[ProAudio] VU meter permitted overdrive with a sine wave...

[Richard L. Hess]

Hi, Bob,

I do not know how this applies today, but the intent in 1953 (of which I 
have a good deal of documentation) is most officially summarized in this 
(OCRd):

"IRE Standards on American Recommended Practice for Volume Measurements 
of Electrical Speech and Program Waves, 1953"

Reprinted from the PROCEEDINGS OF THE I.R.E. VOL. 42, NO. S, MAY, 1954

"4.3 Ability to Withstand Overload. The instrument should withstand 
without injury or effect on calibration, a momentary overload of ten 
times the voltage corresponding to reference deflection, and a 
continuous overload of five times that voltage, because of the great 
variation in amplitude which this indicator may encounter."

Note that "reference deflection" refers to 0VU, so +20 VU momentarily 
and +14 VU continuously. Of course, add 4 to those numbers to get dBu in 
today's standards, but when this was written, the reference level for 0 
VU was often +8 dBm (see EIA RS-219, now withdrawn).

This standard also addresses my other pet peeve with VU meters: When 
they are installed across an output line. Now, I know some devices have 
a lower output impedance than 600 ohms, so this doesn't apply, but still...

"4.2 Harmonic Distortion. The root-mean-square value of the harmonic 
distortion produced when the volume indicator is bridged across a 
resistive circuit impedance through which a sinusoidal wave between 25 
and 8,000
cycles per second is being transmitted should not exceed 0.2 per cent of 
the fundamental."

Other drafts of the standard say 0.3 per cent, but still...IMHO all VU 
meters should be buffered.

Cheers,

Richard


On 2021-01-28 12:26 p.m., Bob Katz via ProAudio wrote:
>
> This list has been remarkably silent, but I am very grateful that it's 
> around when I have a question to ask AUDIO EXPERTS.
>
>
> Who here knows the acceptable rating of a standard mechanical VU meter 
> above 0 VU with a sine wave?  If I recall correctly, they are designed 
> to accept as high as +6 VU or even higher. For example, when doing 
> testing of a tape recorder for headroom. It's still ideal to shut off 
> the meter, but in some cases that's not possible.
>
>
>
> Thanks and happy New Year!
>
>
>
> Bob
>
> -- 
> If you want good sound on your album, come to Bob Katz 407-831-0233 
> DIGITAL DOMAIN MASTERING STUDIO Author: *Mastering Audio* Digital 
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> sending of this message. However a large number of electrons were 
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> 	
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-- 
Richard L. Hess                   email: richard at richardhess.com
Aurora, Ontario, Canada           http://www.richardhess.com/
http://www.richardhess.com/tape/contact.htm
Quality tape transfers -- even from hard-to-play tapes.

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