[PD] vu-meter question

[Martin Peach]

Roman Haefeli wrote:
> On Tue, 2008-08-05 at 14:19 -0400, Martin Peach wrote:
>> Frank Barknecht wrote:
>>> For example as in the attachment.
>>>
>> Interesting playing with that...If you give the osc~ a gain of 100dB, 
>> which dbtorms translates to a gain of 1, then [env~] outputs 96.99dB and 
>> the vu meter -3dB. To get 0dB on the meter you need to give 103dB to the 
>> osc~, a gain of 1.413.
>> My onboard soundcard starts clipping with the vu meter at -3dB, so a 
>> gain of 1 on an osc~ is the maximum the hardware can handle.
>> That is, maximum gain allowed by the hardware is 100dB ("sound pressure 
>> level"), 1 ("full scale"), or -3dB (why not 0dB?).
>> Why does env~ give an output that is 3dB less than, or 1/2 of, its input?
> 
> i think (correct me, if i am wrong) this is dependent on the crest
> factor of the signal:
> 
> http://en.wikipedia.org/wiki/Crest_factor
> 
> altough, the peak of the sine is 1, -1 respectively, the average (that
> is what [env~] gives you) is lower,  3dB to be exactly. try the same
> with a square wave and the result should pretty much be 0dB,
> respectively 100dB.

Yes, that makes sense. That's why in analog equipment you try to stay 
around -3dB instead of 0dB because the peaks are at 0dB. In analog you 
can go above 0dB with progressively reduced gain, but in digital 
equipment you get hard clipping at exactly 0dB, so it's nice to know 
when it's happening. [env~] will give different output depending on the 
waveform it's measuring, so it's not too useful for determining if 
overload is occurring.
Maybe [peak~] from flib is appropriate for that, but it seems to involve 
doing an fft so it may be computationally heavy.
I'm still not able to instantiate [>~] from zexy...

Martin