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#1
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
davew wrote:
On Nov 22, 7:14*am, PStamler wrote: [...] 1V into 1k, at the input, means 1mA, so the power is 1mW. 10V into 100k at the output means 0.1mA, so the power is 1mW again. There's 0dB power gain, but there's voltage gain of 10x, which is coded +20dB in the voltage-gain realm of decibel calculation. This deviates from the "real" standard of what decibels are, by divorcing the voltage and power gains, but using the same unit for them, dB. But, if I were to double the input voltage I would see a 6dB increase in power output would I not? In context, the use of dB for voltage gain is entirely correct and acceptable. It can be used that way as long as the two voltages being compared are across the same impedance and both relate to the power in the same way. This was the case in your example. You can compare voltages at two different places as long as the impedances at those places are the same. So an amplifier working with 600-ohm input and output terminations could have its power gain expressed in dB by just measuring the voltage gain. What you must not do (which is all-too frequently done) is to use the same calculation to compare voltages in different impedances. For that, you must use the voltages to calculate the power at each point and then compare the powers to get a result in dB. If you want to use a logarithmic scale to compare voltages, call it something different (dBu, dBV) because it is not "dB". The convenient shortcut dB formula relating voltages is often taught with insufficient emphasis being placed on the conditions for which it is valid. After a while, people begin to believe that the voltage formula is the true representation of dB until a situation arises where it generates nonsense results - then they get confused and sometimes adamant. At least one well-known textbook has got it wrong. -- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk |
#2
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
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#3
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
On Nov 22, 12:41*pm, (Adrian
Tuddenham) wrote: davew wrote: On Nov 22, 7:14*am, PStamler wrote: [...] 1V into 1k, at the input, means 1mA, so the power is 1mW. 10V into 100k at the output means 0.1mA, so the power is 1mW again. There's 0dB power gain, but there's voltage gain of 10x, which is coded +20dB in the voltage-gain realm of decibel calculation. This deviates from the "real" standard of what decibels are, by divorcing the voltage and power gains, but using the same unit for them, dB. But, if I were to double the input voltage I would see a 6dB increase in power output would I not? *In context, the use of dB for voltage gain is entirely correct and acceptable. It can be used that way as long as the two voltages being compared are across the same impedance and both relate to the power in the same way. This was the case in your example. You can compare voltages at two different places as long as the impedances at those places are the same. *So an amplifier working with 600-ohm input and output terminations could have its power gain expressed in dB by just measuring the voltage gain. What you must not do (which is all-too frequently done) is to use the same calculation to compare voltages in different impedances. *For that, you must use the voltages to calculate the power at each point and then compare the powers to get a result in dB. *If you want to use a logarithmic scale to compare voltages, call it something different (dBu, dBV) because it is not "dB". The convenient shortcut dB formula relating voltages is often taught with insufficient emphasis being placed on the conditions for which it is valid. *After a while, people begin to believe that the voltage formula is the true representation of dB until a situation arises where it generates nonsense results - then they get confused and sometimes adamant. *At least one well-known textbook has got it wrong. -- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply)www.poppyrecords.co.uk This is all well and good, but in a lot of small signal territory and in DSP, we don't care about power or impedances. We may worry about output impedance and power drain of a small signal stage, for example, but we would usually say that a given stage has a voltage gain, not a power gain i.e. where voltage gain, not power gain, is the object. So, a gain of 10 in the signal chain would result in an end output power increase (assuming we were feeding some form of power amplifier) of 20dB. Consider a FET source follower as an example. Tremendous gain i.e. power gain in dB but voltage gain of a little under unity or 0dB. Now you ask a hundred analogue electronics engineers what the gain is in dB and (yes it's a trick question) 99/100 will say a bit less than 0dB. |
#4
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
On 11/22/2010 7:41 AM, Adrian Tuddenham wrote:
The convenient shortcut dB formula relating voltages is often taught with insufficient emphasis being placed on the conditions for which it is valid. This is why it's only been used over the last 30 or maybe 40 years, while the "power dB" goes back 80 years or so. It's only been since the adoption of the nearly universal voltage-not-power scheme of interfacing. This is primarily a result of the use of solid state electronics which characteristically have nearly zero output (source) impedance but without the ability to supply any significant amount of current. It's just how things work now. After a while, people begin to believe that the voltage formula is the true representation of dB until a situation arises where it generates nonsense results Not a lot of confusion, because, in general, audio people TODAY work with dB of gain or voltage levels, and absolute (not relative) watts when dealing with power levels. Someone will understand a 100W or 200W amplifier, but won't think of one having 3 dB more (power) gain than the other. -- "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
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
"Mike Rivers" wrote in message
Someone will understand a 100W or 200W amplifier, but won't think of one having 3 dB more (power) gain than the other. That would be their loss. If you compare amp power in dBs, and remember that it takes about 10 dB to create the perception of "twice as loud", then a lot of things in the real world that may seem mysterious become understandable. |
#6
Posted to rec.audio.tech,rec.audio.pro
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Questions on Levels
davew wrote:
On Nov 22, 12:41*pm, (Adrian Tuddenham) wrote: davew wrote: On Nov 22, 7:14*am, PStamler wrote: [...] 1V into 1k, at the input, means 1mA, so the power is 1mW. 10V into 100k at the output means 0.1mA, so the power is 1mW again. There's 0dB power gain, but there's voltage gain of 10x, which is coded +20dB in the voltage-gain realm of decibel calculation. This deviates from the "real" standard of what decibels are, by divorcing the voltage and power gains, but using the same unit for them, dB. But, if I were to double the input voltage I would see a 6dB increase in power output would I not? *In context, the use of dB for voltage gain is entirely correct and acceptable. It can be used that way as long as the two voltages being compared are across the same impedance and both relate to the power in the same way. This was the case in your example. You can compare voltages at two different places as long as the impedances at those places are the same. *So an amplifier working with 600-ohm input and output terminations could have its power gain expressed in dB by just measuring the voltage gain. What you must not do (which is all-too frequently done) is to use the same calculation to compare voltages in different impedances. *For that, you must use the voltages to calculate the power at each point and then compare the powers to get a result in dB. *If you want to use a logarithmic scale to compare voltages, call it something different (dBu, dBV) because it is not "dB". The convenient shortcut dB formula relating voltages is often taught with insufficient emphasis being placed on the conditions for which it is valid. *After a while, people begin to believe that the voltage formula is the true representation of dB until a situation arises where it generates nonsense results - then they get confused and sometimes adamant. *At least one well-known textbook has got it wrong. This is all well and good, but in a lot of small signal territory and in DSP, we don't care about power or impedances. There is nothing wrong with that approach, but you must not then express the ratios as 'dB'. You may include the letters 'dB' to suggest a logarithmic scale as long as they also have some sort of qualifier prefix or suffix to warn the student or the pedant (or anyone in between) that you are not comparing powers and these measurements do not follow the correct definition of a decibel. Much confusion has been caused over the years by the omission of that qualifier. Try explaining to a student who is used to thinking of dB in terms of voltage alone where the gain occurs in the following example: http://www.poppyrecords.co.uk/other/images/Gain.gif -- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk |
#7
Posted to rec.audio.tech,rec.audio.pro,comp.dsp
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Questions on Levels
In article , "Arny Krueger" wrote:
"Mike Rivers" wrote in message Someone will understand a 100W or 200W amplifier, but won't think of one having 3 dB more (power) gain than the other. That would be their loss. If you compare amp power in dBs, and remember that it takes about 10 dB to create the perception of "twice as loud", then a lot of things in the real world that may seem mysterious become understandable. You got to consider that. But, for understanding basics, knowing 6 dB is twice the voltage is what people should know and use, and talk about. The only thing power dB is good for is speaker dissapation and cost of amplifiers, and indirectly, mains power. Forget volume levels. greg |
#8
Posted to rec.audio.tech,rec.audio.pro
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Questions on Levels
GS wrote:
In article , "Arny Krueger" wrote: "Mike Rivers" wrote in message Someone will understand a 100W or 200W amplifier, but won't think of one having 3 dB more (power) gain than the other. That would be their loss. If you compare amp power in dBs, and remember that it takes about 10 dB to create the perception of "twice as loud", then a lot of things in the real world that may seem mysterious become understandable. You got to consider that. But, for understanding basics, knowing 6 dB is twice the voltage is what people should know.... They should first know that dB is a power measurement, this is the fundamental fact on which the rest is based. When they have grasped the basics, then they can be shown that the voltage is a handy way of comparing two power levels in the right circumstances (and they can remember some handy voltage ratios if appropriate). -- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk |
#9
Posted to rec.audio.tech,rec.audio.pro
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Questions on Levels
On 11/22/2010 11:36 AM, Adrian Tuddenham wrote:
They should first know that dB is a power measurement, this is the fundamental fact on which the rest is based. When they have grasped the basics, then they can be shown that the voltage is a handy way of comparing two power levels in the right circumstances (and they can remember some handy voltage ratios if appropriate). But most people working in audio don't need to compare power levels, they need to compare voltage levels. I'd teach them the "20" formula first. But then you may be more into theory and less into practice than I am. -- "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 |
#10
Posted to rec.audio.tech,rec.audio.pro
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Questions on Levels
Mike Rivers wrote:
On 11/22/2010 11:36 AM, Adrian Tuddenham wrote: They should first know that dB is a power measurement, this is the fundamental fact on which the rest is based. When they have grasped the basics, then they can be shown that the voltage is a handy way of comparing two power levels in the right circumstances (and they can remember some handy voltage ratios if appropriate). But most people working in audio don't need to compare power levels, they need to compare voltage levels. I'd teach them the "20" formula first. But then you may be more into theory and less into practice than I am. I remember the struggles I had in my earlier days, trying to get a grasp on how to use the dB system. There was so much misinformation and so many things that didn't add up when you came to match theory with practice. Nobody seemed to be able to explain what was really going on. Now most of my recording chain is of my own manufacture and I design specialist equipment for others. I am in a good position to look back at my earlier mistakes and misunderstandings and see where I went wrong (and where I was sent up the wrong path by others). I am also in a good position to spot when other people are in the same quandry for the same reasons as I was. That's why I tend to be a bit pedantic about the use of dB; I know from experience that a little pedantry at the outset will pay-off later, when the student moves on and becomes a true professional. -- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk |
#11
Posted to rec.audio.tech,rec.audio.pro
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Questions on Levels
"Mike Rivers" wrote in message
On 11/22/2010 11:36 AM, Adrian Tuddenham wrote: They should first know that dB is a power measurement, this is the fundamental fact on which the rest is based. When they have grasped the basics, then they can be shown that the voltage is a handy way of comparing two power levels in the right circumstances (and they can remember some handy voltage ratios if appropriate). But most people working in audio don't need to compare power levels, they need to compare voltage levels. I'd teach them the "20" formula first. But then you may be more into theory and less into practice than I am. Agreed, since in my world, voltmeters vastly outnumber true watt meters. QED. |
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