2 modules ideas

[Hairy Harry]

I think the owner of the "Modulus" pages has described a very
HF VCO, as have several other list menbers... I seem to remember they were 
still capable above 200KHz. We would NOT care about waveform
integrity... just a pulse would be OK.

You might try waveform multiplication using a sawtooth wave
(like Grant Richter's "Waveform City") to get a high frequency
clock...

I agree with Michael about the sine shaping. I've seen a really good method 
using a counter, 8 resistors, some transmission gates, and some xor gates. 
The idea was to step through one quadrant, then reverse the counter, then 
reverse the polarity, and then count backwards. It looked complex until I 
realized I could probably not design the same with twice the components. Now 
the filtering requirements are minimal but still there. I bet a fixed filter 
would be good enough. BUT now you need an extra 32X frequency....

The "top octave" comment is also good... but since the chips are mostly 
unobtainable today (and they are equally tempered) maybe we
should use our own divider strings. THIS approach will toss the frequency 
well into the MHz...

Anyone thinking of trying the harmonic synthesis should spend some
quality time playing with the drawbars of a Hammond Organ... to see if they 
want to bother.  Additive synthesis also requires the ability to have NON 
harmonic tones, as well as control of phase of each sinewave.

This is why (IMHO) few units have used additive sucessfully. The programming 
is a real bitch !!!  It might be easier to just make
a bunch of good sine-wave VCO's and use them. If you them locked to a 
harmonic sequence, then sync them. Then use the VCA on each one.

Good VCO's are not too hard these days...

H^)  harry

>From: WeAreAs1 at aol.com
>To: jbv.silences at wanadoo.fr, synth-diy at node12b53.a2000.nl
>Subject: Re: 2 modules ideas
>Date: Thu, 10 Aug 2000 15:39:46 EDT
>
>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

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