[sdiy] Vocoder dabblings

[Eric Brombaugh]

On Oct 23, 2012, at 5:05 AM, Richie Burnett wrote:

> It wouldn't be possible without multi-rate tricks. 

Aha! That explains a lot, especially that you were able to get the low frequency bands to work well. Good engineering!

>> Are you doing any kind of nonlinear processing on the detected envelopes? I found that can also help with intelligibility.
> 
> Not at the moment.  What type of non-linear processing do you mean?

Changing the relationship between a detected envelope and the gain on the associated carrier band to something other than 1:1 can yield some useful improvements.

> Would also like to try simple AGC on all of the carrier bands to "whiten" the carrier spectrum before vocoding.  This should make the output spectrum more like the modulator alone, instead of being like the modulator spectrum multiplied by the carrier's own spectrum.  Might sound clearer?  ...but might also boost noise in carriers with a sparse spectrum!

Sounds like an interesting idea.

> Did you do any broadband compression on either the modulator, carrier or both inputs in your vocoder Eric?

There is a type of compression applied to the modulator input which affects all bands. It is helpful under some signal conditions. Also I found a squelch / noise gate was useful to quiet the output when there was no input signal.

Eric