[sdiy] Analog polyphony-how does it work?again-sorry.

[Rainer Buchty]

> How does the polyphony on Roland Juno 6, 60, Jupiter
> 6, Jupiter 8, Prophet 5--work?
>
>[...]
>
> Particularly, how a single knob can control the CV of
> four, six, eight... voices?

The key magic is multiplexing. A single analog voltage source (a DAC
controlled by a uC) is routed through (a bunch of) analog multiplexers
or S&H multiplexers like 4051, SSM2300 or similar to the destination.

Your single knob's value is sampled, processed, and ends up in some
parameter table which in term is used for generating the proper voltages
and routing them to their destinations.

Can't speak for the Rolands, but the Ensoniq ESQ1/SQ80 machines compute
two tables; one holds channel-based parameters like incoming MIDI
controllers, note value, etc. Since the (E)SQs are 9-times multitimbral, 9
of these tables exist. From these tables real-time parameter tables for
each physical voice (8 in total) are computed which hold the actual
modulator values (LFO, EG, MIDI controllers) as well as the control
voltage values computed from that. The CV values are then stored at
distinct locations to allow destination addressing and value transfer at
once.

The analog input values (pitch bend, mod wheel, foot controller, data
slider etc.) are routed through a 4051 to a single ADC through which the
CPU periodically scans and samples the current values. Where necessary
these are added into the channel-based parameter list.

Rainer

-- 

Rainer Buchty, LRR, Technical University of Munich
Phone: +49 89 289-28401, Fax +49 89 289-28232, Room S3240