[sdiy] Analog polyphony question

[Paul Maddox]

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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