[sdiy] modular synth "standards"

[Don Tillman]

   From: "Happy Harry" <paia2720 at hotmail.com>
   Date: Wed, 21 Feb 2001 22:32:05 -0000

   >From: Kenneth Martinez <kmartinez at bency.com>
   >Date: Wed, 21 Feb 2001 15:10:07 -0700
   >
   >So, what else?   What are "typical" p-p audio signal levels in the
   >system? 

   2 schools of thought:
   #1: All signal levels are Control Voltage levels... 
   #2: Signal levels at line level... maybe 500mV to 1V pk-pk. 

Yes, it's major philosphical decision that has to be made at the very
beginning of a synthesizer design and it determines how most of the
modules will be built.

Using the same voltages for both control and audio signals is mighty
compelling in theory -- one can remove preconceptions and assumptions
about how a module or signal will be used.  A control signal can be
used as an audio signal or a VCO can be used as an LFO.  Cool.  But
I'll claim that in practice it's not all that useful.

Take for example a audio VCO with a range switch so it can also be
used as an LFO.  Audio range VCO waveforms need to have a zero DC
component to avoid VCA thumping while LFO waveforms need to "start" at
zero to be useful as a modulation source.

For a 10V p-p output, the audio sawtooth wave would then go from -5V
to +5V volts while the LFO modulation sawtooth would go from 0V to
+10V -- it's much more practical for sawtooth modulation to go from
our starting point up than to start at some negative value, fly
through our normalized point, and continue up to some positive value.
Similiarly an audio square wave would go from -5V to +5V while an LFO
square wave would go from 0V to +10V for trilling effects.

A PWM square wave for audio needs to have some DC restoration (mixing
some of the PWM control into the output) for a zero DC component,
while that wouldn't be very practical as a modulation source.

There are other examples -- a VCF might be optimized to sound good for
audio and have a nonlinear transfer characteristic (like the Moog
ladder) while that wouldn't be accurate enough for a control voltage
portmento.

Now, you *could* build a deluxe VCO with a range switch that not only
tuned it down to LFO frequencies but also adjusted all the output
voltages, but that's getting pretty weird as a conceptual building
block. 

So I don't really see any practical advantages to using the same
voltage range for audio and control signals.  But using standard
studio audio signal levels mean that you can patch in studio effects
and mixers without a problem.  I think that's a huge advantage.

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com