothing less. "dB" is a ratio of powers. Quite, but provided you don't change the impedances in the meantime it is also a ratio of voltages or currents. http://www.digitalsignallabs.com/db.pdf "dBm", "dBV", etc., are units. The "m" and the "V" are the units, not the "dB" part. d |
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#62
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
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#63
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
"rickman" wrote in message ... None of the blind men are really right and none are wrong. In the meantime no coherent picture of the dBFS elephant has emerged and more disjointed statements are made on the topic. As I've said and repeat again |
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#64
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
In article , Randy Yates wrote:
(Don Pearce) writes: [...] FFS. The dB is NOT a unit of anything. It is a ratio expressed in log form for convenience (to avoid huge numbers). Nothing more and nothing less. "dB" is a ratio of powers. dB is a ratio. dBft and dBlb to talk about decibels with respect to a foot or a pound are perfectly reasonable. Probably the most common measurement, dBSPL, is actually referenced to a pressure. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
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#65
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
On Nov 20, 4:04*pm, Randy Yates wrote:
(Eric Jacobsen) writes: [...] It can be anything. Thanks for your input, Eric. I realize this is honestly what you believe, but I'm not sure I agree with it. -- Randy Yates * * * * * * * * * * *% "How's life on earth? Digital Signal Labs * * * * * * *% *... What is it worth?" * * * * *% 'Mission (A World Record)',http://www.digitalsignallabs.com% *A New World Record*, ELO Why do you have a problem with instantaneous power? Power is a rate, just like speed. Energy per unit time / distance per unit time. Of course no measurement of any kind can be done instantaneously, but that is more an issue of quantum mechanics than a theoretical issue. If you can perform measurements at a point in time that allow power to be calculated such as voltage/current/resistance, you can calculate instantaneous power. RMS is just a way to calculating an average power of a varying signal. But it is found by using an integral of an infinite number of power points or in the discrete domain, a sum of many discrete powers. If the instantaneous or discrete powers don't exist, how can the integral or sum exist? Rick |
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#66
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
"Scott Dorsey" wrote in message ... dBFS has not got a damn thing to do with sine waves or reference levels or anything in the analogue world. Well actually *dB Full Scale*, by simple definition can be the *full scale dB value* of any digital OR analog system. It's use now is more commonly connected to digital systems of course. What is the full scale of an analogue system? Is it where it starts to get nonlinear, where it gets really nonlinear, or where it stops and won't go any more at all? Right. With an analog meter it's full scale point is obvious. But power supply limitations mean there is always a maximum point that cannot be exceeded in any system. It gets a bit harder to define when you consider power supply regulation etc. But as I said, it's the *definition* that counts. not necessarily the real world practical implementations. MrT. |
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#67
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
Randy Yates wrote:
(Don Pearce) writes: [...] FFS. The dB is NOT a unit of anything. It is a ratio expressed in log form for convenience (to avoid huge numbers). Nothing more and nothing less. "dB" is a ratio of powers. .... into a fixed 600 ohm impedance, so they have a mathematical dual in swings in voltage (which is much more convenient to measure ). http://www.digitalsignallabs.com/db.pdf "dBm", "dBV", etc., are units. They're still unitless. The specializations just let you know some measure of detail about how many dimensions are represented for cases like power vs. voltage... -- Les Cargill |
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#68
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
On Sat, 20 Nov 2010 17:19:53 -0600, John O'Flaherty
wrote: On Sat, 20 Nov 2010 20:44:55 GMT, (Eric Jacobsen) wrote: On Sat, 20 Nov 2010 15:09:19 -0500, Randy Yates wrote: John O'Flaherty writes: On Fri, 19 Nov 2010 21:46:25 -0500, Randy Yates wrote: (Scott Dorsey) writes: In article , Randy Yates wrote: If dBFS is defined as dBFS = 20 * log_10(XRMS / (RMS value of full-scale sine wave), where XRMS is the RMS value of the digital data stream, and you're generating a "digital square wave," then you are wrong. The digital square wave can go to +3dBFS as defined above. dBFS has not got a damn thing to do with sine waves or reference levels or anything in the analogue world. Again, I'm not asking how it's not defined, I'm asking how it is defined. You guys have danced around this one all day. It's getting humorous. It has ONLY to do with how far a digital level is below the point at which the digital value reaches full scale (all bits on). If you know what it means, and you're literate, then you should be able to come up with a precise definition. I haven't seen one yet. The problem is that dB is defined as a unit of power, usually applied to signals with some time duration. YES!!! Thank you, John! Except he's wrong. As others have said, dB is a way of rescaling and is independent of the units involved or the characteristics of the measurement. It is simply the scaled log of a ratio, where one of the terms in the ratio is a reference level. If the reference level and the measurement have units of power, then the resulting dB value will have units of power, and will usually reflect that, e.g., dBm, dBW, etc. If the reference level and measurement have units of amplitude, then the output will generally reflect that as well, e.g., dBV. But it isn't an independent way of rescaling a measurement. If it were, then the formula for dB as a ratio of voltages would have the same form as that for a ratio of powers: 10 * log(v2/v1). The fact that it has 20 means that it is squaring the voltage ratio to make it a power ratio (implicitly assuming constant impedance). It's a hybrid system of units when it is dBV, but it still represents a power ratio. Wikipedia offers this definition for "decibel": "A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities.", citing this: " IEEE Standard 100 Dictionary of IEEE Standards Terms, Seventh Edition, The Institute of Electrical and Electronics Engineering, New York, 2000; ISBN 0-7381-2601-2; page 288" This isn't to say that it's not used otherwise, but that's the definition. The issue is that it's muddy and not consistently used and hasn't been nearly since inception. Antenna gains, e.g., dBi, Sound pressures, e.g., dBSPL, radar cross sectional area, dBsm, bandwidth, e.g., dBHz, and the current topic, dBFS, are often used with or implemented with power measurements, but they aren't really power, and sometimes don't have anything to do with power. The point is that it's just an equation to plug numbers into, and the meaning is only relevant to the interpretation of what got plugged in. Things like dBFS, or even dBC or other common applications of deciBels, are very often ambiguous and have to have additional context or explanation if one really wants to remove all ambiguity. Get a group of comm engineers in a room and see if anybody agrees on the definition of SNR. Hint: don't get people started. There is no single definition. deciBels are a similar animal. e.g., what is power? What kind of power? RMS? Peak? Which is appropriate for dB? There are common uses that usually apply, but there are enough inconsistencies that one has to be very careful. When writing, if in doubt, spell it out. When reading, if in doubt, don't assume anything, because it could be anything. Perhaps the confusion is that FS is unitless and can be anything; power, amplitude, time, price, whatever. Since it is just a reference to a number within a number system, the output will then have the units of whatever that number represents. Meanwhile, since it is just a number within a particular dynamic range indicated by FS, dBFS is still a useful expression for evaluating a system. But it is not inherently power or amplitude or anything. It takes on the units (or unitlessness) of whatever the number system represents. Obviously, a square wave at full scale of a converter has more power than, say, a sine wave or a 1% duty cycle signal at full scale. So, how can one define dBFS so it represents how the figure is actually used? Not a bad question, but I was hoping there was _THE_ definition. Apparently there is not. And this is really the crux of the issue (for dBFS). Some people say it's a peak (instantaneous) measurement, yet I see meters that use it for RMS measurements. I'm afraid the truth is that there is no universal meaning for it like there is for dBm, dBV, and several other dB units. dBx always takes on the units of the input values. The reference and the measurement have to have the same units for the result to be meaningful. It's true that the input units must be the same, but dB is actually unitless, since it's a ratio of two like units. Again, from Wikipedia: "Being a ratio of two measurements of a physical quantity in the same units, it is a dimensionless unit." It is a dimensionless unit, but it can preserve the dimensions of the input value. Or reflect them, whatever you want to call it. Provide any quantity in dBm, or dBW, and without any other information you also know the power level dimensions without ambiguity. That's an odd thing to be able to do with a dimensionless number or a dimensionles unit, whatever you wish to call it. So one has to keep track of what's going on, regardless of what Wikipedia says. Experienced people still get tripped up on it all the time. How about "a signal at 0 dBFS is one whose instantaneous power I'm not comfortable with the concept of "instantaneous power." Rather, I think we have to just concede that the "dB" sometimes breaks tradition and works with instantanous quantities rather than power. It can be anything. Instantaneous, averaged, glacial, whatever. Time may not be involved at all, or it might be. reaches but never exceeds the instantaneous power associated with full scale of the converter"? Modifying your formula above, dBFS = 20 * log_10(peak signal voltage / converter maximum voltage) That is essentially what I wrote last night. Thanks for your input, John. -- John Eric Jacobsen Minister of Algorithms Abineau Communications http://www.abineau.com |
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#69
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
On Sat, 20 Nov 2010 15:57:15 -0500, Randy Yates
wrote: (Don Pearce) writes: [...] FFS. The dB is NOT a unit of anything. It is a ratio expressed in log form for convenience (to avoid huge numbers). Nothing more and nothing less. "dB" is a ratio of powers. http://www.digitalsignallabs.com/db.pdf "dBm", "dBV", etc., are units. Except it's not always. See my note to John O'Flaherty. Not all dBs are created equal. -- Randy Yates % "...the answer lies within your soul Digital Signal Labs % 'cause no one knows which side % the coin will fall." http://www.digitalsignallabs.com % 'Big Wheels', *Out of the Blue*, ELO Eric Jacobsen Minister of Algorithms Abineau Communications http://www.abineau.com |
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#70
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
On Nov 21, 5:31*am, (Eric Jacobsen) wrote:
On Sat, 20 Nov 2010 15:57:15 -0500, Randy Yates wrote: (Don Pearce) writes: [...] FFS. The dB is NOT a unit of anything. It is a ratio expressed in log form for convenience (to avoid huge numbers). Nothing more and nothing less. "dB" is a ratio of powers. *http://www.digitalsignallabs.com/db.pdf "dBm", "dBV", etc., are units. Except it's not always. * See my note to John O'Flaherty. *Not all dBs are created equal. -- Randy Yates * * * * * * * * * * *% "...the answer lies within your soul Digital Signal Labs * * * * * * *% * * * 'cause no one knows which side * * * * *% * * * * * * * * * the coin will fall." http://www.digitalsignallabs.com% *'Big Wheels', *Out of the Blue*, ELO Eric Jacobsen Minister of Algorithms Abineau Communicationshttp://www.abineau.com I don't know why there is so much confusion in this thread. 0 dBV = 1V. It is unambiguously 1V i.e. one volt, not just "one" That's why it has a "V" slapped on the end. 0dB = 1 - just "one" Dave. |
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#71
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
On 11/20/2010 3:09 PM, Randy Yates wrote:
(for dBFS). Some people say it's a peak (instantaneous) measurement, yet I see meters that use it for RMS measurements. I'm afraid the truth is that there is no universal meaning for it like there is for dBm, dBV, and several other dB units. You're still not getting it, Randy. 0 dBFS has a precise definition. What it doesn't have (and you seem to object to "not definitions") is a magnitude, either voltage or power, that relates the maximum digital number that a system component can deal with to a physical property that can be measured. You don't MEASURE dBFS, you look at the number represented by the bits at some time and that's it. If you were to take a bunch of samples of program material over time, represent them as dB relative to digital full scale, and plug them into the general RMS formula, you could indeed come up with an RMS value for that set of numbers. But what would be the value of that information? It will always be below zero, but you can't just crank up the level until your "RMS dBFS" is closer to zero unless you don't care about clipping or you're working with a known, continuous waveform. So you're getting engineering answers. We tend to be practical folk, and use concepts that are physically meaningful, not purely theoretical. I'm not comfortable with the concept of "instantaneous power." Rather, I think we have to just concede that the "dB" sometimes breaks tradition and works with instantanous quantities rather than power. Initially dB referred to power because the Bel was a measure of acoustic energy (which becomes power when related by time). But it's always been a ratio to a given reference. The Telephone Company (tm) defined a Transmission Unit as the amount of attenuation in a mile of cable that could just be detected by an average listener. This was important in the days when you had to talk louder when making a long distance call. It turned out that 1/10 of a Bel was about equivalent to a Transmission Unit, so the deciBEL became a useful measure. As commonly used today, dB without any modifiers is usually understood to be sound pressure level referenced to a specific pressure in Pascals. We have "units" like dBA, which means sound pressure measured through a bandpass filter of a known transfer function. We have the "20" formula for dB as a ratio since power is the product of two physical quantities (voltage and current) where voltage is only one, so we make then numbers work by compensating for the "squared" term in the power equation. If you have a dollar and I have fifty cents, you can say you have 6 dB more money than I have (or maybe 3 dB more spending power). If a TV station increases its power from 50,000 watts to 100,000 watts, that's a 3 dB increase. If the digitized value of a sample is 1 bit smaller than another sample, that's half the value, so we say that its amplitude is 6 dB lower. If 1111111111111111 (that's 15 bits plus the first bit representing the sign) is full scale, then 111111111111111 is -6 dBFS. -- "Today's production equipment is IT based and cannot be operated without a passing knowledge of computing, although it seems that it can be operated without a passing knowledge of audio." - John Watkinson http://mikeriversaudio.wordpress.com - useful and interesting audio stuff |
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#72
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
On Sun, 21 Nov 2010 08:46:10 -0500, Mike Rivers
wrote: On 11/20/2010 3:09 PM, Randy Yates wrote: (for dBFS). Some people say it's a peak (instantaneous) measurement, yet I see meters that use it for RMS measurements. I'm afraid the truth is that there is no universal meaning for it like there is for dBm, dBV, and several other dB units. You're still not getting it, Randy. 0 dBFS has a precise definition. What it doesn't have (and you seem to object to "not definitions") is a magnitude, either voltage or power, that relates the maximum digital number that a system component can deal with to a physical property that can be measured. You don't MEASURE dBFS, you look at the number represented by the bits at some time and that's it. If you were to take a bunch of samples of program material over time, represent them as dB relative to digital full scale, and plug them into the general RMS formula, you could indeed come up with an RMS value for that set of numbers. But what would be the value of that information? It will always be below zero, but you can't just crank up the level until your "RMS dBFS" is closer to zero unless you don't care about clipping or you're working with a known, continuous waveform. So you're getting engineering answers. We tend to be practical folk, and use concepts that are physically meaningful, not purely theoretical. I'm not comfortable with the concept of "instantaneous power." Rather, I think we have to just concede that the "dB" sometimes breaks tradition and works with instantanous quantities rather than power. Initially dB referred to power because the Bel was a measure of acoustic energy (which becomes power when related by time). But it's always been a ratio to a given reference. The Telephone Company (tm) defined a Transmission Unit as the amount of attenuation in a mile of cable that could just be detected by an average listener. This was important in the days when you had to talk louder when making a long distance call. It turned out that 1/10 of a Bel was about equivalent to a Transmission Unit, so the deciBEL became a useful measure. As commonly used today, dB without any modifiers is usually understood to be sound pressure level referenced to a specific pressure in Pascals. I think this is exemplary of one issue. This may be true in your area of work, that dB without a modifier has to do with sound pressure, but in communications dB without a modifier is generally representative of a unitless scale factor in a system. e.g., an amplifier that increases the signal power by a factor or ten has 10dB of gain. If 0dBW goes in, 10dBW comes out, if 0dBm goes in, 10dBm comes out. Since logarithms convert multiplication to addition, any application of a scale factor in a signal chain, due to gain (e.g., amplifier) or attenuation, (e.g., cable loss), can be represented in dB (without a modifier). Antenna gain has this characteristic, but then it gets a modifier (usually dBi) to indicate which sort of antenna provides the reference gain. So dB without a modifier is usually representative of a dimensionless scale factor in the signal chain. But not always.  We have "units" like dBA, which means sound pressure measured through a bandpass filter of a known transfer function. We have the "20" formula for dB as a ratio since power is the product of two physical quantities (voltage and current) where voltage is only one, so we make then numbers work by compensating for the "squared" term in the power equation. If you have a dollar and I have fifty cents, you can say you have 6 dB more money than I have (or maybe 3 dB more spending power). If a TV station increases its power from 50,000 watts to 100,000 watts, that's a 3 dB increase. If the digitized value of a sample is 1 bit smaller than another sample, that's half the value, so we say that its amplitude is 6 dB lower. If 1111111111111111 (that's 15 bits plus the first bit representing the sign) is full scale, then 111111111111111 is -6 dBFS. -- "Today's production equipment is IT based and cannot be operated without a passing knowledge of computing, although it seems that it can be operated without a passing knowledge of audio." - John Watkinson http://mikeriversaudio.wordpress.com - useful and interesting audio stuff Eric Jacobsen Minister of Algorithms Abineau Communications http://www.abineau.com |
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#73
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
Eric Jacobsen wrote:
On Sun, 21 Nov 2010 08:46:10 -0500, Mike Rivers wrote: As commonly used today, dB without any modifiers is usually understood to be sound pressure level referenced to a specific pressure in Pascals. I think this is exemplary of one issue. This may be true in your area of work, that dB without a modifier has to do with sound pressure, but in communications dB without a modifier is generally representative of a unitless scale factor in a system. e.g., an amplifier that increases the signal power by a factor or ten has 10dB of gain. If 0dBW goes in, 10dBW comes out, if 0dBm goes in, 10dBm comes out. I believe that Mike is incorrect in this. Some people DO use "dB" to mean "dBSPL." However, those people are wrong. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
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#74
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
On 11/21/2010 11:01 AM, Eric Jacobsen wrote:
in communications dB without a modifier is generally representative of a unitless scale factor in a system. You mean they actually get it right? Really, audio people for the most part understand this as well, and they know that gain or signal-to-noise ratio is expressed simply as "dB." However then you see spec sheets that read: "Noise: -86 dB" and you don't know what they're talking about. If it's electronics, it's surely not SPL. It's probably dBu but only your hairdresser knows for sure. amplifier that increases the signal power by a factor or ten has 10dB of gain. If 0dBW goes in, 10dBW comes out, if 0dBm goes in, 10dBm comes out. In the RF world, you put power into an amplifier and get more power out. You have to. It goes with the territory. At the frequencies where you're working, it's important to provide the proper load impedance for the feedline in order to avoid loss from standing waves. But this tread initiated (at least for me) in rec.audio.pro and had audio connotations, so let's stick to audio. In audio you put voltage into a signal processor (like an equalizer, compressor, or even a mic preamp) and you get voltage out. You can make sense of a gain specification or measurement in dB. You put voltage into a power amplifier and you (intend to) get power out, so dB of gain doesn't make much sense. Nor does it when you put in voltage and get a digital word out. So dB without a modifier is usually representative of a dimensionless scale factor in the signal chain. But not always. My statement that it CONVENTIONALLY represented sound pressure level doesn't mean that it's correct. It's a reasonably well understood mistake, or bad shorthand. Take your pick. -- "Today's production equipment is IT based and cannot be operated without a passing knowledge of computing, although it seems that it can be operated without a passing knowledge of audio." - John Watkinson http://mikeriversaudio.wordpress.com - useful and interesting audio stuff |
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#75
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
On 11/21/2010 10:16 AM, Scott Dorsey wrote:
Eric wrote: On Sun, 21 Nov 2010 08:46:10 -0500, Mike wrote: As commonly used today, dB without any modifiers is usually understood to be sound pressure level referenced to a specific pressure in Pascals. I think this is exemplary of one issue. This may be true in your area of work, that dB without a modifier has to do with sound pressure, but in communications dB without a modifier is generally representative of a unitless scale factor in a system. e.g., an amplifier that increases the signal power by a factor or ten has 10dB of gain. If 0dBW goes in, 10dBW comes out, if 0dBm goes in, 10dBm comes out. I believe that Mike is incorrect in this. Some people DO use "dB" to mean "dBSPL." However, those people are wrong. --scott In decades of working with sound, I have always heard coloquially "dB" but it was always understood to mean "dB" relative to 1 micropascal for underwater applications (up to 1970 it had been the microbar so we had to add 100dB to absolute levels thereafter) and relative to 20 micropascals for airborne sound. You can Google enough references to the need for knowing the particular reference system you're using.... When you're in a system or physical context then it's shorthand to say "dB" for absolute levels - but everyone who has thought about it even just a little bit understands what they really mean. It was one of the *first* things I learned out of school in the real world of acoustics. Then, as one switches from underwater to air and vice versa, we understand that the SPL absolute reference changes as above. I don't think that Mike is wrong - he did say "referenced to a specific pressure". In communications had been pretty typical to talk about "dB" in reference to *particular* voltage levels. That said, I won't argue against it being a unitless measure in a system - as it is, after all, all about ratios. It depends on your context. The amplifier example is a good one. But, in that case we're talking in the context of out/in ratio. In system examples we often talk about *absolute* levels and need a reference level to do so. In other cases we do talk about out/in ratios: e.g. transmission loss and the absolute level issue isn't included. Here's a system example - it could be sonar or space communications or .......: We start with a transmitter with output of "150 dB". Well, that means relative to something - it's a statement of absolute level. Then, we run the transmitter output through a channel that attenuates the signal by 100 dB. This statement of "dB" is purely a ratio with no reference level involved as it is the ratio of in/out . Then, we receive the signal and we're interested in the absolute level being received because we have a transducer conversion to deal with and environmental noise to overcome and system noise to overcome. The absolute level received in this case is: 150 - 100 = 50 dB relative to our original reference. Notice that here we mix references to fixed absolute levels with references to pure ratios in order to get what we need. So, both uses are appropriate. The logs just make it easier to compute and to comprehend when one is used to it. This is no different than saying: 200upa/10^5 = 0.002upa or 7w/m^2/10^10 = 0.007uW/m^2 which both use a ratio equivalent to 100dB. Fred |
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#76
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
On Sun, 21 Nov 2010 05:23:37 GMT, (Eric
Jacobsen) wrote: On Sat, 20 Nov 2010 17:19:53 -0600, John O'Flaherty wrote: On Sat, 20 Nov 2010 20:44:55 GMT, (Eric Jacobsen) wrote: On Sat, 20 Nov 2010 15:09:19 -0500, Randy Yates wrote: John O'Flaherty writes: On Fri, 19 Nov 2010 21:46:25 -0500, Randy Yates wrote: (Scott Dorsey) writes: In article , Randy Yates wrote: If dBFS is defined as dBFS = 20 * log_10(XRMS / (RMS value of full-scale sine wave), where XRMS is the RMS value of the digital data stream, and you're generating a "digital square wave," then you are wrong. The digital square wave can go to +3dBFS as defined above. dBFS has not got a damn thing to do with sine waves or reference levels or anything in the analogue world. Again, I'm not asking how it's not defined, I'm asking how it is defined. You guys have danced around this one all day. It's getting humorous. It has ONLY to do with how far a digital level is below the point at which the digital value reaches full scale (all bits on). If you know what it means, and you're literate, then you should be able to come up with a precise definition. I haven't seen one yet. The problem is that dB is defined as a unit of power, usually applied to signals with some time duration. YES!!! Thank you, John! Except he's wrong. As others have said, dB is a way of rescaling and is independent of the units involved or the characteristics of the measurement. It is simply the scaled log of a ratio, where one of the terms in the ratio is a reference level. If the reference level and the measurement have units of power, then the resulting dB value will have units of power, and will usually reflect that, e.g., dBm, dBW, etc. If the reference level and measurement have units of amplitude, then the output will generally reflect that as well, e.g., dBV. But it isn't an independent way of rescaling a measurement. If it were, then the formula for dB as a ratio of voltages would have the same form as that for a ratio of powers: 10 * log(v2/v1). The fact that it has 20 means that it is squaring the voltage ratio to make it a power ratio (implicitly assuming constant impedance). It's a hybrid system of units when it is dBV, but it still represents a power ratio. Wikipedia offers this definition for "decibel": "A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities.", citing this: " IEEE Standard 100 Dictionary of IEEE Standards Terms, Seventh Edition, The Institute of Electrical and Electronics Engineering, New York, 2000; ISBN 0-7381-2601-2; page 288" This isn't to say that it's not used otherwise, but that's the definition. The issue is that it's muddy and not consistently used and hasn't been nearly since inception. Antenna gains, e.g., dBi, Sound pressures, e.g., dBSPL, radar cross sectional area, dBsm, bandwidth, e.g., dBHz, and the current topic, dBFS, are often used with or implemented with power measurements, but they aren't really power, and sometimes don't have anything to do with power. But antenna gains are compared to the power provided by an isotropic antenna, aren't they? For dBSPL, it's a power measurement too. Though it's called a pressure level, the defining formula involves pressure squared, so it should be interpreted as the pressure corresponding to a particular power level. What criterion do you use to decide whether to include a factor of 10 or a factor of 20 in your formula? The point is that it's just an equation to plug numbers into, and the meaning is only relevant to the interpretation of what got plugged in. Things like dBFS, or even dBC or other common applications of deciBels, are very often ambiguous and have to have additional context or explanation if one really wants to remove all ambiguity. Get a group of comm engineers in a room and see if anybody agrees on the definition of SNR. Hint: don't get people started. There is no single definition. deciBels are a similar animal. e.g., what is power? What kind of power? RMS? Peak? Which is appropriate for dB? Power is rate of transfer of energy, and its time distribution, its form, and its location of measurement require further specification, but I don't see why dB shouldn't be applicable to all cases. There are common uses that usually apply, but there are enough inconsistencies that one has to be very careful. When writing, if in doubt, spell it out. When reading, if in doubt, don't assume anything, because it could be anything. Perhaps the confusion is that FS is unitless and can be anything; power, amplitude, time, price, whatever. Since it is just a reference to a number within a number system, the output will then have the units of whatever that number represents. Meanwhile, since it is just a number within a particular dynamic range indicated by FS, dBFS is still a useful expression for evaluating a system. But it is not inherently power or amplitude or anything. It takes on the units (or unitlessness) of whatever the number system represents. Why then is a factor of 20 used for voltages rather than a factor of 10? Are there any actual examples of the use of dBFS that don't relate to a full-scale voltage or current? Of course, the FS has to be defined- voltage current, pressure. But I bet that anyone who was using a full scale defined in terms of power would use a formula with a factor of 10, not 20. Obviously, a square wave at full scale of a converter has more power than, say, a sine wave or a 1% duty cycle signal at full scale. So, how can one define dBFS so it represents how the figure is actually used? Not a bad question, but I was hoping there was _THE_ definition. Apparently there is not. And this is really the crux of the issue (for dBFS). Some people say it's a peak (instantaneous) measurement, yet I see meters that use it for RMS measurements. I'm afraid the truth is that there is no universal meaning for it like there is for dBm, dBV, and several other dB units. dBx always takes on the units of the input values. The reference and the measurement have to have the same units for the result to be meaningful. It's true that the input units must be the same, but dB is actually unitless, since it's a ratio of two like units. Again, from Wikipedia: "Being a ratio of two measurements of a physical quantity in the same units, it is a dimensionless unit." It is a dimensionless unit, but it can preserve the dimensions of the input value. Or reflect them, whatever you want to call it. Provide any quantity in dBm, or dBW, and without any other information you also know the power level dimensions without ambiguity. That's an odd thing to be able to do with a dimensionless number or a dimensionles unit, whatever you wish to call it. So one has to keep track of what's going on, regardless of what Wikipedia says. Experienced people still get tripped up on it all the time. Yes, dB per se is unitless but dBm and dBW aren't. +20 dB has no units, but +20 dBm means 100 milliwatts. If you append RMS to dB, that's a procedural specification, and you can have +10 dBVRMS, where a unit is specified as well as the measurement procedure. I agree that everything should be specified; nevertheless, if dB is used for something that is not power, or not directly relatable to power, I think it's being misused. How about "a signal at 0 dBFS is one whose instantaneous power I'm not comfortable with the concept of "instantaneous power." Rather, I think we have to just concede that the "dB" sometimes breaks tradition and works with instantanous quantities rather than power. It can be anything. Instantaneous, averaged, glacial, whatever. Time may not be involved at all, or it might be. reaches but never exceeds the instantaneous power associated with full scale of the converter"? Modifying your formula above, dBFS = 20 * log_10(peak signal voltage / converter maximum voltage) That is essentially what I wrote last night. Thanks for your input, John. -- John |
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#77
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
On Nov 21, 12:16*pm, (Scott Dorsey) wrote:
I believe that Mike is incorrect in this. *Some people DO use "dB" to mean "dBSPL." *However, those people are wrong. Scott, we had this debate here a couple of years ago, and what it came down to was a discussion between what would in the world of lexicography be called prescriptivists and descriptivists. A prescriptivist writes a dictionary to tell people what words mean and how they should be used. A descriptivist writes a dictionary to tell what people mean by words and how they use them. It's a philosophical and practical division. So a descriptivist would say that one meaning for dB is as a shorthand for dBSPL, which is how a lot of audio engineers use it. A prescriptivist would say, as you did, that's wrong, because it doesn't correspond with the officially-defined meaning of dB. More to the point, someone who uses "dB" to mean the voltage gain of something, ignoring the power aspects, is violating the official definition. But that usage is near-universal among audio circuit designers, who talk about opanp circuits with "20dB of gain", and mean a voltage gain of 10x, with no reference to impedance or power. They may even refer to dB of gain in a transformer, which can never have any power gain, being a passive device. Whether you accept that this is the usage of the population, or condemn it as wrong, is a choice, just as a dictionary-maker must choose whether to be prescriptivist or descriptivist. The fact is that the speech of the community has taken a turn which deviates significantly from the official standards. Parenthetically, we invented dBu as a standard when people stopped using dBm; perhaps it's time to invent dBG for voltage gain situations. Peace, Paul |
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Questions on Levels
On Sun, 21 Nov 2010 15:18:08 -0800 (PST), PStamler
wrote: On Nov 21, 12:16*pm, (Scott Dorsey) wrote: I believe that Mike is incorrect in this. *Some people DO use "dB" to mean "dBSPL." *However, those people are wrong. Scott, we had this debate here a couple of years ago, and what it came down to was a discussion between what would in the world of lexicography be called prescriptivists and descriptivists. A prescriptivist writes a dictionary to tell people what words mean and how they should be used. A descriptivist writes a dictionary to tell what people mean by words and how they use them. It's a philosophical and practical division. So a descriptivist would say that one meaning for dB is as a shorthand for dBSPL, which is how a lot of audio engineers use it. A prescriptivist would say, as you did, that's wrong, because it doesn't correspond with the officially-defined meaning of dB. More to the point, someone who uses "dB" to mean the voltage gain of something, ignoring the power aspects, is violating the official definition. But that usage is near-universal among audio circuit designers, who talk about opanp circuits with "20dB of gain", and mean a voltage gain of 10x, with no reference to impedance or power. They may even refer to dB of gain in a transformer, which can never have any power gain, being a passive device. Whether you accept that this is the usage of the population, or condemn it as wrong, is a choice, just as a dictionary-maker must choose whether to be prescriptivist or descriptivist. The fact is that the speech of the community has taken a turn which deviates significantly from the official standards. Parenthetically, we invented dBu as a standard when people stopped using dBm; perhaps it's time to invent dBG for voltage gain situations. There is a sense in which calling a voltage gain of 10 a gain of 20 dB does refer to power. In a circuit in which nothing is changed but that gain (including output loading and input signal level), if that gain is reduced to 0 dB, the output power level will be reduced by a factor of 100. Similarly, suppose a converter is fed a signal that runs it at a level of -6 dBFS. Halving the input power (f.e., by decreasing a voltage input by a factor of 1.414) will shift the converter to -9 dBFS. Quadrupling the input power by doubling the input level will move the converter to 0 dBFS. The output powers will show the same dB changes (assuming linearity and no tricks). I believe these examples show the power nature of dB measurements. -- John |
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Questions on Levels
On Sun, 21 Nov 2010 22:41:07 -0600, John O'Flaherty
wrote: On Sun, 21 Nov 2010 15:18:08 -0800 (PST), PStamler wrote: On Nov 21, 12:16*pm, (Scott Dorsey) wrote: I believe that Mike is incorrect in this. *Some people DO use "dB" to mean "dBSPL." *However, those people are wrong. Scott, we had this debate here a couple of years ago, and what it came down to was a discussion between what would in the world of lexicography be called prescriptivists and descriptivists. A prescriptivist writes a dictionary to tell people what words mean and how they should be used. A descriptivist writes a dictionary to tell what people mean by words and how they use them. It's a philosophical and practical division. So a descriptivist would say that one meaning for dB is as a shorthand for dBSPL, which is how a lot of audio engineers use it. A prescriptivist would say, as you did, that's wrong, because it doesn't correspond with the officially-defined meaning of dB. More to the point, someone who uses "dB" to mean the voltage gain of something, ignoring the power aspects, is violating the official definition. But that usage is near-universal among audio circuit designers, who talk about opanp circuits with "20dB of gain", and mean a voltage gain of 10x, with no reference to impedance or power. They may even refer to dB of gain in a transformer, which can never have any power gain, being a passive device. Whether you accept that this is the usage of the population, or condemn it as wrong, is a choice, just as a dictionary-maker must choose whether to be prescriptivist or descriptivist. The fact is that the speech of the community has taken a turn which deviates significantly from the official standards. Parenthetically, we invented dBu as a standard when people stopped using dBm; perhaps it's time to invent dBG for voltage gain situations. There is a sense in which calling a voltage gain of 10 a gain of 20 dB does refer to power. In a circuit in which nothing is changed but that gain (including output loading and input signal level), if that gain is reduced to 0 dB, the output power level will be reduced by a factor of 100. Similarly, suppose a converter is fed a signal that runs it at a level of -6 dBFS. Halving the input power (f.e., by decreasing a voltage input by a factor of 1.414) will shift the converter to -9 dBFS. Quadrupling the input power by doubling the input level will move the converter to 0 dBFS. The output powers will show the same dB changes (assuming linearity and no tricks). I misspoke on that last sentence; there is no physical power associated with the numbers from a converter. But if and when the output is converted to analog again, the system output will reflect the power changes. I believe these examples show the power nature of dB measurements. -- John |
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Questions on Levels
On Sun, 21 Nov 2010 15:39:11 -0600, John O'Flaherty
wrote: On Sun, 21 Nov 2010 05:23:37 GMT, (Eric Jacobsen) wrote: On Sat, 20 Nov 2010 17:19:53 -0600, John O'Flaherty wrote: On Sat, 20 Nov 2010 20:44:55 GMT, (Eric Jacobsen) wrote: On Sat, 20 Nov 2010 15:09:19 -0500, Randy Yates wrote: John O'Flaherty writes: On Fri, 19 Nov 2010 21:46:25 -0500, Randy Yates wrote: (Scott Dorsey) writes: In article , Randy Yates wrote: If dBFS is defined as dBFS = 20 * log_10(XRMS / (RMS value of full-scale sine wave), where XRMS is the RMS value of the digital data stream, and you're generating a "digital square wave," then you are wrong. The digital square wave can go to +3dBFS as defined above. dBFS has not got a damn thing to do with sine waves or reference levels or anything in the analogue world. Again, I'm not asking how it's not defined, I'm asking how it is defined. You guys have danced around this one all day. It's getting humorous. It has ONLY to do with how far a digital level is below the point at which the digital value reaches full scale (all bits on). If you know what it means, and you're literate, then you should be able to come up with a precise definition. I haven't seen one yet. The problem is that dB is defined as a unit of power, usually applied to signals with some time duration. YES!!! Thank you, John! Except he's wrong. As others have said, dB is a way of rescaling and is independent of the units involved or the characteristics of the measurement. It is simply the scaled log of a ratio, where one of the terms in the ratio is a reference level. If the reference level and the measurement have units of power, then the resulting dB value will have units of power, and will usually reflect that, e.g., dBm, dBW, etc. If the reference level and measurement have units of amplitude, then the output will generally reflect that as well, e.g., dBV. But it isn't an independent way of rescaling a measurement. If it were, then the formula for dB as a ratio of voltages would have the same form as that for a ratio of powers: 10 * log(v2/v1). The fact that it has 20 means that it is squaring the voltage ratio to make it a power ratio (implicitly assuming constant impedance). It's a hybrid system of units when it is dBV, but it still represents a power ratio. Wikipedia offers this definition for "decibel": "A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities.", citing this: " IEEE Standard 100 Dictionary of IEEE Standards Terms, Seventh Edition, The Institute of Electrical and Electronics Engineering, New York, 2000; ISBN 0-7381-2601-2; page 288" This isn't to say that it's not used otherwise, but that's the definition. The issue is that it's muddy and not consistently used and hasn't been nearly since inception. Antenna gains, e.g., dBi, Sound pressures, e.g., dBSPL, radar cross sectional area, dBsm, bandwidth, e.g., dBHz, and the current topic, dBFS, are often used with or implemented with power measurements, but they aren't really power, and sometimes don't have anything to do with power. But antenna gains are compared to the power provided by an isotropic antenna, aren't they? For dBSPL, it's a power measurement too. Though it's called a pressure level, the defining formula involves pressure squared, so it should be interpreted as the pressure corresponding to a particular power level. What criterion do you use to decide whether to include a factor of 10 or a factor of 20 in your formula? Like any computation, dimensional analysis suggests one uses whatever makes the computation consistent so that the result is useful. Consider an amplifer (which is pretty much the same as an antenna for this purpose, gain is gain). If one were comparing voltage gain, then one has to be consistent with that. If one is comparing (or computing) power gain, then one has to be consistent with that. Power is the most convenient partly because it removes the ambiguity associated with impedance. So that gets used most often, hence the additional factor of two when using voltages for most cases. The "10" is pretty much an arbitrary scale factor, and it turns into "20" so that people can mix power and voltage measures (with careful assumptions). The point is that it's just an equation to plug numbers into, and the meaning is only relevant to the interpretation of what got plugged in. Things like dBFS, or even dBC or other common applications of deciBels, are very often ambiguous and have to have additional context or explanation if one really wants to remove all ambiguity. Get a group of comm engineers in a room and see if anybody agrees on the definition of SNR. Hint: don't get people started. There is no single definition. deciBels are a similar animal. e.g., what is power? What kind of power? RMS? Peak? Which is appropriate for dB? Power is rate of transfer of energy, and its time distribution, its form, and its location of measurement require further specification, but I don't see why dB shouldn't be applicable to all cases. It is applicable, but it's not as clearly defined as some think or are at least expressing here. Power measurement, as you just said, requires integration over time. How much time? It is often (usually) not specified, so there's already ambiguity in the "definition" or "standard". "Instantaneous power" is a hand-wavy way around that, but you can't measure that practically, so time integration is required. How much is up to the implementer. There are common uses that usually apply, but there are enough inconsistencies that one has to be very careful. When writing, if in doubt, spell it out. When reading, if in doubt, don't assume anything, because it could be anything. Perhaps the confusion is that FS is unitless and can be anything; power, amplitude, time, price, whatever. Since it is just a reference to a number within a number system, the output will then have the units of whatever that number represents. Meanwhile, since it is just a number within a particular dynamic range indicated by FS, dBFS is still a useful expression for evaluating a system. But it is not inherently power or amplitude or anything. It takes on the units (or unitlessness) of whatever the number system represents. Why then is a factor of 20 used for voltages rather than a factor of 10? To make voltage and power measurements compatible. Are there any actual examples of the use of dBFS that don't relate to a full-scale voltage or current? Of course, the FS has to be defined- voltage current, pressure. But I bet that anyone who was using a full scale defined in terms of power would use a formula with a factor of 10, not 20. Actually, dBFS implies a digital number scale system, so the traditional notions of voltage or current or power don't really even apply any more. The analysis is performed on a numeric sequence, which could represent anything. A single sample can be taken from the numeric sequence, say X, and dBFS could be computed as either ans = 10*log(X/FS) if one were interested in interpreting X as an instantaneous power measurement (and ADCs often have internal integration over some fraction of the sample period so that can be argued). This follows the definition of RMS for a numeric sequence when n = 1, as long as X is positive. or ans = 10*log(X/FS) if one were interested in interpreting X as a voltage. I'd suggest, though, that one use whatever is consistent with the rest of the analysis being performed. There's nothing magical about the factor of 10 or 20. As always, one just has to keep track of what one is doing and be consistent to get a useful result. Obviously, a square wave at full scale of a converter has more power than, say, a sine wave or a 1% duty cycle signal at full scale. So, how can one define dBFS so it represents how the figure is actually used? Not a bad question, but I was hoping there was _THE_ definition. Apparently there is not. And this is really the crux of the issue (for dBFS). Some people say it's a peak (instantaneous) measurement, yet I see meters that use it for RMS measurements. I'm afraid the truth is that there is no universal meaning for it like there is for dBm, dBV, and several other dB units. dBx always takes on the units of the input values. The reference and the measurement have to have the same units for the result to be meaningful. It's true that the input units must be the same, but dB is actually unitless, since it's a ratio of two like units. Again, from Wikipedia: "Being a ratio of two measurements of a physical quantity in the same units, it is a dimensionless unit." It is a dimensionless unit, but it can preserve the dimensions of the input value. Or reflect them, whatever you want to call it. Provide any quantity in dBm, or dBW, and without any other information you also know the power level dimensions without ambiguity. That's an odd thing to be able to do with a dimensionless number or a dimensionles unit, whatever you wish to call it. So one has to keep track of what's going on, regardless of what Wikipedia says. Experienced people still get tripped up on it all the time. Yes, dB per se is unitless but dBm and dBW aren't. +20 dB has no units, but +20 dBm means 100 milliwatts. If you append RMS to dB, that's a procedural specification, and you can have +10 dBVRMS, where a unit is specified as well as the measurement procedure. I agree that everything should be specified; nevertheless, if dB is used for something that is not power, or not directly relatable to power, I think it's being misused. dBm and dBW are, actually, strictly speaking, still unitless or dimensionless. The units cancel in the ratio of the reference and the measurement, which HAVE to have the same units to get a meaningful result. dBm and dBW (and others, but definitely not all) have the odd property that they completely define a dimension, despite being dimensionless. They still carry or reflect (or whatever) the indicated dimensional unit with the quantity conveyed. Sort of. IMHO, that's actually a hint that one has to pay attention to what one is doing to get usable results. How about "a signal at 0 dBFS is one whose instantaneous power I'm not comfortable with the concept of "instantaneous power." Rather, I think we have to just concede that the "dB" sometimes breaks tradition and works with instantanous quantities rather than power. It can be anything. Instantaneous, averaged, glacial, whatever. Time may not be involved at all, or it might be. reaches but never exceeds the instantaneous power associated with full scale of the converter"? Modifying your formula above, dBFS = 20 * log_10(peak signal voltage / converter maximum voltage) That is essentially what I wrote last night. Thanks for your input, John. -- John Eric Jacobsen Minister of Algorithms Abineau Communications http://www.abineau.com |
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