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From: malcolm@Apple.COM (Malcolm Slaney)
Newsgroups: comp.dsp
Subject: Re: Digital Output and VU meters
Summary: VU meters in the 1940s
Keywords: digital, DA, VU, video
Message-ID: <50285@apple.Apple.COM>
Date: 15 Mar 91 06:47:34 GMT
References: <27517@netcom.COM> <50146@apple.Apple.COM> <1991Mar13.103618.15519@corp.telecom.co.nz>
Organization: Apple Computer Inc., Cupertino, CA
Lines: 41

In article <1991Mar13.103618.15519@corp.telecom.co.nz> stephen@corp.telecom.co.nz (Richard Stephen) writes:
>CHINN HA, GANNETT DK, MORRIS RM : A New Standard Volume Indicator and
>Reference Level, Bell System Tech. Journal, Vol 19 No 1, 1940,
>pp 94-137 (January).

Oooh, good article.  It was fun to read about how this idea (VU) which is
taken so casually now has a very well thought out rationale.

Turns out they spent most of their time comparing RMS meters to peak reading.
They decided that peak meters are more accurate (give you an extra dB of level
before distortion becomes noticeable) but the peaks could be badly messed up
in a transmission network (due to different phase delays).  In the end they
concluded
	The tests of aural distortion due to overload showed a slight 
	disadvantage for the r-m-s instrument and the experiments on peak
	checking showes such a marked advantage for this type (rms) as
	compared to the peak instrument, that it was decided to develop
	the r-m-s type of instrument.
They continue to say (!!!!)
	Another consideration was that, with the advances in copper-oxide
	types of instruments, it has become possible to make r-m-s 
	instruments of sufficient sensitivity for most purposes without
	the use of vacuum tubes and their attendent need of power supply....

They also talk about the normal difference in levels between a peak meter
and an RMS meter.  This is relavent for digital systems since signals are
normalized to be full scale in the word
	Male Speech			10dB
	Female Speech			8dB
	Piano				7dB
	Brass Band			7dB
	Dance Orchestra			4dB
	Violin				5dB

Somebody suggested calibrating my system with a sine wave that was 8dB down
from the digital (full-scale) peak.  That sounds like a pretty good rule
of thumb.

Many thanks for the help!

								Malcolm