[sdiy] Schmitz ADSR replace pots with CV control?

[rsdio at sounds.wa.com]

It seems best to use the appropriate technology for the task, and  
also to save yourself trouble when building. Whether it's discrete  
transistors, op-amps, vintage synth chips (that are no longer  
manufactured), digital logic, analog-compatible MOS FET logic gates,  
8-bit processors, 16-bit or 32-bit processors - each has advantages  
and shortcomings.

If you want to use a PIC to replace the 8048, 8049 or 8051 CPU in an  
old synth, then you're likely to have only advantages. I think that's  
a great choice, but I wouldn't recommend a PIC for every task.

Replacing discrete logic with a PIC is basically a mistake. I don't  
see logic chips as "messy" because they do what they're supposed to  
do, practically instantaneously. With a PIC or other CPU, the  
response is not instant, but rather has to wait for the software to  
catch up to what the hardware is doing. It's only when you can't fit  
all the logic on the board, or when you need to change the logic  
significantly, that a CPU makes more sense.

One big disadvantage of a CPU solution is that the board doesn't do  
anything when you first solder it together. You have to load the code  
into memory. For me, a big advantage of discrete logic is that no  
"programming" is necessary when assembling hundreds of boards. They  
just work as soon as they're soldered and powered.

Also, I've saved designs from other engineers that were based on an  
ARM chip with hundreds of I/O pins, because even several hundred  
megahertz is not fast enough to update that many pins without a  
substantial latency and therefore delay in reaction. Discrete logic  
can implement counters, loops, decoding, and other tasks instantly  
when a CPU would require several steps to produce the same result.

By the way, everybody talks about FPGA because it is the latest and  
most powerful variation on the theme of programmable logic. However,  
FPGA tends to be expensive overkill. Instead, take a look at SPLD  
(Simple Programmable Logic Devices), CPLD (Complex Programmable Logic  
Devices) starting at $0.90 each, and EEPLD (Electronically Erasable  
Programmable Logic Devices). These can take the "mess" out of logic  
without sacrificing the instantaneous response of discrete logic. If  
you want to implement a processor from basic building blocks, then an  
FPGA has what you need, although it will cost more than someone's  
already-designed processor chip. But if you just have a handful of  
discrete logic chips then you should look into other programmable  
logic chips that cost much less and still have all you need.

Brian


On Nov 26, 2013, at 04:01, Roman Sowa wrote:
> yes, and frankly I thought about making semi-digital envelope  
> recently.
> But:
> 1. PIC firmware retention is only 40 years ;)
> 2. pure logic control exhibits beauty of cleanness and reliability
> 3. I like it the way it is...
>
> How about this usage of single SSM2164:
> cell 1 - VCO
> cell 2 - envelope
> cell 2+3 - multimode filter
> plus 1 transistor VCA ala Korg MS
>
> Who's going to do the PCB for that?
>
> Roman
>
> W dniu 2013-11-26 12:26, Tom Wiltshire pisze:
>> Now that's a circuit that is crying out for a little PIC with a  
>> comparator on it to get rid of all that messy logic.
>> You could clear up the two 4011's and the 4013 bistable, and  
>> probably reduce the number of op-amps too.
>