2 modules ideas

[WeAreAs1 at aol.com]

In a message dated 8/10/00 9:30:49 AM Pacific Daylight Time, 
jbv.silences at wanadoo.fr writes:

<< 1) "additive" synthesis
 
 the square output of a VCO is fed into a logic divider
 (CMOS or TTL) with at least the following outputs :
 F/2, F/4, F/8, F/16, all squares and in sync.
 Each of these SQ outputs is sent to a Square-to-sine
 converter, which should give synched and octave
 tuned sines.
 Then, each sine is followed by a VCA (the hex-VCA
 recently discussed here could be a cheap solution). >>

Yes, the VCA's could be cheap, but remember that it's not easy to convert 
square waves to sine waves over a wide frequency range.  Unless you can come 
up with a more elegant solution, you might also need to have six 
exponential-response lowpass filters (one after each square wave) to filter 
the squares down to sines.  The filters would need to be exponential so they 
could track the pitch of the driving oscillator over a wide range.  Which 
brings us to:

<< As for the input signal, one could use the SQ
 of a VCO or any signal through a comparator
 to get a SQ... >>

I think that the driving oscillator would need to be a very high frequency 
oscillator, for this module to be very useful.  For instance, if you plan to 
derive your lowest harmonic by dividing by 16, you would need a driving 
oscillator that could accurately track CV at 16 times the frequency of a 
"normal" VCO.  (normal VCO:  10 Hz to at least 10 kHz, high-frequency VCO: 
160 Hz to at least 160 kHz)

Are there any good circuits for very high frequency expo-response VCO's 
floating around ?  I imagine someone here must have done some experiments in 
this area, since there is a similar requirement when driving a 
Voltage-Controlled Digital Oscillator (which typically need a VCO that runs 
at 8X the desired audio frequency).  What about the CEM3340?  I've heard that 
they track well at very high frequencies.  What about using a 
Phase-Locked-Loop following a "normal" VCO to generate the high-frequency 
clock?  (loop error, overshoot, and delayed response might be problematic..I 
don't know)

I don't mean to sound like I think it's a bad idea.  Actually, I think such a 
module (sine wave harmonic stack) would be lots of fun.  I just think that it 
might not be as easy to build as one might first think.

Michael Bacich

P.S. - I think that this module should also have musical intervals other than 
just octaves, such as fifths and thirds, etc.  It would be really great if 
you could duplicate the entire natural harmonic series, up to about the ninth 
or tenth harmonic.  This could be done (sort of) with the use of a 
Top-Octave-Divider chip and an even higher frequency driving VCO.  However, 
we must remember that Top-Octave-Divider chips do not create the exact 
pitches from the true natural harmonic series - they create a series of 
pitches based on the equal-tempered scale.  This may or may not be a 
problem...  MJB