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Randy Yates Randy Yates is offline
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Default 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
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Randy Yates Randy Yates is offline
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Default 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
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Randy Yates Randy Yates is offline
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Default 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
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Mike Rivers Mike Rivers is offline
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Default 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
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rickman rickman is offline
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Default 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




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Scott Dorsey Scott Dorsey is offline
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Default 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."
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glen herrmannsfeldt glen herrmannsfeldt is offline
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Default 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
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Don Pearce[_3_] Don Pearce[_3_] is offline
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Default 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
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Al Clark Al Clark is offline
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Default 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


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Randy Yates Randy Yates is offline
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Default 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


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Mike Rivers Mike Rivers is offline
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Default 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
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Default 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.
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Randy Yates Randy Yates is offline
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Default 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
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Randy Yates Randy Yates is offline
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Default 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
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Randy Yates Randy Yates is offline
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Default 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
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Default 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)
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Mike Rivers Mike Rivers is offline
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Default 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


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Mike Rivers Mike Rivers is offline
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Default 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
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Mike Rivers Mike Rivers is offline
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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
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Randy Yates Randy Yates is offline
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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
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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
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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




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Default 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
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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.
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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."
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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."
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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."


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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
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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."
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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
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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.
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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


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(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
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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
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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
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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
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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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