[sdiy] Vocoder dabblings

[Richie Burnett]

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,