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From: mc@flutter.tv.tek.com (Mike Coleman)
Newsgroups: comp.dsp
Subject: Re: Digital Output and VU meters
Message-ID: <MC.91Mar13104330@flutter.tv.tek.com>
Date: 13 Mar 91 18:43:30 GMT
References: <50010@apple.Apple.COM> <27517@netcom.COM> <50146@apple.Apple.COM>
Sender: root@sail.LABS.TEK.COM
Organization: Tektronix TV Products
Lines: 48
In-reply-to: malcolm@Apple.COM's message of 12 Mar 91 17:36:42 GMT

In article <50146@apple.Apple.COM> malcolm@Apple.COM (Malcolm Slaney) writes:

   What does 0VU really mean?  I assume it is power related but I'm not sure of
   all the nuances.....and I suspect that there is a temporal dimension to it.
   Otherwise why would Nackamichi make such a big deal about their peak VU
   meters?  Are VU's constant across frequencies?

   So, what is 0 VU really mean?  I want to know so that I can consistently 
   normalize my speech signals.  The energy in a digital impulse train is very
   different from a full-scale sine wave.  Speech is somewhere in between.  Just
   how high can I crank the analog gain before I saturate the tape with random
   speech data?


If you want to know what a VU meter is, there is an ANSI/IEEE standard
(152-1953) describing it.  There IS a 'temporal aspect' to it, described by
the indicator's response to tone bursts:

-  A VU meter must respond to 99% of its final value within 300
   milliseconds +/- 10%.

- It must overshoot at least 1% but not more than 1.5%.

- The indicator must fall to a rest position from 0 VU in 300 milliseconds
  +/- 10% after removal of a signal.

This can be achieved with a damped two-pole response, with a frequency of
2.1 Hz and a Q of 0.62 according to one of our analogue designers.

As far as a description of the actual steady-state electrical level
necessary to deflect the meter to 0 VU, this differs among users.  It is
customary to make 0 VU correspond to the level of a sinewave that is 8 dB
below the "peak program level" on an audio line.  0 VU should be at least 8
dB below clipping, for example.

Modern broadcast studio practice seems to put 0 VU at about 4 or 8 dBu.  A
dBu is a voltage measurement, relative to 0.775 volts RMS.

There is no standard I am aware of that relates a digital audio level to a
desired VU level.  If I were making a digital '0 VU' tone, I would make it
a sinewave at 8 dB below the largest sinewave you can represent with your
digital code.

Note that there are other, better metering standards than VU for most
purposes.  If you have questions, send email to me.

Mike Coleman  (mc@flutter.tv.tek.com)
Tektronix