Re: [sc-users] make your digital oscillators sound like a moog!

["Sean Costello" <seancostello2003@xxxxxxxxxxx>]

From: "kernel" <le_kernel@xxxxxxxxxx>

> I've got a doepfer modular catalogue (as near as I got to owning one)
> which bangs on about the SSM & CEM chips that they use in their filter
> modules.
> so why are these things called 'chips'?  are they analog devices under
> stable digital control?  just curious.

They are called "chips" because they are a single IC (usually 14 to 18 pins)
that can act as a VCO, VCF, VCA, etc. with a minimum of external components.
Usually you would hook up a few resistors, and a capacitor or 4, and you
would have a complete voltage controlled whatever.

In many of the synths that used SSM and Curtis components, the voltage
controlled components were under digital control. For a synth like the
Prophet 5, the knobs and panel controls were demuxed (i.e. a multiplexer
like the CD4051 was used to select a single knob at a time), the resulting
value was digitized, and processed by a microprocessor. The microprocessor
would then send out a corresponding value into a single DAC, that was
multiplexed into the proper voltage control location. Even synths that were
not programmable would use a digital scanning keyboard (such as the SH-101).
The chips were also used in modular synths like the Digisound series, where
the control might be from a pure analog source.

The earlier chips assumed that the voltages coming in would be from an
analog source, even if that source was the output of a D/A, so the voltage
response was usually a very accurate 1 volt/octave. Later CEM chips were
assumed to be used with a microprocessor, so the tuning per octave was
relaxed, with the idea that the uP would run diagnostics and send out the
proper voltage to play the oscillator in a tempered scale (this was done in
the CEM 3372, if memory serves me correctly). Not sure how that works in
practice.

As far as CEM and SSM chips being a plus, I would think of using them as a
drawback for modern analogs, simply because they are no longer made and hard
to find. Of course, many of the components used in creating analog synths
are now going away (CA3080, matched transistor pairs, non-surface mount
components).

SC3 content: The Moog ladder filter someone posted a while back does an
EXCELLENT job of reproducing the behavior of an overdriven Moog filter. One
of my someday projects is to optimize that code to use table lookup for the
tanh() in that code, and to rearrange the filter structures such that only 5
tanh() are used per filter, instead of 9.

Sean Costello