Using an /|\ ST as a...

[Mikko Helin]

On Thu, 2 Dec 1999, Guido Goebertus wrote:

> 1. I'm playing with the idea to use an old Atari ST to control my
> filter stuff, because it has midi ports as a standard, and because it is
> easy to program. 

Go for it.

> 2. Also, I would like the Atari to control several envelopes by software.
> and I figured I only need a DAC  as a programmable currentsource for
> each ADSR, charching a cap . Am I right?

Yes, a standard unbuffered multiplying DAC is indeed a currect source,
but then you need one DAC for each CV output. Usually one 12-16 bit
DAC output (voltage) is multiplexed with several S/H stages
(cap + buffer). If you use DAC as current source, you must have
some reference voltage as input to DAC (the voltage to charge
the cap). Also some comparator is needed to tell when the cap is
charged. So you need at least two DAC's (ref + current source)
plus a comparator / ADSR. With this design you can use 8-bit
DAC's, so you have 256 levels and 256 time units. I think there
are some dual DAC's which help saving space on pcb (AD75xx, I think).
One problem will be the physical layout on pcb (you'll need 8-bit bus
with chip select lines for each DAC separately). You could build
one ADSR on one PCB and use ribbon cable and header to connect
multiple cards. Also use a latch ('259) for the chip select bits, so you
can use the data out also for address select. Each DAC also needs
a latch.

Heavy thing to build, I would recommend the usual one DAC +
multiplexing scheme. Use a serial 16-bit DAC (MAX 541 from Maxim
or other) and the rest of lines to select the channel address
for multiplexer. Though it may be that the parallel port
is too slow for 1 ms DAC channel updates. In that case
use a parallel DAC with latches (two for the DAC and one
for the multiplexer) and the control  lines
to select the output mode (address / data msb / data lsb) 
and enabling the latches, multiplexer etc. Then in code
generate the CV (EG, LFO etc), disable the multiplexer, output DAC msb, 
output DAC lsb, output address (this is last and gives DAC 
some time to settle), then enable the multiplexer again.
If you can you could also use a PIC or AVR to replace
all the latches and logics except the multiplexer, DAC and
S/H's (but then you would need the Atari at all).

-Mikko


> 
> Any ideas, comments, help or even webpages on one of these subjects??
> 
> Thanks,
> Guido
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