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  #1  
Old November 22nd 10, 02:32 PM posted to rec.audio.tech,rec.audio.pro,comp.dsp
Arny Krueger
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Posts: 17,262
Default dBFS

"Randy Yates" > wrote in message


> What units would a typical professional digital audio
> system use to measure RMS values of digital signals?


The well known DAW software packages Cool Edit Pro and Adobe Audition both
have a measurement tool that produces both peak, average, and RMS values.
The measurement interval is the range of the wave that is selected and
highlighted on the screen. They provide their measurements in dB FS.

Signal values in DAW programs are typically dimensionless because they are
dependent on the gain of the ADC that was used to digitize them, perchance
they once existed as analog voltages. The gain of ADCs is not standardized
and is often not even carefully specified. Pro audio ADCs often have
continuously-variable analog or digital attenuators on their inputs. If you
stabilize the settings of the input attenuators, then Pro Audio ADCs can be
calibrated. I use an analog meter and sine waves for the purpose.

The RMS and peak values of a dimensionless quantity are themselves both
dimensionless.


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  #2  
Old November 22nd 10, 02:50 PM posted to rec.audio.tech,rec.audio.pro,comp.dsp
Mark
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Posts: 966
Default dBFS


some comments:

1) Any meter can be calibrated to read the RMS value of a steady sine
wave. i.e. Simpson 260 reads the RMS value of a steady sine wave but
is not true RMS responding.

2) True RMS meters will read the RMS value of any (within its
limitation) STEADY waveform i.e. a steady square wave or steady
triangle wave etc.

3) There is no standardized reading of RMS for a time varying waveform
like real audio, as has been mentioned the integration time and
weighting needs to be defined and I don't think there is such a
standard?

4) dBFS meters on most digital equipment are not RMS or average but
are more peak reading, that (try to) capture the peak value of even an
individual sample.

5) Most of us given a choice I suspect would choose to a dual meter
system, one that shows both the peak so that we can be assured there
is no clipping combined with another display that show the LOUDNESS.
LOUDNESS is not the same as RMS. There are some standards on metering
of loudness of audio.

Those are the two reasons we meter audio, to assure the equipment is
not overloaded even on peaks and for some gauge of the loudness.

Note: As has been discussed in another thread on intersampling peaks,
even the definition of peak is ambiguous, the peak SAMPLE value is not
always the same as the peak value of the reconstructed waveform. I
don't know, but I suspect that most peak meters calibrated in dBFS are
reading the peak SAMPLE value and not the true wavefomr peak therfore
the true audio peak can be even higher. I don't know if this is what
Randy is trying to get at or not.

Mark




  #3  
Old November 22nd 10, 03:17 PM posted to rec.audio.tech,rec.audio.pro,comp.dsp
Arny Krueger
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Posts: 17,262
Default dBFS

"Mark" > wrote in message

> some comments:
>
> 1) Any meter can be calibrated to read the RMS value of a
> steady sine wave. i.e. Simpson 260 reads the RMS value of
> a steady sine wave but is not true RMS responding.


Yes, and most of the meters in use, whether as test equipment or part of a
piece of audio gear, is average-responding, calibrated as if it was RMS.
IOW as we both knows, its only accurate for sine waves.

> 2) True RMS meters will read the RMS value of any (within
> its limitation) STEADY waveform i.e. a steady square wave
> or steady triangle wave etc.


That depends on the TRMS meter. In another post I pointed out that some DAW
software metering facilities respond to the portion of the wave that is
selected which can range from a whole song to one or just a few samples.

> 3) There is no standardized reading of RMS for a time
> varying waveform like real audio, as has been mentioned
> the integration time and weighting needs to be defined
> and I don't think there is such a standard?


I'm pretty comfortable with the metering in my DAW software because it
leaves the integration time up to me and delivers both peak and average
readings.

> 4) dBFS meters on most digital equipment are not RMS or
> average but are more peak reading, that (try to) capture
> the peak value of even an individual sample.


Yes, or they respond to both peak and average values.

> 5) Most of us given a choice I suspect would choose to a
> dual meter system, one that shows both the peak so that
> we can be assured there is no clipping combined with
> another display that show the LOUDNESS. LOUDNESS is not
> the same as RMS. There are some standards on metering of
> loudness of audio.


I have a lot of equipment and software that works this way.

> Those are the two reasons we meter audio, to assure the
> equipment is not overloaded even on peaks and for some
> gauge of the loudness.


Agreed. Our ears response is closely approximated by True RMS as
calculated over various periods of time.

> Note: As has been discussed in another thread on
> intersampling peaks, even the definition of peak is
> ambiguous, the peak SAMPLE value is not always the same
> as the peak value of the reconstructed waveform.


I've been aware of this issue for years. I take intersample peaks to be
freaks of nature that respond well to being ignored.

> I don't
> know, but I suspect that most peak meters calibrated in
> dBFS are reading the peak SAMPLE value and not the true
> wavefomr peak therfore the true audio peak can be even
> higher. I don't know if this is what Randy is trying to
> get at or not.


Some software applies a Sinc function to the samples and integrates under
it.


 




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