[sdiy] Top Octave Modular

[Ullrich Peter]

Hi!

An option for switching such a big amount of matrix connections could be the MT8816 matrix switch (8x16 matrix with internal latches, can work with +/- 5V audio signals for example.). 
I used them for a universal (for for my Korg Poly61 MIDI interface).

They are not the cheapest chips but very fine - they can even work with video signals...
Just be careful when controlling them: 2 matrix lines are swapped in the addressing scheme... 
I simply changed my CAD symbol for logical control...

https://www.microsemi.com/products/switches/analog-cross-point-switches/mt8816

you can buy them from here:
http://www2.mouser.com/Search/Refine.aspx?Keyword=MT8816

Ciao
Peter

-----Ursprüngliche Nachricht-----
Von: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] Im Auftrag von rsdio at audiobanshee.com
Gesendet: Dienstag, 04. April 2017 01:25
An: Elain Klopke <functionofform at gmail.com>
Cc: *SYNTH DIY <synth-diy at synth-diy.org>
Betreff: Re: [sdiy] Top Octave Modular

On Apr 2, 2017, at 6:04 AM, Elain Klopke <functionofform at gmail.com> wrote:
> So, more stupid questions.... In having 8 octaves of notes to switch to 8 busses, each with their own switch, that's 832 switches to control. At first I thought that daisy chaining 74HC595s would work, and it probably would, but there has to be a simpler way of controlling all those switches.
> 
> CPLD, FPGA, Raspberry Pi?
> 
> I'm getting pretty comfortable with the analog bits of the design (except maybe being a little fuzzy on  how to turn the 0-5V output to something centered around 0V that other modules would be expecting), but the programming bits of this are still over my head.

I suppose that you may already be aware that the vintage Hammond keyboards have 8 switches per key? I still haven't bothered to trace the complete signal flow diagram, but the Hammond organ maintains several audio busses for the various oscillators and drawbars, and then still needs 8 switches per key to bring in the right set of harmonics in the selected proportions to get the final sound.

Of course, you don't necessarily need to be limited to 8 mechanical switches when a single switch can control any number of analog CMOS gates.

As Tom has pointed out, if you stick to square waves then you have a lot of options for digital combinatorial gates that might work to simplify your circuit. If you have sawtooth (or even sine) waveforms, instead, then it still might be possible to simplify the switching.

The question of CPLD versus FPGA versus R-Pi is best left as a decision after you've designed the logic. I recently designed a complex interface between two parts and found that the cheapest CPLD was still more expensive than two or three discrete logic chips. In my case, I used the 74LS32 2-input OR, 74LS27 3-input NOR, and a weird NL7SZ19 1-to-2 address decoder chip. Vintage logic chip designs like those are so cheap that a CPLD is expensive in comparison. Another huge disadvantage of CPLD or FPGA is that the chips do nothing when soldered on the board - you have to program them first, and that can be a pain for manufacturing. In contrast, OR gates and NOR gates do what they're designed to do without any extra programming steps. Just design the PCB, assemble the chips, and apply power. However, it depends upon the complexity of your logic. At a certain level, a CPLD or even an FPGA makes sense because we no longer build giant PCBs. If you want to avoid programming, that's !
 another vote for discrete logic.

By the way, most of those 74LS* parts that I mentioned have equivalents in the CMOS 4000 series, and those run on various voltages instead of being fixed to just 5V or 3.3V. Depending upon your mix of analog and digital signals, you might want CMOS logic chips.

Brian Willoughby
Sound Consulting


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