[music-dsp] Polyphony - how do you mix N number of signals andkeep a consistent volume?

Didier Dambrin didid at skynet.be
Mon Oct 25 05:54:44 EDT 2010



> As someone who designs AGCs for a living, I think using an AGC or limiter 
> is just wrong for something like this, especially since it is very 
> possible that it really isn't needed. By definition, AGC, even when 
> artfully accomplished, is a modification to the signal(s)'s original 
> dynamics.
>
> For some reason, the ongoing mindset seems to be that all note values have 
> to be produced at the highest level possible, and therefore when you add 
> several together they clip. But why do they have to be at highest level in 
> the first place? To enjoy the best signal-to-noise? To provide the purest 
> waveform?
>

Marketing..
You'd be surprised how musicians are too fooled by loudness=quality.
So it doesn't hurt for a synth to have its own limiter (especially for 
filter resonance), as long as pros can disable it.
Besides, if it really has a big resonance, it will have to be limited in 
some way on its own, whether it's in the plugin or outside (or even in the 
filter itself. On top of this, AGC is often kinda built-in filters, it's 
common to lower the level along with resonance).



> The fact is, if you must eventually reduce their levels anyway to avoid 
> clipping under summation, then perhaps they shouldn't (and needn't) be so 
> loud in the first place.
>
> What we should be thinking is backwards from "all notes produced should be 
> as close to clipping as possible." We should be thinking worst-case all 
> inputs full level and summed, and simply scale our inputs accordingly (see 
> below) from the beginning.
>
> Yes, the individual signals will be quieter. But this is the reason we 
> have lots of bits. 24 bits of resolution allows for 144dB of dynamic 
> range, WELL beyond the limits of human perception, never mind comfort. 
> [Even 16 bits with its 96 dB clipping-to-noise range (perceived to be much 
> greater with dither) has proven in practice to allow for 20 dB of headroom 
> with a generally acceptable signal-to-noise, even for full orchestral 
> recordings with quiet interludes and solos.]
>
> Also, keep in mind that unless all the signals are (1) identical, (2) at 
> full value AND (3) in perfect phase coherence with each other (a very 
> unusual circumstance), they will never sum at the peak theoretical value 
> anyway, but tend to add up to a level of, for "N" summed channels, (N-1) * 
> 3dB. So for eight inputs at full volume, each input could be scaled: 
> ((8-1) *3) db, or -21 dB.
>
> So, no: ACG is a band-aid. The real world has pretty much shown that if 
> you just mix at a reasonably lower level, the problem is greatly minimized 
> if not eliminated, with little audible penalty, and does not require the 
> complexity of creating an AGC and artfully implementing it.
>
>
> Kind Regards,
> David Reaves
>
>
> On Sun, 24 Oct 2010 20:26:25 -0400,  robert bristow-johnson 
> <rbj at audioimagination.com> wrote:
>>
>>
>> On Oct 24, 2010, at 5:23 PM, Alan Wolfe wrote:
>>
>>> How I understand mixing is that if you have N signals, all in the
>>> range of -1,1 that you want to add all the signals together then
>>> divide by N to get an output signal still in the range of -1,1.
>>>
>>> What do you do then if you are making a live playing synthesizer and
>>> want to play 1 to N notes at the same time?
>>>
>>> If i cap the max number of notes to 8, add all the channels together
>>> (some of which may be silent) and then divide by 8, single notes
>>> played alone are very quiet.
>>>
>>> If i only divide by the number of non silent channels, there is an
>>> audible pop when a new note is added or removed and the rest of the
>>> audible channels suddenly increase or decrease in volume.
>>
>> would you expect something different?
>>
>>> How is this normally handled?
>>
>>
>> i dunno what this is, a synthesized organ or similar?
>>
>> normally, whether you have 0 or 1 or 2 or N notes that may or may not
>> have simultaneous onset or release, you do *no* scaling per se
>> regarding the number of notes.  just add 'em together.  we assume that
>> the note onset and release is click free by virtue of the synthesis or
>> sample playback alg.
>>
>> now, to keep a consistent volume, that is level compression or AGC.
>> you apply that to the summed channel or channels (in case of stereo).
>> the gain change applied by the compressor or AGC better be designed to
>> be reasonably smooth, by use of some ramping or LPF on the gain
>> control signal.  sometimes you cannot avoid some artifact of the gain
>> change (usually called "pumping") but you should always be able to
>> avoid clicking or popping.  clicks or pops are an indication that some
>> mistake has happened.
>>
>> that's my spin on it.
>
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