[sdiy] Interesting article on top octave generators.

Sat Jan 4 18:09:38 CET 2025

All this discussion about tuning and temperaments reminded me of the Fender Rhodes service manual, where they take a dive into stretch-tuning. See https://www.fenderrhodes.com/org/manual/ch5.html#5-3 

Quote: "Stretch tuning is a procedure widely followed by the piano tuning profession. It recognizes a phenomenon of the human ear whereby tones in the upper range of a keyboard will sound "flat" even though they are calibrated with extreme precision. Fortunately for all, a consensus has long since been agreed upon as to the exact amount of stretching. A piano so tuned creates the impression of great tonal brilliance."

With this chart:

This obviously can never be done with the divide by 2 approach.

But are there any analog VCO’s (or synths) that actually implement some sort of stretch tuning? (Aside from the unwanted flattening in the high end which is opposite from stretch tuning.)

Ben

> On 2 Jan 2025, at 21:25, Michael E Caloroso via Synth-diy <synth-diy at synth-diy.org> wrote:
> 
> The Moog Polymoog was a dual VCO rank TOS system using a pair of TOGs but with an interesting twist: one TOG was offset from the other by a semitone (per US patent  4,145,943 and 4,228,717).  In other words when you pressed a key with both ranks engaged, you were hearing one TOG at its nth divider output plus the other TOG as its n+1th divider output.  The master oscillators were offset so that n and n+1 divider pairs produced the (approximate) same pitches.  Because the TOG can't produce equal 12th root of 2 temperaments at all its dividers, this offset resulted in dissimilar scaling offsets per semitone with both ranks engaged.  With pitch modulation, it produced a richer string ensemble chorus effect and deviated the waveshapes of each rank.  The cent difference of ranks when a key was depressed was not uniform between keys, the largest being ~3.3 cents.  This gave the Polymoog an organic sound quality and probably prevented dividers from syncing up.
> 
> Before he joined Moog Music in 1972, Dave Luce had partnered with Melville Clark Jr to design a TOS polyphonic keyboard.  I haven't yet read the patent descriptions (US 4,365,533 and 3,969,968) but from the circuit blocks it looks pretty convoluted (the description text runs 20-40 pages long).  At Moog, Luce was tasked with designing the Polymoog; when Clark learned of this he made angry infringement noises so Luce was forced to design a Polymoog without infringing his work with Clark.  The Clark/Luce patents were granted close to the Polymoog era, probably by Clark to establish prior art in case of legal conflict.  No matter, nothing was ever seen of the Clark/Luce polyphonic system nor was there any infringement case ever filed.
> 
> The Polymoog did have one ace that the Clark system did not: the custom IC under each key of the Polymoog which drastically reduced the size (and cost) of a fully polyphonic synthesizer.
> 
> MC
> 
> On Thu, Jan 2, 2025 at 10:25 AM Roman Sowa via Synth-diy <synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org>> wrote:
>> The thresholds in 40106, which define oscillator stability, are not even 
>> specified with regards to temperature or long term drift or any other 
>> drift. They may differ a lot from part to part, and nobody cares, 
>> because it's not the intention  of 40106 to be a timing device. Its 
>> operation relies on Vgs voltages and who knows what else, and that 
>> voltage greatly depends on temperature.
>> 
>> Sure it's nice simple oscillator, like many others, but not all of them 
>> would be my first choice for tuning source. But for a drone, why not. 
>> Last year I've made a board which contains 24 such oscillators, voltage 
>> controlled, for a drone. And it's less than 1 inch squared.
>> 
>> Roman
>> 
>> 
>> W dniu 2024-12-30 o 19:49, David G Dixon pisze:
>> > A CD40106 oscillator is just a schmitt trigger, a cap, and a resistor.  
>> > Presuming the supplied voltages are well regulated and the caps are of 
>> > decent quality, then why should one expect anything other than perfect 
>> > stability?  If I were looking for fixed-frequency square-wave 
>> > oscillators, that would also be my first choice, cuz it's so simple, and 
>> > one can fit all 12 oscillators onto a board that's about 4 inches 
>> > squared, even with through-hole components.
>> > 
>> > ------------------------------------------------------------------------
>> > *From:* Synth-diy [mailto:synth-diy-bounces at synth-diy.org <mailto:synth-diy-bounces at synth-diy.org>] *On Behalf Of 
>> > *Roman via Synth-diy
>> > *Sent:* Monday, December 30, 2024 12:01 AM
>> > *Cc:* synth-diy at synth-diy org
>> > *Subject:* Re: [sdiy] Interesting article on top octave generators.
>> > 
>> > Not only from the 70's. There's this one guy who makes combo organ 
>> > eurorack module the hard way, with TOG, dividers and discrete circuitry 
>> > all the way down from there. He uses 12 tunable oscillators. And what is 
>> > most surprising, he uses 2 hex Schmit hex inverter chips for that 
>> > without any problems of stability.
>> > 
>> > 
>> > Roman
>> > 
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