MOTM

On Fri, 12 Jul 2002 groovyshaman@... wrote:

> I just finished listening to the MOTM-480 sound bites - wow!  (I know, a
> week late and a dollar short.)   Lots of nice resonance, nearly self-osc.  I
> know Paul has indicated this module will not self-oscillate, but it sounds
> nearly there.  I've never had the chance to play with a CS-80, alas, but I
> have read that its filter does not self-osc.  So I'm wondering, does the 480
> closely match the resonance capability of the CS-80, or are we getting more
> res for "free"?

  The filters can't resonate as they have feedback limiting components in
the circuit specifically to prevent oscillation.  I suppose one could
build the circuit board without the six feedback limiting networks around
the six 'cells' (four filter stages, two resonance control stages).  
Someone once asked if a "self-osc/non-self-osc" switchable mode could be
done:  in theory it is possible, but in practice it would be cumbersome as
all six networks would have to be disconnected, and the resulting extra
circuit traces to cell nodes from analog switches might start to affect
the response.

  Since the actual CS-80 filter circuits are linear CV response and MOTM 
is exponential response, changes were made to the CV front end so as to 
provide expo response for the frequencies and resonances.  On a 
CS-50/60/80, there are separate frequency and resonance controls for the 
high-pass and low-pass filters.  There are also "global" frequency (called 
'brightness' on a CS instrument) and resonance controls on the CS 
machines.  These are provided for on the MOTM-480 as the 1V/Oct input that 
affects both the HP and LP filters simultaneously and the resonance CV 
input affects both HP and LP resonance simultaneously.

  The CS VCF filter resonance controls provide about 80% of the resonance 
range for the HP and LP filters.  On a CS machine, setting these to 
maximum and then using the global resonance control allows for the 
remaining 20% of resonance range to be used.  On the MOTM-480, setting the 
individual resonance controls to maximum provides the 80%-of-range, and 
applying some (positive) control voltage to the resonance CV jack provides 
the remaining 20%.  I think the maximum "Q factor" is 15 or so (and the 
minimum around 0.5).

  The MOTM-480 resonance is the same as the CS-80's.  Just try the "funky" 
tone selectors (that is, presets literally labeled 'Funky 1' to 'Funky 
4') on a CS-80.  Or, listen to the Vangelis track "Chung Kuo" from the 
China album at around 1 minute into the piece--the low-frequency notes are 
all done using one of the "funky" presets with aftertouch control of 
filter frequency.  In fact, listen to the entire track for a 
nearly-all-CS80 experience.


Crow
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