[sdiy] Analog polyphony question

Mon Aug 13 06:50:39 CEST 2001

If the frequency range is limited square to saw conversion only takes a few
components.
For a TOS based organ the frequency range for each key is _very_ limited, so
it should be doable to add saw converters to each key.
Spaced saw is even easier to do: diode//resistor, capacitor to gnd and
probably a opamp or transistor buffer.
And when you got (spaced) saw then PWM is only a opamp/comparator away.
When done with spaced saw the pwm range would be limited between 0 and 50%,
but this should be not too much of a problem.
Ofcause for every waveform you will need a additional keyboard contact or
switch IC.

Just some thoughts.
Theo

From: Dr Strangelove <phdinfunk at hotmail.com>

> 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
> > > PPG Synthesizer pages;-
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> > > Waveterm C Project page;-
> > >     Http://www.waveterm.com
> >
>
>
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