Since starting in January, I've thought about a lot, done a lot of experiments, made mistakes, committed to some decisions, and now that a few modules are in place, I'm starting to get a picture of how it'll all fit together. Here is that picture!
The biggest change since January, is that I want to incorporate a second Casio MT-240 into the organ. There are some cool things I want to do using two MT-240s. But this also means a lot more physical space and A LOT more work. But one thing at a time.
- Synth Body with Auxiliary bends
- uC interface
- ROM Logic Controller/Patch Bay
- 5 Octave Keyboard
- Frame
This is the main body of the synth, with the original MT-240 circuit boards. I've removed the back and made the circuit board vertical so I can access it easier.
The body is home to many miscellaneous bends that are auxiliary to the main ROM bends. The controls for these auxiliary bends is on the top (where the original keyboard used to be), and the circuits are mounted to the bottom.
The aux. bends circuit can be broken down into a couple of smaller circuits:
- Clock circuit
- LC pitch circuit
- Hidden functions
- ROM disregulate and disengage
- DAC overdrive & squealers
A menu interface with a rotary dial, select, return buttons to choose a uC function. The idea is, for example: 5 => LC pitch circuit; 5.7 => LC tremolo circuit; 5.7.9 => fastest tremolo speed, send. I'm very happy with how this turned out. I think it'll also be a good framework for programming the microcontroller.
There's also an ignition to turn on the casio & uC simultaneously... vroom vroom!!
to do:
- add indicator LEDs (indicates when critical things are functioning)
The combined patch bay and logic controller for the main ROM bends. Each pin on the ROM gets its own line. Each line has a couple banana jacks that make the ROM patch bay. It also has a "Logic" switch that "forces" each line to either a logical high or low. In series is a 200 ohm 10 turn pot that controls how much of the "forced logic" gets through to the line. The third switch is used to bypass this pot for a full "force". The pot will also work with the planned Switchboard controller to allow for the option to increase a connection between two ROM pins to 400 (200 on each logic controller line) + 100 (from the Switchboard) = 500 ohms, which I've found from experiments seems to be the upper limit to affecting a ROM bend.
todo:
- add LEDs for logic forcing
- finish banana jacks
- think of a snazzier name
A five octave keyboard torn from a different Casio. I added switches to every single key for individual note sustains. It's good and reliable for now, but I think I will replace this in the future with a more advanced keyboard.
todo:
- add pressure sensitivity to volume control
- add simpler uC sequencer interface, perhaps combine with individual note sustain
- add LED ROM-Patch range indicators
- reconfigurable keys for different ROM-patches? would have to have keyboard piped to uC before synth
The frame for the synth is currently happily holding all the pieces. I now have an idea for the constraints of how big it can be, so I think it's gonna get WIDE. I anticipate it getting deeper than it should be (without becoming a problem fitting thru doors), so my idea for it now is to have the deepest parts fold over the top. I also removed the legs for now.
The clock is powered by an external ltc1799 squarewave oscillator chip. The speed of the oscillator is currently controlled by a resistance up to 1M. A rotary dial is used to select between modes: potentiometer, ldr, or steps (controlled by another rotary dial). A switch controls the range of the oscillator. I believe I've covered the entire range of clock speeds possible with this chip and this Casio, so I'm satisfied with this.
todo:
- Try uC pulsing an LED to control the LDR
- once the second MT-240 is up, chain this oscillator to that clock circuit to sync both synth's clocks. May need to "duplicate" this wave, maybe use an op amp unity gain follower?
- far future: divide the frequency to have clk speed ratios between synths. Also, maybe the uC can then interpret this to do things based on the synth clock speed
The pitch of the synth is based on the frequency of a resonating LC circuit. I've added two variable capacitors to greatly vary the pitch, one as trimmer, other as main pitch control. I'm really happy with the range, almost a P5, and just at the upper limit of before the synth begins to crash. The lower limit can still be explored. I did some experiments and found a weird way to use a 10 turn 100ohm pot (I think as a variable center-tapped inductor, since the 10 turns are coiled) to reach this super low, awesomely unstable pitch range. This really is one of the best effects, but this isn't quite as reliable as I'd like, and it does not work with the pitch varicap. I am considering using a rotary dial to choose between these two modes, but that wouldn't be ideal. I need to do a lot more research, or get lucky.
todo:
- find a way to access that super duper low range reliably
- stepped switches for pitches, maybe also uC pulsed reed option
- touch node
- uC controller motor to turn trimmer varicap to make tremolo effect
There's a bunch of useful hidden functions that are on other Casio synths but no hardware for on the MT-240. These are simple switches & diodes between points on the circuit, but connecting them efficiently was a challenge, since many of them share points. One of the more interesting things is that there are two additional super secret TONEs in addition to the known 10 hidden TONEs. They are glitchy and not very pleasant, but they act as a preset TONE. I think this is because there are 2^5 = 32 Address "chapters" in the ROM that correspond to TONEs, but only 30 that are actually programmed. I think I can make buttons for those extra two with the help of the uC.
todo:
- add two glitch TONEs
- add 8 'atmosphere' buttons & control buttons (part of MT-540 ROM- maybe also add optional 540 ROM??)
- add tritone spam button (a special effect coming from the way the keyboard matrix is layed out)
These controls affect the BHE, OE, Vss, Gnd pins on the ROM chip. The BHE and OE pins have stable digital signals that enable the ROM, but using a pot like this causes the signals to enter a scary disregulated mode which may glitch out or disable the ROM. Some improvements can be made so that they aren't just leaking current when not being used. The Vss & Gnd disengage switches disengage the V & G signals on the Logic Patch Bay from the ROM chip, useful for a type of quick reset with help from the uC.
todo:
- optimize BHE, OE pot circuits
These circuits affect the DAC. I've experimented, but haven't built them yet. The Squealer scream and have all sorts of oscillating effects by creating feedback paths to the DAC inputs. These will probably affect the functionality of some or all of the other bends, when enabled, because they sort of take precedence over everything else being at the end of the audio chain. The Overdrive will probably be the most stable, and can be adjusted for intensity, it can get LOUD. Excited to work on these. They can be controlled by LDRs and uC by pulsing LEDs.
- Switchboard (to connect to patch bay)
- Timbre Controller
- Gliss Controller
- Power distribution
- uC keyboard sequencer
- 4x4 active matrix mixer
- pedal rack
- foot pedals
- Second MT-240 with a few aux bends, small patch bay, and logic array controller
After the ROM logic controller/patch bay, I want to work on the Timbre controller. I've been thinking about this for a while. The idea is that grounding Address lines 13-17 only affect the timbre, and nothing else. With transistors and pots, I can use the other 14 address lines to gate the grounding of A13-17. This way I can have an array of 14x4 pots dedicated to controlling the timbre. But before I work on that, I need to make a couple banana cables with built-in diodes to connect the patch bay.