[sdiy] Vocoder dabblings

Tom Wiltshire tom at electricdruid.net
Mon Oct 22 17:50:38 CEST 2012

Hi Richie,

I think that sounds really good! It's indisputably "vocodery", which is what you're after. I suppose having got it working and the structure of it sorted out, the question would then turn to "how does it sound?" and that's where the art takes over from the technical side. Presumably "well-thought of" or "classic" analog vocoders have their own specific characteristics of timing and frequency responses or distortion that give them their particular "sound". Adding modelling to this level of complexity would presumably give a digital vocoder a similar level of character.

> Processor is dsPIC30F6012A 16-bit DSC from Microchip running at 25MHz. Audio sample rate is 24kHz.  Vocoder is 20 bands (20 analysis + 20 synthesis), each 1/3rd octave, filters are 8th order Butterworth band-pass response (24dB/oct slope each side)  (160 seperate state-variable filters in total!)

So each filter is two SVFs in series, right? Are these typical 'Chamberlin' SVFs or did you do anything clever to them? The Chamberlin SVF is only supposed to be stable to 1/6th of the sample rate, so I'm guessing you used an alternative algorithm to avoid the need to oversample the filters.
I'm not up to date on digital filtering, so I'd be interested to hear your thought process when working out what filter design to choose for this application.

> Lowest band is 60Hz, highest band is 6kHz, although the bands in the lowest octave do little with speech signals.  Sibilance from modulator above 6kHz is high-pass filtered and mixed in with the modulated carrier to increase intelligibility.  +3dB/oct spectral tilt is added for classic sawtooth/square based carriers, and -3dB/oct spectral tilt is added for white noise carrier.  No user-interface or spectral display.
> Modulator audio was either Radio-4 news, or audio sample played from Akai sampler.  Carriers are various waveforms or white noise from Roland SH-09, or hoover sound from Juno-2.
> Never built a vocoder before, either analog or digital so please be kind...

A vocoder is not a simple thing in any incarnation, so I think you should be proud of having pulled it off at all!!


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