[sdiy] Understanding 80s Synth Architectures

[Ben Bradley]

" They also usually just have 1 DAC, time-domain multiplexed to all of
the different parameters via more logic chips."

Looks like you already have some sort of idea of how they work,
thought if you don't know, some of the "logic chips" are analog
switches (older ones from back then are CMOS 4016/4066's, and some
similar CMOS multiplexer types, newer/better/more expensive ones are
DGxxx) used to multiplex the DAC to the various analog control
voltages, with a capacitor on each output for the hold part of
sample-and-hold.

I just looked up the Rhodes Chroma (I've never even seen one in
person, though I had a Chroma Polaris) and I see it used a 6809,
apparently the last and likely most powerful 8-bit microprocessor
(more recent 8-bitters are microcontrollers). The Chroma Polaris used
an 80186, and I still don't think it was fast enough.I could put my
hand down to play maybe 10 notes at once, and there was an audible
delay before the notes played, I presume it would send out all the
MIDI note-ons before activating the internal voices.

I agree that modern analog polysynths aren't that different
conceptually, just using 32-bit microcontrollers (having many
peripherals on chip), largely or mostly ARM Cortex M's. They do the
same function of deciding which voice to activate for a new keypress
and what to do when more keys are pressed than there are voices and
such. Another significant difference is the code is written in the C
language (or possibly C++, but C is still the main language for
embedded ) for modern chips instead of assembly for the old 8-bit
processors and controllers.

Someone mentioned 1960s synths - I'm only familiar with the Moog
modular and Minimoog from back then, and they're mostly "discrete"
transistors instead of chips. You better know pretty well how a BJT
works if you're going to work on one.. The Micromoog came later and
was better at using the 1970s chip technology. I recall it used the
LM3046 transistor array as a temperature regulator for the exponential
converter transistor.

On Fri, 4 Feb 2022 at 05:25, ackolonges fds via Synth-diy
<synth-diy at synth-diy.org> wrote:
>
> Hi Everyone,
>
> As the local SDIYer I sometimes get asked to try and fix synths from the 80s like various Rolands, Korgs, Oberheims etc. and I generally just try to pinpoint the rough area of the issue and replace logic chips until the issue is resolved...
>
> Most of these synths have a CPU connected to everything via a parallel address bus and a parallel data bus, with all sorts of glue logic chips doing various things. They also usually just have 1 DAC, time-domain multiplexed to all of the different parameters via more logic chips.
>
> Obviously this is very different to the modern ways that microcontrollers and DACs are used in synths, and since I wasn't around in the 80s, these older architectures are very foreign to me. To aid in my troubleshooting efforts, I would love to better understand the details of how these architectures work, so I was wondering if anyone on here would be able to point me to any resources that could explain these types of systems to me, be they websites, articles, or books?
>
> Thanks a lot for any advice you might have.
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