[sdiy] Understanding 80s Synth Architectures
Michael E Caloroso
mec.forumreader at gmail.com
Sat Feb 5 07:29:24 CET 2022
Note that the first edition of MAM does not have the Chroma section.
On 2/5/22, Ben Bradley via Synth-diy <synth-diy at synth-diy.org> wrote:
> " 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
> 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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