[sdiy] Analog polyphony question

Mon Aug 13 06:24:03 CEST 2001

Au Contraire... Dr Strangelove  (surpresses involuntary nazi salute...)

The Top Octave Generator does exactly that... generates a square wave in the
top octave... about 7-8KHz.  Remember that you can't hear harmonics above
20KHz (or if you are a little older, a little less... progressively)   ;^P

So if you sum the square waves in a 1-2, 1-2-4, or 1-2-4-8 sequence... you
can get quasi sawtooth waves at 4K (the first harmonic that would not be quite
right
would be 12KHz... it would be there but at the wrong amplitude... 2K with even
better fidelity... etc.

This is exactly what TOG organs do... additive synthesis using the available
waves.

If you were not going to modulate the 2MHz signal very far... you could generate

a real sawtooth for the top octave and add it to the staircase waves and get the
real
deal...

Juergen Haible had a schematic for his version of the MS-20 oscillator... which
uses
this approach.  You'd have to make the ramps for each top octave note...

Or... Walsh Functions !

H^) harry  (back to my deep mine shaft for 99 years....)

Dr Strangelove wrote:

> I was reading recently about TOS based organs, Thomas Organ Co., Farfisa,
> Wersi, etc, The Vox Jaguar used a similer approach but didn't use a TOS IC
> so it didn't have JUST square waves I think.  I was also considering my Korg
> Poly-800II, it only has 1 VCF for the whole synthesizer, rarely do I miss
> one VCF per not though So I was thinking that a person could make something
> like a hybrid, 70s organ/synthesizer, itd be taking the cheapo route, you
> couldn't LFO moodulate the oscs I don't think.  Basically the TOS ICs would
> divide a 1 or 2 Mhz square wave down to create the top octave then they
> would have other ICs to divide by 2, 4, 8, 16 etc for the lower octaves, If
> you could tune the 1 or 2 Mhz square wave then you could detune two TOS
> organs from each other, which would be kinda cool.  What Wouldn't be cool
> would be the fact that you'd only have square waves and that you'd only have
> constant triggering for the filter that you had, IE your filter could follow
> an ADSR that would gate when you pressed the first key, but any other keys
> you pressed would just come in without retriggering the filter, your VCA
> would have the same property.  The Korg Poly-800 overcomes this by having a
> Multi-trigger switch that retriggers the filter ADSR with each key pressed,
> I never use this function because I rarely like the results.
>
>          -=<Jonathan Pratt>=-
>        (Phdinfunk at hotmail.com)
>
> >From: harry <harrybissell at prodigy.net>
> >To: Paul Maddox <Paul.Maddox at wavesynth.com>
> >CC: Jon Darby <jdarby at lplizard.com>, synth-diy at dropmix.xs4all.nl
> >Subject: Re: [sdiy] Analog polyphony question
> >Date: Wed, 08 Aug 2001 22:57:42 -0400
> >
> >Hi Paul (and all)
> >
> >I agree with 'most' of this... except the part about the Prophet V choosing
> >the
> >economy approach (5 voices) as opposed to the common Top Octave Generator
> >of the day.
> >
> >The Top Octave Generator makes all the voices track the same exact note...
> >to
> >arbitrary precision following the equally tempered scale. Usually it was
> >about
> >+/-
> >2.5 cents (100th of a semitone).  So all pitches were digitally locked to
> >each
> >other...
> >whatever beating there was between notes would be absolutely constant.
> >The Prophet V used microprocessor assisted tuning to keep all the voices
> >reasonably
> >the same, but still have random pitch fluctuations.  User specified tunings
> >were
> >possible as well... making the P5 the axe of choice for most ethnic
> >music....
> >
> >Cross modulation would be very hard...  in fact most of the Prophet V voice
> >patches would be next to impossible.
> >
> >Top octave generator usually says "electronic organ".  Few synths use this
> >approach.  I think the Polymoog (strange beast that it is...) is one of
> >them
> >
> >The Matrix 6 from Oberheim used two high frequency VCO's to feed a digital
> >pitch generation system.
> >
> >Why did I ever sell my P5 (oh yeah... no velocity, no midi and one note
> >short of
> >a guitar synth....)
> >
> >H^) harry
> >
> >
> >
> >Paul Maddox wrote:
> >
> > > Jon,
> > >
> > > >     I was rereading the old Penfold book today where it was comparing
> > > modern
> > > > synthesis techniques to modern electronic organs. It mentioned the
> > > polyphony
> > > > is obtained from a single high frequency oscillation and divided down
> >to
> > > the
> > > > appropriate frequency determined by the key pressed. My question is
> >what
> > > > module would be required per key to divide off the frequency of the
> >master
> > > > oscillator into the custom frequency before being mixed together to
> > > output?
> > >
> > > if you do it that way, just a divide by two circuit (4013 has two of
> >them).
> > > There are some disadvantages..
> > >
> > > > What parts are involved, another oscillator that oscillates at a
> >frequency
> > > > based on the frequency of the master? Hmmm, just curious how complex
> >of a
> > > > circuit is required at each key. Thanks a ton.
> > >
> > > the top octave generator (TOG) needs to generate the top octave, ie 12
> > > notes.
> > > each of these is then fed into a seirs of divide by two chips (4024 has
> >12
> > > stages , IIRC)
> > > so with a TOG and 12 4024's you could have 13 octaves..
> > > now heres the catch...
> > > you need a VCF and VCA and EG for EACH key, yes , EACH key....
> > > as each key is able to generate sound simply by pressing it you have
> > > unlimited polyphoney (as many keys as your keyboard has).
> > >
> > > Synth makers soon realised (back the early days) this would be far to
> >big
> > > and costly...
> > > so they used (in the case of the prophet5) 5 oscillators, each could be
> > > assigned a note and it would play it..
> > > this meant you could have 5 notes playing at the same time , a MIRACLE
> >at
> > > the time...
> > > you would only need five oscillators (ok so you need more than the other
> > > mothed which had one) but you now only need 5 VCFs and 5 VCAs, much much
> > > less electronics that the other method..
> > > BUT you then had added complexity of voice assignment to contend with...
> > >
> > > MOST synths now use the latter method, processors are cheap and fast
> >now,
> > > and 16note polyphonic assignment is easy enough to do. and most people
> >only
> > > have 10 fingers !
> > >
> > > Hope this helps a little
> > > Paul Maddox
> > > _______________________________________
> > > Wavesynth home page;-
> > >     Http://www.wavesynth.com
> > > Modulus synthesizers home page;-
> > >     Http://www.modulus.wavesynth.com
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> >
>
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