14 band analog vocoder

This design is based upon the Okita vocoder.
I increased the number of bands to 14 (instead of 10). The ETI vocoder schematic gave
component values for this modification. I also added a voiced/unvoiced detector
based upon the Elektor vocoder and the pseudo random noise source from the ETI design.
Both inputs (analysis and excitation) pass vactrol compressors.


Layout for 14 analyse/synthesise bands	Finished analyse/sythesise board	Finished input/output board	Schematic for the I/O board

Testing during contruction	Inside	Front view	Measured frequency response

german poem / bass voice (poly saw on EMU sampler)	same poem / high chords (poly saw on EMU sampler)	voice/distorted guitar	analysis in: sine sweep (bleeds through) synthesis set to unvoiced (white noise)

I was concerned with the performance of the envelope followers and vcas that are realised with
seven NE571 ICs. It turned out that noise, especially in the high frequency bands, tends to open
the vcas, which can be heard as excitation input bleedthrough on the output. I solved this problem
by connecting Pin1/16 (NE571/rect cap) via a resistor (470k) to ground. In addition the gain cell
behaves very ungraciously when it's overdriven. Signal levels have to be compressed or limited to
prevent ugly pops and glitches. The complexity of the cirquit is nicely reduced by the NE571 though.
Perhaps the THAT 4305 integrated RMS level sensor (log) and VCA (exp) could be used for improved design.
Whats interesting to notice is that the ETI Vocoder inverts Bands 3,5,7,9,11,13 before mixing them with
the other bands. I adopted this feature as it seemed to sound smoother.