Audio RMS versus Peak

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terkio
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Audio RMS versus Peak

Post by terkio » Sat Sep 14, 2019 5:57 am

In line level gear what is considered signal peak value of a given signal rms value ?
The answer for a Sine signal is Vpeak /Vrms = √2; What about audio signals rms values ?
In line level gear, when considering headroom and clipping, what is the ratio used by pro designers?

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mediatechnology
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Re: Audio RMS versus Peak

Post by mediatechnology » Sat Sep 14, 2019 6:25 am

Headroom and clipping points are limited by supply rails so the peak value of the signal, in volts, is what is most relevant.
But, at least in Pro Audio, the level is usually expressed in RMS referenced to a sine wave and converted to dBu.
Without knowing what an "audio waveform" is it's hard to reproduce RMS test results.
For this reason a sine wave is used as a test signal.

A 5532 for example running on +/-15V may clip at +/- 12V peak.
Converting to RMS gives us about 8.5V RMS.
Expressed in dBu the clipping point is +20.8 dBu.

Now you could just say the clipping point is +/-12V peak and the signal level is +/- 1.2V so I therefore have 20 dB of headroom.
Both make sense.

I think the use of an RMS-equivalent (or average measurement calibrated to RMS) using a sine wave is both practical and historic and dating back to analog metering, the measurement of power and loudness perception.
Peak level measurement wasn't that common except for things like modulation of transmitters or cutterheads.
In most cases a 'scope was required to read peak level.

Now, in a digital context in particular, peak level, and the amount of headroom between 0 dbFS and signal peak level make the most sense for monitoring headroom in a medium.

For measuring loudness there's still a lot of benefit in average or RMS measurement.
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Re: Audio RMS versus Peak

Post by terkio » Sat Sep 14, 2019 7:39 am

I see, in this matter audio rms refers to sine signals. I wanted to make sure.
Another related issue ( or same ?).
RMS-equivalent (or average measurement calibrated to RMS) using a sine wave
.
When measuring AC, multimeters display a RMS value. They actually measure the mean value of the rectified signal that is converted into a rms value assuming a sine signal.
When measuring audio, this mean DC to AC rms is questionable. Is it accepted as is, or with some correction factor.

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mediatechnology
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Re: Audio RMS versus Peak

Post by mediatechnology » Sat Sep 14, 2019 8:05 am

They actually measure the mean value of the rectified signal that is converted into a rms value assuming a sine signal.
Not necessarily, depends on the meter.
The classic Fluke 8050A is an example.
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Re: Audio RMS versus Peak

Post by JR. » Sat Sep 14, 2019 8:25 am

terkio wrote:
Sat Sep 14, 2019 5:57 am
In line level gear what is considered signal peak value of a given signal rms value ?
The answer for a Sine signal is Vpeak /Vrms = √2; What about audio signals rms values ?
In line level gear, when considering headroom and clipping, what is the ratio used by pro designers?
There is only a simple relationship for sine waves (peak= 1.4x ave). Complex waveforms can vary dramatically wrt average or rms value.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

terkio
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Re: Audio RMS versus Peak

Post by terkio » Sat Sep 14, 2019 10:33 am

JR. wrote:
Sat Sep 14, 2019 8:25 am
terkio wrote:
Sat Sep 14, 2019 5:57 am
In line level gear what is considered signal peak value of a given signal rms value ?
The answer for a Sine signal is Vpeak /Vrms = √2; What about audio signals rms values ?
In line level gear, when considering headroom and clipping, what is the ratio used by pro designers?
There is only a simple relationship for sine waves (peak= 1.4x ave). Complex waveforms can vary dramatically wrt average or rms value.

JR
This is why I wonder wether there exists some thumb rule, some correcting factor applicable to audio giving better estimated values.
I am asked to design a VCA with very high headroom, I will use rail to rail op amps and rails at max ratings....and worry about clipping.

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mediatechnology
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Re: Audio RMS versus Peak

Post by mediatechnology » Sat Sep 14, 2019 11:25 am

Have you considered using the VCAs in a balanced configuration to double the headroom?

What are the available supply voltages?

Are HV op amps an option?
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Re: Audio RMS versus Peak

Post by JR. » Sat Sep 14, 2019 11:26 am

terkio wrote:
Sat Sep 14, 2019 10:33 am
JR. wrote:
Sat Sep 14, 2019 8:25 am
terkio wrote:
Sat Sep 14, 2019 5:57 am
In line level gear what is considered signal peak value of a given signal rms value ?
The answer for a Sine signal is Vpeak /Vrms = √2; What about audio signals rms values ?
In line level gear, when considering headroom and clipping, what is the ratio used by pro designers?
There is only a simple relationship for sine waves (peak= 1.4x ave). Complex waveforms can vary dramatically wrt average or rms value.

JR
This is why I wonder wether there exists some thumb rule, some correcting factor applicable to audio giving better estimated values.
I am asked to design a VCA with very high headroom, I will use rail to rail op amps and rails at max ratings....and worry about clipping.
I designed a VCA back in 1989 and got a patent (US04818951 Roberts). These days you can buy a high performance (analog) VCA off the shelf from THAT Corp. So it doesn't make much sense to design your own.

THAT corp publishes comprehensive application notes, that will get you a competent design.

JR

PS: About the only thing that almost applies to you question is setting the nominal 0VU operating point (current) for the VCA. You can hit it hotter for better S/N, or cooler for lower distortion. Again I would suggest you take the THAT corp advice for nominal operating point.
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

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Re: Audio RMS versus Peak

Post by Audio1Man » Sat Sep 14, 2019 11:40 am

In Pro Audio I would not use rail to rail op amps. Look @ the THD/SPECTRUM of the output as output level increases. Many op amps will go nonlinear when the output gets near to 3 volts from the rails voltage. To get clean 8v rms the op amp rails need to be slightly larger than 15 vdc.
Duke

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JR.
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Re: Audio RMS versus Peak

Post by JR. » Sat Sep 14, 2019 11:42 am

mediatechnology wrote:
Sat Sep 14, 2019 11:25 am
Have you considered using the VCAs in a balanced configuration to double the headroom?

What are the available supply voltages?

Are HV op amps an option?
I am not sure what exactly a balanced VCA configuration is. Using two (one for positive zig, and another for negative zag) will sum to +6dB more signal, while noise should combine incoherently to raise noise floor +3dB. So net 3dB S/N improvement for 2x cost.

To effectively realize +6dB more headroom, you would need to run the VCAs down 6dB. (FWIW I routinely ran my big consoles at nominal -6dB VU to take advantage of extra signal swing from differential outputs.)

A modern VCA should not be the weakest link for typical signal processing applications, it only suffers in comparison to a simple fader. Higher voltage rails generally deliver diminishing returns.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

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