SC filters for vocoding

[James Joseph Clark]

Has anybody used Switched-capacitor filter chips in vocoders? A while
back I built a 4 formant resonator, for demonstration in a computer
speech course I taught, using two National Semiconductor MF-10
chips. These are dual SC filter chips. Each of the filters has 2
poles. The data sheet claims that one can make Bessel filters of
arbitrary order using multiple filter blocks, so it would seem that
you could use these for vocoders. The clock frequency is 100x the
centre frequency, so for the lower frequency filters (such as a 50 Hz
centre) the clock noise could appear in the audio range. It would be
simple even to construct a divider chain to produce a nice
distribution of centre frequencies.

Anybody tried this? The MF-10 chips are about $4 each, so if you used
a 4-pole Bessel filter for both analysis and synthesis, a 16 band
vocoder would need 32 of the MF-10s, costing about $130 US.

Jim
clark at cim.mcgill.ca