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

robert bristow-johnson rbj at audioimagination.com
Mon Oct 25 14:32:30 EDT 2010


On Oct 25, 2010, at 11:03 AM, Tom O'Hara wrote:

> Actually, it's Log2(n) * -3dB, which is the same as dividing by  
> sqrt(n).

Tom's right.  David R is right (except for the (N-1)*(-3 dB)).  Dave H  
is right.  Didier is right.

and, in my opinion, Johannes is also right.

i would think that a useful *option* for an organ or sampler (not a B3  
simulation, that's a different animal) with potentially 128 note  
polyphony, that if the musicians were to lay both his entire forearms  
down on the keyboard, that maybe there should be something other than  
the volume pedal to limit the volume.  for an internal compressor/ 
limiter, perhaps level detection wouldn't be needed as it would be for  
an outboard unit because the compressor/limiter would know the number  
of keys pressed.

if the number of keys pressed is N, i might suggest a linear scaling  
factor of 1 when N < M, where M is some preset number where this  
scaling would begin to kick in.  for N > M, perhaps it the linear  
scaling factor should be sqrt(M/N), but perhaps some other heuristic  
should be considered.  this gain signal should always have some kinda  
LPF applied to it so that it slews or ramps from one value to the  
other reasonably slowly.

but there should always be the volume pedal, in case the scaling  
applied by the compressor is not what the musician wants.

i am not sure, but i think a B3 simulation would be applying some  
kinda overall scaling based on the number of keys pressed, but i don't  
know what it is.

--

r b-j                  rbj at audioimagination.com

"Imagination is more important than knowledge."






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