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#1
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dBFS
Hi,
Some had responded here to my recent inquiry on levels that dBFS is a peak measurement. If an RMS measurement needs to be made for a digital signal (i.e., on a digital mixing console or a ProTools plugin), what units are utilized? I thought they were dBFS, i.e., that dBFS was an RMS measurement. Apparently I am incorrect. Somebody please set me straight. -- Randy Yates % "Ticket to the moon, flight leaves here today Digital Signal Labs % from Satellite 2" % 'Ticket To The Moon' http://www.digitalsignallabs.com % *Time*, Electric Light Orchestra |
#2
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dBFS
Also, what reference level does an analog peak-reading meter
use? --Randy Randy Yates writes: Hi, Some had responded here to my recent inquiry on levels that dBFS is a peak measurement. If an RMS measurement needs to be made for a digital signal (i.e., on a digital mixing console or a ProTools plugin), what units are utilized? I thought they were dBFS, i.e., that dBFS was an RMS measurement. Apparently I am incorrect. Somebody please set me straight. -- Randy Yates % "She tells me that she likes me very much, Digital Signal Labs % but when I try to touch, she makes it % all too clear." http://www.digitalsignallabs.com % 'Yours Truly, 2095', *Time*, ELO |
#3
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dBFS
Cross-posting to comp.dsp.
--RY Randy Yates writes: Also, what reference level does an analog peak-reading meter use? --Randy Randy Yates writes: Hi, Some had responded here to my recent inquiry on levels that dBFS is a peak measurement. If an RMS measurement needs to be made for a digital signal (i.e., on a digital mixing console or a ProTools plugin), what units are utilized? I thought they were dBFS, i.e., that dBFS was an RMS measurement. Apparently I am incorrect. Somebody please set me straight. -- Randy Yates % "Ticket to the moon, flight leaves here today Digital Signal Labs % from Satellite 2" % 'Ticket To The Moon' http://www.digitalsignallabs.com % *Time*, Electric Light Orchestra |
#4
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dBFS
On 11/19/2010 4:33 PM, Randy Yates wrote:
If an RMS measurement needs to be made for a digital signal (i.e., on a digital mixing console or a ProTools plugin), what units are utilized? I thought they were dBFS, i.e., that dBFS was an RMS measurement. Apparently I am incorrect. Somebody please set me straight. What, exactly, are you trying to do? 0 dBFS is a peak level. It can be only one sample long, and you'll never come up with a useful RMS value for that. I explained some instances where you'd know both the peak level (full scale) and the RMS value of the waveform with those peaks. But the two aren't really the same kind of measurement. -- "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 |
#5
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dBFS
Since this seems to be a top posted thread and I don't like being
yelled at for top posting, maybe I shouldn't contribute... ;^) I can't resist. I would not think by saying dBFS you are specifying one way or another about whether a value is RMS or peak. The dB is just a power measurement. Hmmm.... maybe I'm not thinking enough. Because of the reference to meters, I am thinking of peak in the sense of a vu meter that holds a peak value, not the peak level of an AC signal. If you mean the latter, I expect all power measurements would be RMS and dBFS is a power measurement, no? Rick On Nov 19, 5:09*pm, Randy Yates wrote: Cross-posting to comp.dsp. --RY Randy Yates writes: Also, what reference level does an analog peak-reading meter use? --Randy Randy Yates writes: Hi, Some had responded here to my recent inquiry on levels that dBFS is a peak measurement. If an RMS measurement needs to be made for a digital signal (i.e., on a digital mixing console or a ProTools plugin), what units are utilized? I thought they were dBFS, i.e., that dBFS was an RMS measurement. Apparently I am incorrect. Somebody please set me straight. -- Randy Yates * * * * * * * * * * *% "Ticket to the moon, flight leaves here today Digital Signal Labs * * * * * * *% *from Satellite 2" * * * * *% 'Ticket To The Moon'http://www.digitalsignallabs.com% *Time*, Electric Light Orchestra |
#6
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dBFS
In article , Randy Yates wrote:
If an RMS measurement needs to be made for a digital signal (i.e., on a digital mixing console or a ProTools plugin), what units are utilized? I thought they were dBFS, i.e., that dBFS was an RMS measurement. Apparently I am incorrect. Somebody please set me straight. That depends entirely on which averaging standard you decide to use. Most common is LKFS according to ITU BS.1771 loudness standard. You will never, never see this in the US, but RTW standalone meters can display it. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#7
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dBFS
In comp.dsp rickman wrote:
Since this seems to be a top posted thread and I don't like being yelled at for top posting, maybe I shouldn't contribute... ;^) I can't resist. I would not think by saying dBFS you are specifying one way or another about whether a value is RMS or peak. The dB is just a power measurement. Hmmm.... maybe I'm not thinking enough. Because of the reference to meters, I am thinking of peak in the sense of a vu meter that holds a peak value, not the peak level of an AC signal. If you mean the latter, I expect all power measurements would be RMS and dBFS is a power measurement, no? It does seem that the FS applies to measuring devices, either analog or digital meters. I believe, though, that it is usual to put a dB scale on peak-to-peak reading VTVMs, with an assumed lead impedance and that the signal is sinusoidal. It seems to me that there is still some uncertainty in the meaning of dBFS. Well, consider that CDs are considered to have 96dB (or some similar number) of dynamic range. That is comparing a full scale signal (just about impossible in a live recording) to one that is all quantization noise and no signal. That doesn't seem quite fair. -- glen |
#8
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dBFS
On Fri, 19 Nov 2010 16:33:53 -0500, Randy Yates
wrote: Hi, Some had responded here to my recent inquiry on levels that dBFS is a peak measurement. If an RMS measurement needs to be made for a digital signal (i.e., on a digital mixing console or a ProTools plugin), what units are utilized? I thought they were dBFS, i.e., that dBFS was an RMS measurement. Apparently I am incorrect. Somebody please set me straight. dBFS answers the question "by how many dB must the signal be increased for the highest peak to hit full scale"? I think that should answer it for you. d |
#9
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dBFS
0 dBFS is a digital specification that represents the maximum level that a
data converter can convert. For example 0x7FFFFF... or 0x800000.. assuming twos complement. It follows that the level of all signals will be = 0 dBFS It has nothing to do with the rms level at all. The relationship between nominal rms levels and dBFS is loose. The more bits you assign for headroom, the less bits you have for low levels. A common professional audio tradeoff is 4dBu = -18dBFS. This would mean that a +22dBu sine wave would just fit into the converter range without clipping. It is also common that 0dBu = -18dBFS. This means the maximum input level is +18dBu. Al Clark www.danvillesignal.com Rick On Nov 19, 5:09*pm, Randy Yates wrote: Cross-posting to comp.dsp. --RY Randy Yates writes: Also, what reference level does an analog peak-reading meter use? --Randy Randy Yates writes: Hi, Some had responded here to my recent inquiry on levels that dBFS is a peak measurement. If an RMS measurement needs to be made for a digital signal (i.e., on a digital mixing console or a ProTools plugin), what units are utilized? I thought they were dBFS, i.e., that dBFS was an RMS measurement. Apparently I am incorrect. Somebody please set me straight. -- Randy Yates * * * * * * * * * * *% "Ticket to the m oon, flight leaves here today Digital Signal Labs * * * * * * *% *from Satellite 2" * * * * *% 'Ticket To The Moon'http://w ww.digitalsignallabs.com% *Time*, Electric Light Orchestra |
#10
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dBFS
Al Clark writes:
0 dBFS is a digital specification that represents the maximum level that a data converter can convert. For example 0x7FFFFF... or 0x800000.. assuming twos complement. It follows that the level of all signals will be = 0 dBFS It has nothing to do with the rms level at all. The relationship between nominal rms levels and dBFS is loose. The more bits you assign for headroom, the less bits you have for low levels. A common professional audio tradeoff is 4dBu = -18dBFS. This would mean that a +22dBu sine wave would just fit into the converter range without clipping. It is also common that 0dBu = -18dBFS. This means the maximum input level is +18dBu. Hey Al, I'm trying hard to see an answer to my question in what you wrote and failing. Let me respond to you with this question: If you had a meter that -24 dBFS with a Fs/4 sine wave, what would the peak value of the sine wave be? --Randy Al Clark www.danvillesignal.com Rick On Nov 19, 5:09Â*pm, Randy Yates wrote: Cross-posting to comp.dsp. --RY Randy Yates writes: Also, what reference level does an analog peak-reading meter use? --Randy Randy Yates writes: Hi, Some had responded here to my recent inquiry on levels that dBFS is a peak measurement. If an RMS measurement needs to be made for a digital signal (i.e., on a digital mixing console or a ProTools plugin), what units are utilized? I thought they were dBFS, i.e., that dBFS was an RMS measurement. Apparently I am incorrect. Somebody please set me straight. -- Randy Yates Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â*% "Ticket to the m oon, flight leaves here today Digital Signal Labs Â* Â* Â* Â* Â* Â* Â*% Â*from Satellite 2" Â* Â* Â* Â* Â*% 'Ticket To The Moon'http://w ww.digitalsignallabs.com% *Time*, Electric Light Orchestra -- Randy Yates % "She has an IQ of 1001, she has a jumpsuit Digital Signal Labs % on, and she's also a telephone." % http://www.digitalsignallabs.com % 'Yours Truly, 2095', *Time*, ELO |
#11
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dBFS
On 11/19/2010 5:36 PM, glen herrmannsfeldt wrote:
It seems to me that there is still some uncertainty in the meaning of dBFS. I think that's the only thing certain in this thread, but if you're here from the comp.dsp crosspost, you may not have encountered the concept. -- "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 |
#12
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dBFS
On Nov 19, 10:36*pm, glen herrmannsfeldt
wrote: In comp.dsp rickman wrote: Since this seems to be a top posted thread and I don't like being yelled at for top posting, maybe I shouldn't contribute... ;^) I can't resist. *I would not think by saying dBFS you are specifying one way or another about whether a value is RMS or peak. *The dB is just a power measurement. *Hmmm.... maybe I'm not thinking enough. Because of the reference to meters, I am thinking of peak in the sense of a vu meter that holds a peak value, not the peak level of an AC signal. *If you mean the latter, I expect all power measurements would be RMS and dBFS is a power measurement, no? It does seem that the FS applies to measuring devices, either analog or digital meters. * I believe, though, that it is usual to put a dB scale on peak-to-peak reading VTVMs, with an assumed lead impedance and that the signal is sinusoidal. * It seems to me that there is still some uncertainty in the meaning of dBFS. Well, consider that CDs are considered to have 96dB (or some similar number) of dynamic range. *That is comparing a full scale signal (just about impossible in a live recording) to one that is all quantization noise and no signal. * That doesn't seem quite fair. -- glen On digital audio meters we (the company I work for that is and many others in the audio industry, I believe) use a dBFS scale and display both peak spots (i.e. maximum absolute value latched for a set hold time) and bargraphs with VU/PPM meter ballistics (neither of which are RMS). The VU ballistic requires a 4dB hike to get over the "crest" factor i.e. mean absolute (rectified) value is 2/pi. So a pure test tone at any value will read/indicate the same level as the peak spot. Of course, this does not apply to anything other than a pure test tone. So dBFS refers to peak sine wave level. 0dBFS is the largest level of sine wave prior to clipping. |
#13
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dBFS
rickman writes:
Since this seems to be a top posted thread and I don't like being yelled at for top posting, maybe I shouldn't contribute... ;^) I can't resist. I would not think by saying dBFS you are specifying one way or another about whether a value is RMS or peak. I had the same impression until today when some very experienced people on rec.audio.tech/rec.audio.pro informed me it was a peak measurement. The dB is just a power measurement. Hmmm.... maybe I'm not thinking enough. I think you're thinking sufficiently, rick! In fact you're bringing up a problem I hadn't even considered yet - how can you use dB for a "peak" measurement when it isn't power? More about this below. Because of the reference to meters, I am thinking of peak in the sense of a vu meter that holds a peak value, not the peak level of an AC signal. At this point I am questioning everything myself, but my current understanding of "peak" is the latter, or more precisely, the maximum instantaneous level of a rectified AC signal. Usually there is some sort of "reset" or "decay" so that it's sort of a peak in a local temporal extent. See "Considerations for accurate peak metering of digital audio signals", AES-R7-2006. If you mean the latter, I expect all power measurements would be RMS and dBFS is a power measurement, no? Don't follow you here. --Randy Rick On Nov 19, 5:09Â*pm, Randy Yates wrote: Cross-posting to comp.dsp. --RY Randy Yates writes: Also, what reference level does an analog peak-reading meter use? --Randy Randy Yates writes: Hi, Some had responded here to my recent inquiry on levels that dBFS is a peak measurement. If an RMS measurement needs to be made for a digital signal (i.e., on a digital mixing console or a ProTools plugin), what units are utilized? I thought they were dBFS, i.e., that dBFS was an RMS measurement. Apparently I am incorrect. Somebody please set me straight. -- Randy Yates Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â*% "Ticket to the moon, flight leaves here today Digital Signal Labs Â* Â* Â* Â* Â* Â* Â*% Â*from Satellite 2" Â* Â* Â* Â* Â*% 'Ticket To The Moon'http://www.digitalsignallabs.com% *Time*, Electric Light Orchestra -- Randy Yates % "Remember the good old 1980's, when Digital Signal Labs % things were so uncomplicated?" % 'Ticket To The Moon' http://www.digitalsignallabs.com % *Time*, Electric Light Orchestra |
#14
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dBFS
Randy Yates writes:
[...] Let me respond to you with this question: If you had a meter that -24 dBFS with a Fs/4 sine wave, what would the peak value of the sine wave be? Assume the samples are integer and scaled in fractional format, i.e., between -1 = x +1. -- Randy Yates % "Though you ride on the wheels of tomorrow, Digital Signal Labs % you still wander the fields of your % sorrow." http://www.digitalsignallabs.com % '21st Century Man', *Time*, ELO |
#16
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dBFS
(Don Pearce) writes:
On Fri, 19 Nov 2010 16:33:53 -0500, Randy Yates wrote: Hi, Some had responded here to my recent inquiry on levels that dBFS is a peak measurement. If an RMS measurement needs to be made for a digital signal (i.e., on a digital mixing console or a ProTools plugin), what units are utilized? I thought they were dBFS, i.e., that dBFS was an RMS measurement. Apparently I am incorrect. Somebody please set me straight. dBFS answers the question "by how many dB must the signal be increased for the highest peak to hit full scale"? I think that should answer it for you. That does answer the question of what you think dBFS means. Although that wasn't my question here, it is useful - thanks Don. -- Randy Yates % "She has an IQ of 1001, she has a jumpsuit Digital Signal Labs % on, and she's also a telephone." % http://www.digitalsignallabs.com % 'Yours Truly, 2095', *Time*, ELO |
#17
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dBFS
Randy Yates writes:
(Don Pearce) writes: On Fri, 19 Nov 2010 16:33:53 -0500, Randy Yates wrote: Hi, Some had responded here to my recent inquiry on levels that dBFS is a peak measurement. If an RMS measurement needs to be made for a digital signal (i.e., on a digital mixing console or a ProTools plugin), what units are utilized? I thought they were dBFS, i.e., that dBFS was an RMS measurement. Apparently I am incorrect. Somebody please set me straight. dBFS answers the question "by how many dB must the signal be increased for the highest peak to hit full scale"? I think that should answer it for you. No. If an RMS measurement needs to be made for a digital signal (i.e., on a digital mixing console or a ProTools plugin), what units are utilized? Correction: change that i.e. to a e.g. -- Randy Yates % "Ticket to the moon, flight leaves here today Digital Signal Labs % from Satellite 2" % 'Ticket To The Moon' http://www.digitalsignallabs.com % *Time*, Electric Light Orchestra |
#18
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dBFS
davew writes:
[...] On digital audio meters we (the company I work for that is and many others in the audio industry, I believe) use a dBFS scale and display both peak spots (i.e. maximum absolute value latched for a set hold time) and bargraphs with VU/PPM meter ballistics (neither of which are RMS). Why do you say the VU measurements aren't RMS? Because of the meter ballistics? -- Randy Yates % "Bird, on the wing, Digital Signal Labs % goes floating by % but there's a teardrop in his eye..." http://www.digitalsignallabs.com % 'One Summer Dream', *Face The Music*, ELO |
#19
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dBFS
On Nov 19, 7:32*pm, Randy Yates wrote:
Randy Yates writes: [...] Let me respond to you with this question: If you had a meter that -24 dBFS with a Fs/4 sine wave, what would the peak value of the sine wave be? Assume the samples are integer and scaled in fractional format, i.e., between -1 = x +1. -- Randy Yates * * * * * * * * * * *% "Though you ride on the wheels of tomorrow, Digital Signal Labs * * * * * * *% *you still wander the fields of your * * * * *% *sorrow."http://www.digitalsignallabs.com% '21st Century Man', *Time*, ELO 10^(-24/20) |
#20
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dBFS
On 11/19/2010 7:17 PM, Randy Yates wrote:
Let me respond to you with this question: If you had a meter that -24 dBFS with a Fs/4 sine wave, what would the peak value of the sine wave be? -24 dBFS If you wanted to know it in dBu, you'd need to know the relationship between the digital and analog levels for that particular converter. -- "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 |
#21
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dBFS
On 11/19/2010 7:55 PM, Randy Yates wrote:
Why do you say the VU measurements aren't RMS? Because of the meter ballistics? That's part of it. A VU meter is designed to look good when indicating speech level, and speech is hardly a single sine wave. -- "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 |
#22
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dBFS
On 11/19/2010 7:28 PM, Randy Yates wrote:
you're bringing up a problem I hadn't even considered yet - how can you use dB for a "peak" measurement when it isn't power? Easy . Because you WANT to. At this point I am questioning everything myself, but my current understanding of "peak" is the latter, or more precisely, the maximum instantaneous level of a rectified AC signal. Actually, it's the maximum voltage of the AC signal at the peak portion of the waveform. It needs to be rectified in order to be seen on an analog meter, but when you have an A/D converter looking at the voltage every 1/44,100th of a second (or whatever the sampling rate is) it's looking at the absolute voltage, not an average, not one with all the peaks going in the same direction. The converter needs to convert negative peaks as well as positive ones. -- "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 |
#23
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dBFS
Mike Rivers writes:
On 11/19/2010 7:17 PM, Randy Yates wrote: Let me respond to you with this question: If you had a meter that -24 dBFS with a Fs/4 sine wave, what would the peak value of the sine wave be? -24 dBFS There is 0 information in your response. -- Randy Yates % "Midnight, on the water... Digital Signal Labs % I saw... the ocean's daughter." % 'Can't Get It Out Of My Head' http://www.digitalsignallabs.com % *El Dorado*, Electric Light Orchestra |
#24
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dBFS
On 11/19/2010 7:36 PM, Randy Yates wrote:
If an RMS measurement needs to be made for a digital signal (i.e., on a digital mixing console or a ProTools plugin), what units are utilized? Oh! So what you're really asking is how does a program come up with the level of a signal that's already in the digital domain? Simple - however it wants, and it's usually wrong. -- "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 |
#25
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dBFS
On Nov 19, 7:55*pm, Randy Yates wrote:
davew writes: [...] On digital audio meters we (the company I work for that is and many others in the audio industry, I believe) use a dBFS scale and display both peak spots (i.e. maximum absolute value latched for a set hold time) and bargraphs with VU/PPM meter ballistics (neither of which are RMS). * Why do you say the VU measurements aren't RMS? Because of the meter ballistics? -- Randy Yates * * * * * * * * * * *% "Bird, on the wing, Digital Signal Labs * * * * * * *% * goes floating by * * * * *% * but there's a teardrop in his eye..."http://www.digitalsignallabs.com% 'One Summer Dream', *Face The Music*, ELO Randy, we're not sure what you are trying to figure out. I'll try to give you background info so maybe you can figure it out yourself. In the dark old analog days audio was measured with VU meters that have specified ballistics. Because the meter is slow, a 0 VU tone had to be well below clipping in good hardware. Good electronics would have +12 to +20 dB of "headroom" above a 0 VU tone. This way when you play real audio that moved the meter to around 0 VU, the peaks would not clip. Tape recorders would gradually compress someplace above 0 VU. The key point is that 0VU did not represent a hard ceiling that you can't exceed, instead it sort of represented an average. When digital came along, there of course is a hard ceiling at full scale. This equipment tended to have peak responding meters. Early digital recordists would use the 0 dBFS peak meter like 0 VU and had bad results. Now we know when calibrating with a tone, 0 VU needs to be something like -20dBFS to avoid clipping when audio at 0 VU replaces the tone. To recap...0 VU represents an average level that give good results because there is adequate headroom for the peaks 0 dBFS represents the peak clipping point. The correspondence between 0 VU and 0dBFS therefore depends on the crest factor that you expect. The rule of thumb for professional LIVE recording is about 20 dB. Mark |
#26
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dBFS
Randy Yates wrote:
Mike Rivers writes: On 11/19/2010 7:17 PM, Randy Yates wrote: Let me respond to you with this question: If you had a meter that -24 dBFS with a Fs/4 sine wave, what would the peak value of the sine wave be? -24 dBFS There is 0 information in your response. Randy, I think this info was critical: "If you wanted to know it in dBu, you'd need to know the "relationship between the digital and analog levels for that "particular converter." I think the answer to your question can only be specified for a particular convertor with known input sensitivity. -- shut up and play your guitar * http://hankalrich.com/ http://armadillomusicproductions.com/who'slistening.html http://www.sonicbids.com/HankandShai...withDougHarman |
#27
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dBFS
On Nov 20, 12:55*am, Randy Yates wrote:
davew writes: [...] On digital audio meters we (the company I work for that is and many others in the audio industry, I believe) use a dBFS scale and display both peak spots (i.e. maximum absolute value latched for a set hold time) and bargraphs with VU/PPM meter ballistics (neither of which are RMS). * Why do you say the VU measurements aren't RMS? Because of the meter ballistics? -- Randy Yates * * * * * * * * * * *% "Bird, on the wing, Digital Signal Labs * * * * * * *% * goes floating by * * * * *% * but there's a teardrop in his eye..."http://www.digitalsignallabs.com% 'One Summer Dream', *Face The Music*, ELO The VU meter is basically a bridge rectifier followed by a low pass filter. So it's mean rectified, not mean squared. So a 1dB difference for pure tone. We don't tend to use rms or mean whatever when talking about audio levels though, we just say "level" and that seems to be good enough. It's understood that when you reach 0dBFS you're in trouble shortly thereafter. The only reliable means to know when the limit has been reached is the peak indicator (and then only if it's true peak which was the subject of a recent thread or two). As far as a reasonable measure of loudness, neither VU or peak or PPM are good enough, just a guide. This has all changed no with loudness metering, but that's another subject. |
#28
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dBFS
glen herrmannsfeldt wrote:
It does seem that the FS applies to measuring devices, either analog or digital meters. NO. FS applies ONLY to digital system. When all the bits are set to 1, the meter goes to FS. It seems to me that there is still some uncertainty in the meaning of dBFS. Well, consider that CDs are considered to have 96dB (or some similar number) of dynamic range. That is comparing a full scale signal (just about impossible in a live recording) to one that is all quantization noise and no signal. That doesn't seem quite fair. When all the bits are 1, the meter reads 0dBFS. When the bits are all zero, the meter reads -96dBFS. It doesn't matter how difficult or realistic it is to generate such a signal in the analogue world, the measure is ONLY relevant for digital signals. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#29
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dBFS
In article , Randy Yates wrote:
I'm trying hard to see an answer to my question in what you wrote and failing. Let me respond to you with this question: If you had a meter that -24 dBFS with a Fs/4 sine wave, what would the peak value of the sine wave be? I don't know what a Fs/4 sine wave is. If the meter is reading -24 dBFS, and it's a 16 bit system, that means the peak value of the waveform is 0x1FFF, right? Because it's one-eighth of the full scale value. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#30
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dBFS
In article , Randy Yates wrote:
If an RMS measurement needs to be made for a digital signal (i.e., on a digital mixing console or a ProTools plugin), what units are utilized? Correction: change that i.e. to a e.g. As I said earlier, it depends entirely on what kind of "rms" measurement, weighting, and ballistics you want. There is an IEC recommendation that some people follow. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#31
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dBFS
On Nov 20, 12:17*am, Randy Yates wrote:
Let me respond to you with this question: If you had a meter that -24 dBFS with a Fs/4 sine wave, what would the peak value of the sine wave be? --Randy If it was Full scale/4 then it's: 20log10(0.25) = -12.04dBFS It wouldn't read -24dBFS at all |
#32
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dBFS
In article , Randy Yates wrote:
davew writes: [...] On digital audio meters we (the company I work for that is and many others in the audio industry, I believe) use a dBFS scale and display both peak spots (i.e. maximum absolute value latched for a set hold time) and bargraphs with VU/PPM meter ballistics (neither of which are RMS). Why do you say the VU measurements aren't RMS? Because of the meter ballistics? For THAT, you need to go back and check the original BSTJ paper defining the VU meter. It's not peak, it's not RMS. It is something very specific. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#33
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dBFS
davew writes:
On Nov 20, 12:17Â*am, Randy Yates wrote: Let me respond to you with this question: If you had a meter that -24 dBFS with a Fs/4 sine wave, what would the peak value of the sine wave be? --Randy If it was Full scale/4 then it's: 20log10(0.25) = -12.04dBFS It wouldn't read -24dBFS at all Sorry for not being more specific, Dave. I'm using "Fs" to mean the sample frequency, not "full-scale" - for the latter I write "FS". -- 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 |
#34
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dBFS
On Nov 20, 2:43*am, Randy Yates wrote:
davew writes: On Nov 20, 12:17*am, Randy Yates wrote: Let me respond to you with this question: If you had a meter that -24 dBFS with a Fs/4 sine wave, what would the peak value of the sine wave be? --Randy If it was Full scale/4 then it's: 20log10(0.25) = -12.04dBFS It wouldn't read -24dBFS at all Sorry for not being more specific, Dave. I'm using "Fs" to mean the sample frequency, not "full-scale" - for the latter I write "FS". -- 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 OK I see what you're asking. With an initial phase of pi/4, giving a reading of -24dBFS would mean true peak is -21dBFS I suppose. I may be missing your point though. |
#35
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dBFS
On Fri, 19 Nov 2010 19:17:56 -0500, Randy Yates
wrote: Al Clark writes: 0 dBFS is a digital specification that represents the maximum level that a data converter can convert. For example 0x7FFFFF... or 0x800000.. assuming twos complement. It follows that the level of all signals will be = 0 dBFS It has nothing to do with the rms level at all. The relationship between nominal rms levels and dBFS is loose. The more bits you assign for headroom, the less bits you have for low levels. A common professional audio tradeoff is 4dBu = -18dBFS. This would mean that a +22dBu sine wave would just fit into the converter range without clipping. It is also common that 0dBu = -18dBFS. This means the maximum input level is +18dBu. Hey Al, I'm trying hard to see an answer to my question in what you wrote and failing. Let me respond to you with this question: If you had a meter that -24 dBFS with a Fs/4 sine wave, what would the peak value of the sine wave be? --Randy Four bits below full scale of the converter. Eric Jacobsen Minister of Algorithms Abineau Communications http://www.abineau.com |
#36
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dBFS
(Scott Dorsey) writes:
In article , Randy Yates wrote: I'm trying hard to see an answer to my question in what you wrote and failing. Let me respond to you with this question: If you had a meter that -24 dBFS with a Fs/4 sine wave, what would the peak value of the sine wave be? I don't know what a Fs/4 sine wave is. A sine wave with a frequency that is one-fourth of the sample rate. If the meter is reading -24 dBFS, and it's a 16 bit system, that means the peak value of the waveform is 0x1FFF, right? Because it's one-eighth of the full scale value. The positive full-scale value of a two's complement 16-bit word is 0x7FFF, or 32767. One-eighth of that is (approximately) 0x0FFF, or 4095. Assuming you just made an arithmetic error, that is an answer to my question. Thank you. So to infer from your response, one definition of dBFS would be something like this: dBFS = 20 * log_10(XPEAK / FSPEAK), where XPEAK denotes the instantaneous peak level of the signal, and FSPEAK is the full scale peak value of the digital path. What units would a typical professional digital audio system use to measure RMS values of digital signals? -- Randy Yates % "And all that I can do Digital Signal Labs % is say I'm sorry, % that's the way it goes..." http://www.digitalsignallabs.com % Getting To The Point', *Balance of Power*, ELO |
#37
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dBFS
On Nov 19, 9:44*pm, Randy Yates wrote:
What units would a typical professional digital audio system use to measure RMS values of digital signals? It wouldn't. I'm not just being a smart-aleck; I've never seen such a measurement done on a professional digital audio system. Wait, not quite: there was a Mitsubishi digital recorder back in the 1980s that had VU meters on it. As has been pointed out, they're not really RMS, but that's as close as I've seen on pro digital audio. And I have no idea how they were calibrated, or why they bothered. Peace, Paul |
#38
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dBFS
PStamler writes:
On Nov 19, 9:44Â*pm, Randy Yates wrote: What units would a typical professional digital audio system use to measure RMS values of digital signals? It wouldn't. I'm not just being a smart-aleck; I've never seen such a measurement done on a professional digital audio system. OK, well here are three examples: http://www.digitalsignallabs.com/m-1.png http://www.digitalsignallabs.com/m-2.png http://www.digitalsignallabs.com/m-3.png -- Randy Yates % "Bird, on the wing, Digital Signal Labs % goes floating by % but there's a teardrop in his eye..." http://www.digitalsignallabs.com % 'One Summer Dream', *Face The Music*, ELO |
#39
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dBFS
On Fri, 19 Nov 2010 19:36:10 -0500, Randy Yates
wrote: (Don Pearce) writes: On Fri, 19 Nov 2010 16:33:53 -0500, Randy Yates wrote: Hi, Some had responded here to my recent inquiry on levels that dBFS is a peak measurement. If an RMS measurement needs to be made for a digital signal (i.e., on a digital mixing console or a ProTools plugin), what units are utilized? I thought they were dBFS, i.e., that dBFS was an RMS measurement. Apparently I am incorrect. Somebody please set me straight. dBFS answers the question "by how many dB must the signal be increased for the highest peak to hit full scale"? I think that should answer it for you. No. If an RMS measurement needs to be made for a digital signal (i.e., on a digital mixing console or a ProTools plugin), what units are utilized? The problem here is mainly the general one of what an RMS measurement means. With a sine wave it is easy. You can relate the RMS to the peak, which in its turn relates to full scale - the only fixed point with any significance in the digital domain. With a music waveform that relationship does not exist in any meaningful way. The only ratio of any practical use is how many dB short of full scale is the largest peak - dBFS in other words. And it isn't a matter of dynamics as in a PPM - it is simply a sample-by-sample assessment. "Have I hit full scale or not?". Of course digital console must be used by people who are accustomed to the analogue domain, and want metering that works in the fashion they are used to. This means the compromise of the PPM or VU meter, which never provides that one vital piece of information. As for making an RMS measurement, of course you can always use the units of volts by relating it back to the ADC or forwards to the DAC. d |
#40
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dBFS
In comp.dsp Mark wrote:
On Nov 19, 7:55*pm, Randy Yates wrote: (snip) Why do you say the VU measurements aren't RMS? Because of the meter ballistics? (snip) we're not sure what you are trying to figure out. I'll try to give you background info so maybe you can figure it out yourself. In the dark old analog days audio was measured with VU meters that have specified ballistics. Because the meter is slow, a 0 VU tone had to be well below clipping in good hardware. Good electronics would have +12 to +20 dB of "headroom" above a 0 VU tone. This way when you play real audio that moved the meter to around 0 VU, the peaks would not clip. Tape recorders would gradually compress someplace above 0 VU. The key point is that 0VU did not represent a hard ceiling that you can't exceed, instead it sort of represented an average. OK, analog tape will get more and more non-linear, rounding off the peaks, which doesn't sound near as bad as clipping. When digital came along, there of course is a hard ceiling at full scale. This equipment tended to have peak responding meters. Early digital recordists would use the 0 dBFS peak meter like 0 VU and had bad results. Now we know when calibrating with a tone, 0 VU needs to be something like -20dBFS to avoid clipping when audio at 0 VU replaces the tone. I thought for CDs the number is supposed to be -12dB, but I am not sure what is supposed to be at -12dB. Some CD recorders have an arrow at -12dB. (And the meter isn't likely to have VU ballistics.) To recap...0 VU represents an average level that give good results because there is adequate headroom for the peaks 0 dBFS represents the peak clipping point. The correspondence between 0 VU and 0dBFS therefore depends on the crest factor that you expect. The rule of thumb for professional LIVE recording is about 20 dB. I sometimes record live high-school orchestra concerts. Because it is hard to know the level, I record 24 bit, then find the peak and RMS of each track. Then I figure out how many bits to scale each track by so that peaks stay below FS, and they should sound about right together. (I have thought about scaling in less than whole bits, but so far it works pretty well.) One that I have tried is about the mean of the peak and RMS in dB. This is with a live audience with students and kids, so the background is probably high enough not to worry about the last bits, but I try anyway. -- glen |
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