[sdiy] Fatman module??

[harrybissell]

Hello Richard and all... (inline and snipped)

Richard Wentk wrote:

> If I were going to do this I would:
>
> 1. Create LFOs and envelopes digitally using a single card to emulate a
> bunch of hardware
> 2. Multiplex this and other voice data over the bus so you could have up to
> (say) 32 channels without using a ton of cabling

I think that probably once you reach maybe 8 voices, an analog design is
unlikely
to make any sense.

I was thinking that you would have things like

(8) - note CV
(1) VCO1 offset
(1) VCO2 offset
(2) VCO mix
(4) ADSR 1
(4) ADSR 2
(1) VCF Fc
(1) VCF Q
(1) VCF ENV AMT
(1) VCA ENV AMT
(1) LFO rate
(1) LFO depth

all analog... maybe more.  I'm thinking these are not multiplexed - direct DC
voltages

> Addressing would either be by way of a handful of digital address lines and
> a clock, or you could use two lines - a master clock and a counter reset -
> onto two lines and put demux control counters onto the boards.

Yes I see that.  I'd agree that functions like waveform selects and mod
routings
would be very well handled by a digital multiplex.

> I'd imagine you'd need these lines as a bare minimum, which would
> pre-summed digitally to include all the possible modulation sources (except
> poly mod):
> VCO 1 pitch
> VCO 2 pitch
> VCF cutoff
> VCF Q
> VCA volume
> (Output pan?)
>
> The problem with using discrete lines for individual cards is that you're
> inevitably limited in what you can do. Using the muxed approach you can
> squeeze the information for 32 cards into a handful of lines, and the
> design is inherently multitimbral. Doing it with a discrete backplane I
> doubt you'd be able to get more than 8 voices together.

I agree... but don't think you'd need 32 voices.  I'd even think that you might

want to have different voice cards available.  (designer flavors ???)

> The voice cards themselves would have just a couple of VCOs , sync, polymod
> control (you *have* to have filter and VCO FM on an analogue synth :) ),
> perhaps a ring mod, the VCF and VCA, and a bunch of latches and S&Hs for
> the modulation inputs.
>
> The hard part would be deciding how to handle the possible poly mod
> routings, and also mix control for the VCO waveform outs, the ring mod and
> noise, and so on. Some of these could be switched digitally instead of
> given continuous level controls.
>
> Then again, given how cheap processors are today, it's not so very nuts to
> do *all* the distribution digitally, with a processor on each card making
> sense of the input information from an 8-bit data bus on the backplane.
> That would certainly be more reliable and easier to design than analogue
> muxing, and probably wouldn't be all that much more expensive. (If at all.)
>
> So you'd have:
>
> 1. A master processor, including the envelope and LFO simulation as before,
> and also MIDI decoding and basic pitch calculations. Something moderately
> beefy (but not insanely so) would be needed here.

I'm not a fan of soft envelopes myself. I'd rather go discrete ....

>
> 2. A distribution bus. 8-bit data and 8-bit address plus some control lines
> and a couple of audio out lines would probably be enough at a pinch. More
> would be nicer, but maybe not essential.
> 3. A voice card with two VCOs, one VCF, a VCA, a ring mod, noise, poly mod
> mixers/switchers, a processor, and a 16-way analogue demux to source the
> CVs. All of this should fit onto a Eurocard without too much of a stretch,
> especially if it's based around one of the CEM chips.

I'd probably beware the CEM chips because of availability. They would reduce
board
space quite a bit

> It's still not cheap, but by the time you're done you'd have a home-made
> Rhodes Chroma in a rack of manageable proportions that wouldn't be so
> insanely difficult to make.
>
> Of course it's nothing like a Fatman any more. But if you want a poly one
> of those, I'm thinking the best way would be to make a rack that's 19" high
> and 8U wide and just stack them vertically for effect. :)
>
> The other plus is you can go nuts with the LFO designs, and do a bunch of
> stuff analogue designs can't usually do. (MIDI sync and all that, weird
> waveshapes, and so on.) Likewise it's easy to do things like pitch tracking
> for the envelopes, which is a high-overhead kind of thing for straight
> analogue.

Agreed to most:
Other counties heard from ???

H^) harry