DIY Digital Synth

[Jim Patchell]

jbv wrote:

> >     Glad to hear I am not the only one who is not afraid to utter the "D"
> > word. :-)
> >
>
> he he he...
>
> OK. Here we go.
>
> The 1st thing I'd like to say is that your digital synth project follows the
> same
> path as G. di Giugno's (GdG) researches at IRCAM in the mid 70's. I don't mean
> t
> o be cynical here, I'm just trying to put things in perspective. Furthermore,
> I want to
> point out that it's a pretty interesting field, as I'll try to show below.
>
> Actually, GdG started with a single board full of TTL logic (4A) dedicated to
> additive & FM
> synthesis : it featured a bank 64 (or 128, don't remember) digital oscillators
> (based on the
> Mathews algorithm) and their enveloppes.
>
> The interesting thing is the way the 4A evolved into the 4C (and later the 4X,
> which was a
> combination of various 4C boards). The idea was to replace a fixed hardware
> architecture by
> a programmable and more flexible one.  This was achieved by latching every
> component of the
> board (multiplier, adder/substractor, WT memory, etc.) in order to get a
> pipe-lined structure.
> Furthermore, the use of of a data memory and a micro-program memory allowed
> some
> programmability of the unit : recombining elementary  blocks and allowing
> data-flow between
> blocks through 2 general purpose buses, in order to achieve different
> algorithms : oscillators
> banks of course, but also digital filters, delays, reverbs, waveshapers, and
> even FFTs...
> Somehow the first DSP was born.
> I don't have any article reference at hand, but the 4X system was described in
> several CMJ
> articles between 1975 & 1980. AFAIR a similar system called SYTER was
> developped
> rougly at the same time in the INA-GRM studios.
>

    I have the article on the 4C (and I think the 4B article is in the same book),
but did not know about the 4X.  I will have to see if I can get over to the local
University to see if they have that CMJ.  There was another article in the same
book (I mentioned this book in an earlier post, but dang, it must be too early in
the moringing, I can't remember the name of the book or the editors), which is a
collection of articles from the CMJ, and now I can't remember the author or name
of that article either (BRING ME MY COFFEE!), but anyway, my block diagram is just
an extension of the one in that article.

>
> BTW, some figures are amazing : the 1st prototype of the 4X unit (built circa
> 1980) worked
> at 16 MHz on 24 bits fixed words, weighted 100 Kg and featured 1700 TTL chips
> !!!
> And now, we can we can do the same at home on a single programmable logic
> chip...
> Some of you may ask "why not use a regular DSP instead"...
> Well, first of all there's the pleasure of hacking (this is synth-DIY, isn't
> it ?) and thus
> understanding step by step the basics of DSP, and also because of the
> possibility to design
> your own custom DSP.
>

    I agree.  I even considered using a DSP at one time, the Analog Devices SHARC
looked interesting, but doing the gate array trip looks like a lot more fun.

>
> Now, here's the result of my own brainstorming on the subject. I tried to
> figure out what I could
> keep from GdG work on the 4X boards, how it could be squeezed into
> programmable logic and,
> most important, what features I could add to make this thing more "hybrid" (or
> less dry-digital).
> The 1st thing that came to mind was to add a few ADCs and DACs (for analog
> voltage-control
> and interfacing with analog synths) and MIDI i/o. A part of the WT memory
> should also be
> dedicated to an expo converter.
> As for parameters to be VCed, of course we have freq & amp (as well as
> enveloppes) for digital
> oscillators, but also freq & phase & amp offset between harmonics in case of
> oscillator banks for additive synthesis, as well as number of harmonics...
> Various parameters (such as freq cut-off,
> Q...) can be VCed in case of filters, or delays, reverbs, etc...
> Micro-programs can also be selected by VC (in order to switch between filter
> or reverb
> algorithms for instance).
> Another promising (yet more abstract) approach could be to VCed various
> parameters of each
> algorithms without any straight relationship with the sound itself. Of course,
> the final sound will
> be modified, but what I mean is to voltage-control some less straightforward
> parameters (such as
> temporary data in a specific algorithm, instead of fundamental freq or filter
> Q). I guess it can be
> called "voltage-controled hacking". This has to be tested case by case, but
> I'm sure some weird
> (yet interesting) results can be achieved.
> And last but not least, a more ambitious path is to combine VC, digital world
> and some promising
> fields like alife (especially cellular automata and genetic algorithm), all in
> real time, of course.
> Basically the idea is to use CA or GA to shape the spectrum of a sound (with
> additive synthesis
> for instance), and to allow realtime modification of various parameters of the
> CA or GA algo
> through voltage control...
>

    The voltage controlled aspects sound interesting.  With Delta Sigma converters
being so cheap these days, there are a lot of posibilities there.  One of these
days I need to find out what I can do  with the A/D converters on the Nord
Modular.

>
> If we all know which elementary blocks are necessary for digital sound
> generator / processor
> (1 multipler, 1 adder/sub, 1 WT, data + adress memory, and at least 2 data bus
> - which can be
> pretty tricky to achieve with programmable logic), I'm still trying to figure
> out which basic
> blocks are necessary for CA & GA algo... And am reading about Danny Illis &
> Karl Simms
> work at Thinking Machines...
>
> AFAIK, there isn't any such system on the market. But if anyone knows of
> similar research
> being conducted at the moment, I'd really like to get some info...
>
> jbv

    Sounds like you have lots of interesting ideas.  It is hard to say in either
your case or mine, if the machines we are thinking of would ever be comercially
viable (which may explain why "there isn't any such system on the market).  Back
when the DX-7 came out several people I know got one, I myself went for the TX-816
(same thing, more of the same).  Of everybody I knew that had one, I am the only
one that still has one.  The big complaint was "It is imposible to create your own
patches!".  I tried to show a few people how to program the thing, but unless you
know what it is that FM really does, it is somewhat difficult.  But, I guess that
is another topic.

    -Jim