The Logic Off Topic list

litepipe wrote:
>     This may sound stupid,
Not at all, actually!

>  but I'm wondering why do we even use VU Meters?
>  I understand that it shows us more of the "comon" level but shouldn't we 
> really be watching the peaks to see where the level is too hot? What do 
> we get from VU that we don't get from PPG? I'm just a little confued as 
> to their uses. I understand what they do now, using them is the 
> problem:-P Thanks!!


It's also partly a 'Europe versus USA' thing! The VU 'standard' was cobbled 
together by Bell Labs, CBS and NBC back in the late 30's or early 40's. 
They optimized it for radio broadcasting primarily, but for all sound 
recording in general. There's an ANSI specification for it but I'm afraid I 
don't know it off by heart. In order to determine 0 VU in the ANSI spec, a 
sine wave of +4 dBu is used. As I said earlier, it's deliberately concocted 
to have a slow response: it's intended for it to take 300 ms to reach 99% 
of its full deflection under any given aural input. Its overshooting or 
clipping must also lie within the range of 1-1.5% and not more. Useless for 
peaks.

The PPM system is European. To be honest, it's the more valuable one for 
digital recordings simply because it's far better for preventing clippings 
and overs. Unfortunately, there are several different PPM specs doing the 
rounds in Europe even now! Technically speaking, peak programme meters do 
not respond "instantaneously". Instantaneous must be defined because it's 
basically impossible to realize an electrical signal in which one has 
integrated over an infinite span. PPM's simply must have a finite 
integration time so that peaks wide enough to be audible are displayed. The 
DIN specification is probably the most commonly used although the BBC one 
is quite common too. I looked them up the other day out of interest but I'm 
afraid I can no longer remember them. It's to do, though, with a difference 
in attack times for any transient, and the degree of response to those attacks.

Those don't really answer your question, although I think they establish 
the necessary background. Suppose you've been asked to record (a) a funeral 
conducted by a group of ants who happen to live right by the take-off 
runway at an airport ... and (b) you've been asked to provide some 
background noise from a cocktail party for use in say a theatre production.

In the former case, if you worked only from the peaks (using PPM) then 
there'd be complete silence on your recordings excepting only the 
occasional loud aircraft. Of course, you could also try "riding the meters" 
by watching out for them airplanes and turning your meters down whenever 
they came by, and then returning your focus to the ants at the point in 
between. Which is really rather the point here ... your focus is on the 
occasional airplane that comes by and not upon the ants which you are 
trying to record. On the VU system you would just set your level for the 
ants and pretty much have done with it because the occasional airplane 
wouldn't really trouble you except that you might the first time have to 
nudge your recording level down a notch to make sure that ... in general 
... you recordings as a whole weren't clipping.

As for the second scenario of the cocktail party, since you're definitely 
not interested in any peaks but rather than in the average overall level, 
the VU method is definitely superior. You don't have to worry about the 
occasional clink of glass or any such because you're don't want anything to 
stand out and you don't need anything to stand out.

Then ... the standard broadcasting (and also partly mixing) problem is ... 
how do you get the volume levels of many many different sources to blend 
together into one acceptable continuum (think, adverts in the breaks 
between different radio or TV programmes). The answer is you use VU meters 
to keep the overall general level within certain aesthetically acceptable 
limits. Thus you could have a documentary on the ecological behaviour of 
ants who live near an airport followed immediately by a programme on the 
evolutionary behaviour of human beings who attend cocktail parties, with as 
many advertising breaks as you like between them, and everything would 
match in audio levels without consumers having to keep jumping up and 
turning their volume controls up and down in spite of the widely differing 
acoustic environments.

Peaks and VU's are both useful which is why they are both there, although 
with digital recordings peaks have really become pretty close to essential 
in a way that they weren't before.

Hope that makes sense.

Kool Musick
Keep Musick Kool




>   --Roger
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