Following on from the 4xD, I got to thinking some more about the possibilities of digital processing and synthesis with the hands-on accessibility and immediacy of analog, particularly in the context of a modular synthesiser. I wanted to create a general piece of hardware on which I could try out some of my software ideas.
So the Digi-Mod was born. In contrast to the 4xD which has a digital display and rotary encoders, the emphasis for the Digi-Mod is on direct analog control - by CV/Gate and potentiometer/switch/button. Having said that, it does also have plenty of digital I/O possibilities and with appropriate software there's no reason it can't be controlled digitally / over MIDI / etc.
So what's on board?
- dsPIC33FJ128GP708 processor running at 40 MIPS (DSP friendly architecture, DMA, CODEC interface, etc.)
- Stereo audio I/O with Cirrus Logic CS4270 24bit/192kHz CODEC
- Realtime hardware control of sample rate
- Dual analog VCA input stage / feedback loop based on THAT2162
- 14 analog CV inputs (2 to VCAs, 12 to dsPIC: 12bit 500ksps multiplexed ADC)
- 4 analog CV outputs (12bit quad DAC)
- 8 analog Gate inputs
- 10 general purpose digital I/O (with access to 2 SPI buses and I2C)
- Dual clip LED outputs
- MIDI In/Out/Thru
- 4Mbit SRAM
- 8Mbit EEPROM
- Flexible power supply requirements (+/-9V, +/-12V or +/-15V, plus optional +5V for more efficient operation)
And all this on a compact 98mm x 105mm PCB which should fit into even the smallest format modular synth system.
There are no dependencies between components so, other than the dsPIC and power components, anything can be omitted if not required. Below is my prototype board with no CV outputs or MIDI and only 8 out of the 12 CV inputs.