VS-4 "Votrax" unit, 1972-74; large rackmount unit; this is the unit famously referred to in old unix source code /usr/sys/dev/vs.c "Screw Works Interface via DC-11"; uses Common Votrax Phonemes (same as sc-01); uses an analog chopper for interpolation; did this even HAVE rate control?
VS-5: an large rackmount unit, improved VS-4, somewhat better sounding speech; uses Common Votrax Phonemes (same as sc-01); uses an analog chopper for interpolation and rate control?
VS-6: a smaller rackmount unit, improved vs5 with fifo buffer and rs232 rate control stuff; uses Common Votrax Phonemes (same as sc-01); uses an analog chopper for interpolation and rate control
ML-1: a large rackmount unit, which is a vs-6 with 128 phonemes instead of 64 to better support foreign languages. I actually own one of these! See http://www.kevtris.org/Projects/votraxml1/index.html ; First 64 phonemes match common votrax phonemes; uses an analog chopper for interpolation and rate control
VSB: a medium sized single pcb, apparently used in an obscure old ibm talking typewriter; uses 16-slot feedbackless Mod-9-update-slot PWM for interpolation (quite clever!) and timers for rate control; does NOT use standard votrax phonemes! Kevtris owns one of these.
VSC: updated VSB?
VSK: a small fully potted pcb, uses R/C filters for interpolation, no rate control, runs on +-12vDC; has 4 levels of interpolation like the sc-01
VSL: a small fully potted pcb, IDENTICAL to VSK except lacking several voltage drop diodes (using shunts instead), so runs on +-8VDC. I think Kevtris owns one of these.
VSM: a 6800 based board with an sc-01 on it. NOT DUMPED.
Tandy Radio Shack Speech Module: basically a brutally butchered VSK, with only 2 formants instead of 3 (so speech is garbled and basically uninitelligible), uses r/c filters for interpolation. Does NOT use standard votrax phoneme ORDER, but does have all the votrax phonemes (except for 3) in a scrambled order. Kevtris owns one of these.
The SC-01 can be thought of as a derivative of the VSK/VSL with some digital interpolation ideas (but not the implementation at all) borrowed from VSB. SC-01's interpolation uses a recirculating serial adder/subtracter and shifter with feedback. See https://dl.dropboxusercontent.com/u/79094972/sc01_interp_hex.c for code which simulates the sc-01 interpolator.
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