Single chip VC-ADSR idea

[Fraser, Colin J]

> -----Original Message-----
> From: Jeremy Brookes [mailto:jbrookes at bluebear.freeserve.co.uk]
> Sent: 06 May 1999 10:26
> To: Synth-Diy
> Subject: Single chip VC-ADSR idea
> 
> I've been looking into doing a VC-ADSR based around a PIC processor,
> something like the 16C7x series. These have up to 8 on board 
> ADCs (only
> 8-bit) and 1 or 2 PWM outputs (10 bit). The ADCs could sample 
> the voltage
> for each parameter and then use this to drive the PWM 
> generator. The PWM
> output can run at something like 20kHz and with a low pass 
> filter on it at
> maybe 10kHz, it should be capable of producing a reasonably 
> fast analogue
> voltage.
-snip-

I've looked at this too - but I'm still undecided on the uC family to use...
For very low cost you can get 12 and 16 bit DACs in 8 pin packages with I2C
interfaces.
There's plenty of sample code about for implementing these buses.

Assuming you want midi control over your synth as well as 'knob' control, I
would think you could do the same job with a PIC or other microcontroller
with a UART for midi input - use CC's for ADSR parameters, and use an
external DAC to generate the output voltages.
The serial bus only needs a couple of lines, so if the processor has enough
oomph, you could use the remaining i/o lines to select demultiplexed outputs
for multiple EGs. One of those 20MIPs ATMEL jobs should handle 8 EGs.
Not quite a single chip solution, but still a very low parts count.

I'd prefer to separate the knob scanning from the envelope generation, to
give better EG performance, and to allow midi control over the EGs (and to
simplify development).

I still haven't done much in this area - I've only used the 12c508 and 16f84
PICs, and not in musical applications, but they seem to be getting faster
and cheaper all the time.


Colin f