lørdag 17. januar 2015

Polysynth voice cards

Voices


I intend to spilt the voice cards into two physically separate PCBs, one containing the mcu and DACs for the various required control voltages, and one with the analog parts like VCOs, VCFs, VCAs etc. This way i may develop the two cards separately and change only parts of a voice if i need to. In short, it reduces the consequences should something go wrong or if i want changes. I may even consider a separate filter card.

(Mostly) analog card


At the moment the analog cards should contain the following:
- noise, switchable between white, pink and red, with its own separate VCA.
- three VCOs, each with saw, triangle and pulse outputs. Each waveform should have a separate VCAS.
- one 2 pole state variable filter (HP/BP/LP and possibly notch)
- one 4 pole LP filter
- one sample player/digital oscillator. This can be used for sampled attacks (Roland D-50 style) or digital waveforms.
- cross modulation between oscillator 1 and 2
- sync (soft and maybe hard) between 1 and 2/3
- a separate output VCA connected to the main envelope.

All these elements will be built as separate, chip-style PCBs, once again reducing the complexity and simplifying testing.

I strongly consider making this an Oberheim Matrix-style synth. More about that in a separate post, but this means that I will move LFOs, envelope/ramp generators, sample and hold and other modulation stuff into the digital domain. A consequence of this is that each waveform may possibly have a separate envelope or have its amplitude modulated in other ways.

 A note on the VCAs as well: if possible, I want to keep them linear. This will make them far less temperature sensitive, as well as slightly easier to build. They may also be turned fully off. It does also mean that exponential conversion will have to be done in software. I have to test this to see if it is feasible and if the result is good enough.

In addition to the necessary CV and switch control inputs, the voice should have the following connectors:
- External sound input (possibly with it's own VCA)
- Feedback from noise to mcu for random/sample & hold functionality
- Feedback from VCOs for tuning
- Separate analog ground for each voice card

Filter section


I have thoughts about making it possible to merge two voices into a six (eight) oscillator monster voice to do super saw-like stuff. Originally I only intended a voice to have a single filter, and in the case of voice merging, which would happen before the filter, I would get a spare filter for each voice.

I'm concidering two ways of using this filter, switchable from software:
- in series with the first one, but with separate filter controls
- in parallel, but with separate output VCAs.


As a wrap up for the post, I have sone other ideal and thoughts that may or may not make their way into the voice cards:

- Move the sample player to the digital board to keep nasty digital transients away from the analog board? (Still required for switches, but they will hopefully not be switched in the middle of playing.
- Suboscillators for each VCO
- Analog envelopes for the filter and/or voice outputs. May use both analog and digital.
- Saw polarity switch or separate VCAs for each polarity?
- Filter cassettes to be able to switch between famous filters. Maybe one cassette per voice card?
- FX busses. I want to include at least a juno-106 chorus module, but possibly also a digital effects module.
- ring modulation

Sketches




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