[sdiy] Analog polyphony question

[harry]

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
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