VU question pop quiz

[Magnus Danielson]

From: Rene Schmitz <uzs159 at uni-bonn.de>
Subject: RE: VU question pop quiz
Date: Fri, 25 Feb 2000 02:15:15 +0100

> >1. Absolute VU readings: By all accounts I have read, a Volume Units
> >meter is only relative to a user defined point, there is no absolute
> >mapping between a VU scale and any logarithmic voltage scales (dBV, dBv,
> >dBu, dBm, etc.). Because an arbitrary voltage level can represent a
> >specific sound pressure level, it makes sense that VU is arbitrary and
> >independent of voltage levels and impedance. 
> 
> Right. Usually this refers to the maximum level which the unit can take
> without distortion. (Think about the VU meters on tape decks...)

A VU indication of 0 dB should give you a feel that you are at the reference
level of that equipment. This reference level may be at -10 dBu, 0 dBu, +4 dBu
or whatever. The reference level may be diffrent at diffrent parts of the
same equiment. The 0 dB reading still gives a measure of where you are with
your level relative to whatever reference is suitable. You migth have diffrent
headroom requirements in diffrent equipment, so you may have +3, +6 or +12 dB
(or whatever) headroom above the 0 dB level. The VU reading is improtant since
it allows for a easy adjustmen of a signal to fit in nicely near the reference
level so that you have enougth margin to clip level as well as enougth margin
to the noise floor. It is a measurement, a relative measurement just like ANY
other dB reading is. A VU reading of say 0 dB will say nothing in terms of
dbSPL since it depence on so many other factors, but it does say where you
are with your level at that point in the equipment and this is why it is there.
The VU level helps you keep your signal at an optimal level from a dynamic
point of view for some particular equipment.

A change in reference level migth change my dBu readings, but for a VU level
indication it has not changed, because it is relative to the assigned reference
level. What that reference level is, that a interesting subject only when you
lift on the hood and hook up your dBu meter. The reference level could be at
100 Vrms, but it is rather uninteresting as long as the VU levels give you a
correct feel about how your program signal is using up the available dynamic
range.

Also, dB is dB is dB. It's a relative power measure by definition, and we can
convert it into voltage, current, soundpressure or whatever using extra
formulas. We migth also use diffrent reference levels. Further, we migth use
diffrent filters prior to the detector in order to filter out the frequencies
of interest.

Only in a well-tuned rig I can tell you from a VU indication what dB SPL level
I am to expect at some point. Normally most haven't the faintest idea how these
relates, and actually I think that either you use your ears to judge if the
soundpressure is good or not, or you use a dB SPL meter. The VI indication is
there for that equipments level, not for the total systems level.

Cheers,
Magnus