Interested in changing project topics. Contacted Plogue to see if I could help them with a circuit modeling project. They make a really cool project called ChipSounds, which "authentically emulates 15 vintage 8bit-era sound chips (on top of their variants), down to their smallest idiosyncrasies" in VST/AU/RTAS plugins. In addition to sounding really awesome, the research that goes into developing this product lines up quite well with my research interests (and interest in the compositional restraints, cool sounds, and retro cachet of chiptunes).
David Viens from Plogue responded! They are interested in having me work on an analog circuit modeling, and have suggested a few possible projects. They are currently developing a new plugin that also seems very cool. As a fan of old consumer synths and sound making devices, I noticed a lot of familiar circuits in their promo video (especially excited about the Casio SK-1, Speak&Math, and SNES)!
I've responded and let them know about the two projects that I'm most interested in.
I need to figure out the legal/intellectual property/&c. issues involved with doing a project for a company as coursework in a graduate class. Hoping to get this cleared up with Prof. Chafe (or other folks who are aware... Spencer? Jiffer?) in the class meeting today, so I can move forward in talks with Plogue and start doing some circuit modeling!
I'm starting to do some work on the Casio SK-1, modelling the analog portions of the circuits. What I worked on over the weekend:
I spent some time with the SK-1 service manual, to try to understand the big picture of what is going on, and which analog parts I might work on modeling. Pages 5, 8, and 9 of the service manual will be particularly relevant to what I'm about to discuss, I'll include scans of them with a few of my notes. My understanding of the signal flow of the analog portion of the circuit (the right half of the block diagram on Page 5) is that it comprises four parts.
1. The envelope/pitch mixing circuits (Page 8)...
...which each (there are 4) take in a "pitch signal" and "envelope signal," and combine them together. It looks like there could be all sorts of interesting stuff happening here, transistors T15 and T16 are working as a kind of two-quadrant multiplier, T17 "mixes" the two signals into a "sound signal", and the resistor and capacitor on the output ("to filter circuit") form a high-pass filter. The diodes and transistors could have some non-linear behavior, and the circuit in general seems to do something more interesting than merely multiplying a signal by an envelope.
Page 8 shows 3 example signals, what looks like an envelope (A), what looks like a signal (B), and what looks like a mix of the two (C). Interestingly, the "mix" does not seem to be a multiplication of (A) and (B) but an addition. However, (A) and (B) are not the same as the "pitch signal" and "envelope signal" that are being fed in the right side of the diagram.
To help understand this, I am brushing up on my SPICE, and building a model of this part of the circuit. Something that could help me would be a better understanding of the nature of the "pitch signal" and "envelope signal" that are being sent in. Question, have Plogue done any measurements of these, or could they otherwise advise?
2. The Change-over Circuit (Page 9)...
...which routes (with an analog switch controlled by pins KO15 and PO3 of the CPU, according to whether the keyboard is in auto-accompany mode or not, etc.) the "Melody or Bass sound signal" and "Melody or Chord sound signal" to the Bass Filter or Melody Filter and Chord Filter or Melody Filter, respectively. The "Melody or Obbligato sound signal" and "Melody sound signal" are always routed through the Melody Filter.
There could be something interesting going on in the analog switch, could be worth looking into (IC2 = TC50H4066). I'll be looking up a data sheet soon to start investigating this.
3. The Filter Circuits (Page 9)...
...which filter the Bass, Chord, and Melody portions of the signal with different filters, as well as the "Percussion sound signal" coming from the CPU (pin SC / 97 / "Analog percussion signal output" on the CPU). The Percussion, Bass, and Chord filters seem pretty simple (all resistors and capacitors), but the Melody filter has a transistor in it as well. The transistor seems to be acting as buffer or follower, again there could be some non-linear behavior worth investigating.
4. The Mixer...
.. which mixes all the signals together. There doesn't seem to be much info on this in the service manual, but an investigation of the circuit should reveal what is going on.
There is also the matter of the microphone or 1/8" jack inputs. I haven't look at this at all yet.
Some resources for SPICE, thanks to Jack Perng, Collin Raffel, and Robin Gareus for the tips.