[sdiy] Vocoder dabblings

Eric Brombaugh ebrombaugh1 at cox.net
Sun Oct 21 23:57:00 CEST 2012


Richie,

Very nice samples. Impressive that you were able to get that many 8th-order IIR filters running on a 24MHz dsPIC.

Are you doing any kind of nonlinear processing on the detected envelopes? I found that can also help with intelligibility. Being able to control the attack and release rates is also a nice variable - long release times sound almost like reverb and while they knock down the intelligibility they sound rather dreamy.

Eric

On Oct 21, 2012, at 1:32 PM, Richie Burnett wrote:

> Hi guys and girls,
> 
> Been playing about with coding up a Vocoder just for fun after recently having to design a filter bank for a commercial job.  Have included a few audio examples of the results I'm getting for those interested:
> 
> http://www.richieburnett.co.uk/temp/vocoder1.mp3
> http://www.richieburnett.co.uk/temp/vocoder2.mp3
> http://www.richieburnett.co.uk/temp/vocoder3.mp3
> http://www.richieburnett.co.uk/temp/vocoder4.mp3
> http://www.richieburnett.co.uk/temp/vocoder5.mp3
> http://www.richieburnett.co.uk/temp/vocoder6.mp3
> http://www.richieburnett.co.uk/temp/vocoder7.mp3
> http://www.richieburnett.co.uk/temp/vocoder8.mp3
> 
> Technical specs as follows:
> 
> 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!)  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...
> 
> -Richie, 
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