[sdiy] Additive/Subtractive synthesis?

[izx at xizx.net]

>>>Yes, but IIRC the DACs are in the signal path.  It'd be better to build a
simple OTA-based state-variable filter and generate the CVs from DACs.
Add enough filtering and you should be able to avoid zipper noise as the
DACs change.

Hmm...that's 8 DACs, at about $5 a pop. Once again, the real world intrudes
as we're limited (damn!) to a certain budget. One of the aims is to make
this project modular, so if the deadline looms ahead, and we're still
tinkering with stuff, we don't risk a bad/failed grade--which is why I
think I'll delay the 3-octave implementation, with its associated filter
headaches.

>>>>Well, if your base signals are digital, whu not make a very simple
digital low-pass FIR filter with a shift register and some resistors?
Then you could modify the cut-off frequency by varying the clock
frequency.

I'm actually taking DSP next semester, and I've heard (vaguely) of
Z-transforms and the like, but I don't know about any hardware
implementations of F/IIR filters *without* a DSP. Could you elaborate a
little on how to rig up a shift reg to do that? Clock controlled filtering
would be a godsend, and gain us some virtual "dollar points" by ditching
the (pretty) expensive UAF42s.

>>>>But that would only give you octaves, not the semitones (if I understand
you correctly...possibly not :-)  Also you'd end up with lots of high-Q
bandpass filters, and if they wandered off centre due to temperature
drift, etc, your notes would fade.

True, which is why I'm sticking to the "pseudo-additive all-note polyphony"
machine :-)

>>>>I know... been to my website yet?  Look in the "shop" section....

Just did...the M108 looks (sounds?) lovely, but alas...

>>>>Why not then implement a top-octave divider (ask Tim Ressel about this
:-)
and generate all the tones in parallel.  There's quite a nice project to
keep you busy for a few weeks, especially if you investigate the spectral
purity and frequency accuracy of each tone.

That was one of the first ways to synthesize each note I looked into. The
problem, again, was price...I'd need to do division by pretty odd numbers
of the base clock (say 10-20 KHz), and the only IC I know of that does this
(the '4059 -- divides from anywhere between 3 to 15999) is ~$3.00 or so in
small quantities. The '4060 (which I'm gonna use), is about 30 cents in
comparison, has a built-in ext-RC oscillator (which, I figure I could get
to about 880 Hz, for example after some heavy permutations of standard (5%)
resistors and caps), and divide by 2,4,6,8,10 sections, if I decide to go
for a multi-octave thing.

I'll keep all you guys updated on the progress of this weird hybrid project
-- I'd thought about doing a an updated (better op-amps, etc) version of
one of the homebrew synths that have complete documentation on the C.A.G.
site, but naaaw...that'd be too easy, besides being too analog.

Cheers,
Ishaan

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