[sdiy] super cheap synthisizers

Theo t.hogers at home.nl
Mon Jun 10 09:27:40 CEST 2002

I've been thinking the same thing for a while now.
My idea was to have identical "one uC" mono synths,
programmed to be the 1st, 2nd and so on, voice of a poly synth.
No global note assignment, but each synth running the same assignment
algorithm and then responding to the correct note.

Big problem I ran into is to find a _cheap_ uC with enough timers.
For 2 "juno style" DCOs you already need two 16 bit timers.
With "juno stile" DCO I mean:
Put out the note number to a 4 to 6 bit R2R DAC loading a capacitor
and use the times to reset.
The error will be in the amplitude, frequency is rock solid.

Then you need some more 8 bit timers to generate PWM signals for the cutoff,
resonance and VCA.

Problem is that cheap uC generally don't come with 3 or more hardware
timers and a hardware async serial  interface for the midi..
The PWM signals for VCF and VCA should preferably run above 40khz and
on top of that you want enough uC speed left for decent LFO and ENV.
So away goes the software timer solution, although software timers are
probably the only way out.
One option might be to share one timer for VCF and VCA.

IMHO AVR are a assembly programmers dream, but none of the AVR fit the

Duno, maybe there is a PIC that fits the bill.
But PIC ain't my first choice, bit slow, ugly structure etc.
Then again there _are_ pp who actually seem to like the PICs,
then again again, some pp are into kinky stuff...

SX maybe a better alternative, fast and have "soft" hardware periphery that
you may configure the way you  need.
However they look a bit like the PIC on the programming side :(
And with the "soft" periphery eating some of your cycles it can become
troublesome to keep track of your cycle count for time critical work.
Still SX is probably the best candidate.

There are new fast 8052 look-a-likes from Dallas and Texas.
Some of these are both fast and offer two 16 bit and one 8 bit timer,
but don't think they fit the "cheap" part of the description.

My 2 cnts,

From: Dominic Tarr <dmt10 at waikato.ac.nz>

> I'm contemplating building a monophonic synth from a PIC chip (as the
oscilator) and a vca and vcf and decay eg.
> either controling the vcf through a heavily low pass a pulse width
modulated signal to produce a rough control voltage, or a frequency to
voltage converter.
> I havn't actually done much electronics, so what are your thoughts on the
feasibility of all this?
> cheers - Dom

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