This is old, work-in-progress code! Please don't rely on it as a sterling example for how to write code for Microchip controllers.
This project contains the source code for my first synthesizer design. An overarching goal of this project is to learn more about electronics design as well as microcontroller programming. Accordingly, a key goal of this project is to document the design and implementation as clearly as possible so that others may use it as a resource in their own designs.
The DIAL-1 employs a PIC18F4620 microcontroller to coordinate system activities, which include:
- Receiving input from panel controls (potentiometers, switches, etc.)
- Receiving and transmitting MIDI data via the USART
- Programming the DCOs (digitally controlled oscillators)
- Computing control voltages for DACs to drive VCAs and the VCF, etc.
- Displaying data on the Hitachi 44780-compatible 16x2 Display
- Communicating with the SD-card interface for Save / Load
- Arpeggiator function
The sections below outline the major subsystems.
The DIAL-1 DCO system uses an Intel 8254 programmable timer IC to generate baseline waveforms (square waves) that are transformed via additional circuitry to "more interesting" waveforms such as saw, triangle, and PWM. Benefits of the DCO approach include unparalleled pitch stability as well as the potential for the implementation of precise microtonal tuning tables.
Early MIDI-controlled analog synthesizers (such as the Roland Juno-106 and the Oberheim Matrix 1000) computed envelopes entirely in software which reduced part count and made rich modulation capabilities possible. However, software-based envelopes tended to be sluggish due to the fact that the microcontrollers used were saddled with other duties.
For this reason, in the prototype of the DIAL-1, a dedicated envelope generator will be used to control the VCA. (The envelope generator is in fact a PIC16-series microcontroller running a highly optimized envelope generation routine that allows for fast (~1ms) response times.
The DIAL-1 features an analog noise generator that employs a typical open-collector transistor, as found in many synthesizers.
The filter used in the DIAL-1 is 24db/oct low-pass ladder filter circuit with resonance and cutoff. The filter can be driven to self oscillation.
The DIAL-1 system software is written in C using Microchip Technologies' MPLABX IDE in conjunction with their XC8 compiler for 8-bit PIC products.
Currently, the software is structured into modules that are intended to be reusable:
- IOPORT - Routines for initializing and reading/writing from the USART
- MIDI - Routines for handling MIDI data