[sdiy] Synthex Oscillator

[rsdio at audiobanshee.com]

On Jul 9, 2017, at 1:17 AM, Tom Wiltshire <tom at electricdruid.net> wrote:
> On 9 Jul 2017, at 05:07, rsdio at audiobanshee.com wrote:
>> In my mind, a boardful of LS series logic begs to be put into an FPGA, but only if the cost is right. So far, every design I've done has turned out to be cheaper as discrete LS logic, simply because those parts are so damn cheap and prevalent. The equivalent FPGA has always been more expensive. There's also the inherent advantage that LS logic does what it's supposed to do as soon as it's soldered on the board, whereas programmable logic has to be programmed before it will do what you want.
>> 
>> I mention this because you should consider making your design first, and after you're settled on the logic you could take a look at how it would affect size and price to squeeze all or part of the logic into a CPLD, FPGA, or other device.
> 
> I totally agree about this. Whilst reading the Synthex schematics your brain starts screaming "FPGA! CPLD!". Unfortunately I have no experience with such things, but there's always a first time. The idea of getting the Synthex oscillators down to a chip or two is certainly appealing. I assume that if you'd programmed a pair of oscillators (with all the cross-mod and whathaveyou) copying that seven more times would be easy enough? Perhaps what you'd save on the PCB cost would compensate a bit for the more expensive chip.

Ah, you've got me thinking of an aspect that I hadn't considered before: although the discrete logic might be cheaper for one instance, the fact that you might be able to cram 8 duplicates of the same sub-circuit could tilt the budget in favor of programmable logic. The only remaining challenge here is whether the chip has enough inputs and outputs. Most of the inputs could be shared, though, and some of the clock circuits would be shared (unless you allow separate modulation of voices instead of global pitch bend).

I'm somewhat inclined to design voices on their own PCB, so that you can expand to larger voice counts easily. However, the Synthex supposedly put two voices on a card, and with two oscillators per voice that was four oscillators per card. In light of that, cramming eight oscillators in a single chip might not be too bad. It still ends up being modular, you're just adding four voices per chip.


As for the technical skills of designing FPGA programming, I'm not too worried about learning that. My biggest hurdle is thinking about the process of hooking up each board that comes from the assembler and loading the FPGA program into it. That's a lot of hassle, like loading firmware onto each unit. Sometimes it's nice to just have a board that is fully functional as soon as it cools down from the SMD oven. Then again, if you also design a test jig and give that to the assembly shop, you can let someone else handle that particular headache and get tested boards with the same amount of effort.

Brian