[sdiy] Re: FPGA 2-OP FM MIDI Polysynth

[Scott Gravenhorst]

=?ISO-8859-1?Q?Johannes_=D6berg?= <johannes.oberg at gmail.com> wrote:
>Reply to various posters
>
>4-op FM vs 2-op FM is like toy keyboard vs serious synthesis platform.
>Worlds apart in power. Especially if you have many feedback
>configurations.

Veronica Merryfield mentioned an interesting idea - to allow sound generators with a
phase modulation input to be incommitted to any specific structure and use a patching
program to "connect" them in various ways.  This way N-operator could be done
depending on the robustness of the patching mechanics.  Not being a person experienced
in FM, I don't know at what value it begins to become silly to increase N.

>If you add non-integral carrier/modulation frequency relationship you
>get a whole new dimension of possibilities, not just inharmonic
>sounds, but you can make some of the harmonic sounds *much* better by
>adding a little beating.

Even with only 2 operators, nonintegral C:M ratios are far more interesting than
integral ones (IMO).  Setting two of them for something like 3.501 and 3.499 give
interesting results that are self animated and seem to keep the timbre character of
3.500 for one generator.

>If you have the DSP power for multiplies, my tip would be: use 4-op
>Phase Modulation synthesis with derived waveforms (like the TX81Z)
>with log tables, and use the multipliers for analog style envelopes,
>good subtractive filters and effects (FM sounds generally takes very
>well to a touch of chorus and delay, and some sounds really come to
>life with a bit of overdrive). Make it multitimbral, and voilà, it
>might be the only synthesizer you ever need.

I would like to try this.  I have a bigger FPGA on which I think this would work
comfortably.  It has a Spartan-3A DSP 1800 FPGA which has more than three times the
equivalent gates and four times the multipliers/RAMs as the Spartan-3E I am currently
working with.  I would think that this FPGA should be able to support 4 operator
configurations at least at a proof of concept level and include at least one SVF per
voice.  It would be a very different design, so it's not a simple port and modify...

>> I have a feeling, however, that the DX7's strange/unpleasant 
>(IMO) envelope attack (which I > brought up on another mailing 
>list) is also partly due to the anti-log lookup table. 
>
>I thought this was because of bad (linear?) envelopes and low resolution?

Eric Brombaugh started a thread on FPGA-Synth regarding expo curves in envelope
generators, pointing out that especially the release phase of an ADSR if done linearly
sounds kinda crappy the way it drops off.  This prompted me to play with the ADSR on
another synth before this FM synth and it was amazing how much nicer and more natural
even simple subtractive synth sounds became just by changing only the release phase to
allow an expo curve instead of a straight line.  In my case, I did the curve
arithmetically instead of with a table.  As a result, the same ADSR with selectable
expo/lin release was incorporated into my FM synth.



-- ScottG
________________________________________________________________________
-- Scott Gravenhorst
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