[sdiy] convolution

[Richard Wentk]

At 22:41 17/06/2005, dustin sedlacek wrote:
>i have asked this before ...but i dont know if it made it to the list ... 
>EMU samplers have a effect that they call "transform multiply" ...taken 
>from the E4xt Manual "Transform Multiplication: this function merges two 
>sounds together in a unique way which can create many strange and 
>beautiful sonic textures. Frequencies common to the original sounds are 
>accentuated while uncommon frequencies are discarded"
>
>  it is my understanding that this is called "convolution"

Or spectral multiplication. They're notionally similar but not identical in 
practice.

>and it seems to me that this can be done in the analog realm also. but i 
>am not sure how exactly.

A high quality 1024 channel vocoder would produce a result very similar to 
spectral multiplication, but would obviously be rather challenging as an 
analogue project. :)

The problem with a BBD approach is that each sample in the output includes 
a contribution from every sample in the input window. So if your window is 
a relatively small 128, that means 128 multiply/accumulates. For a longer 
sample window, you need to increase the components correspondingly.

Done discretely it's a lot of hardware, however you do it.

Digitally it's a much easier. The brute force approach works fine for 
non-real time work. Or for faster results you can multiply FFTs.

Richard