[Mark]

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