[BC] Weston VU Meters -

[PeterH5322@aol.com]

>Hmmmm....about 25 years ago I (think) I knew this. It means volume unit, and
>I think the meters were designed to approximate the way we hear. Having a
>meter that sort of showed how loud (and fast ears responded) to constantly
>changing audio.

A true VU meter also has frequency response well into the supersonic 
region, too.

Used in all "classic" Ampex machines to set the bias, where the 200 kHz 
bias signal is fed into the meter, and the "bias current" is thereby 
measured.

The "damping" of the movement is intended to mimic the physiology of the 
ear ... not a "peak" reading, but an "averaged" reading with a defined 
"overshoot". Semi-peak, as the "damping factor" is less than 0.707 
("critical damping"), but no so low as to be "underdamped".

The Europeans prefer a true peak reading meter.

Also, the change was made from a 500 ohm standard (Telco) to a 600 ohm 
standard (Braodcast), hence the need for completely new terminology.

0 VU = +4 dBm, if on its most sensitive scale factor (no attenuation 
between the line and the meter, other than the specified, and required 
"build out" resistor); +8 dBm, or other, if on a less sensitive scale 
factor (more attenuation between the line and the meter).

Before, the terminology was dB, and was presumed to be across a 500 ohm 
line.

After, the terminology is dBm, and is required to be across a resistively 
terminated 600 ohm line. The "m" in "dBm" means "above one milliwatt", as 
this form of dB is a power measurement, not a voltage measurement.

I believe the term "volume unit" was also introduced at the same time.

The Weston catalog sheets I posted earlier gives the "historical" 
references.