391 KCH CD101 Saw Synth :: Quicker, easier and cheaper to make your own chip!

How it works

This project provides a simple digital synth. It consists of a pulse-wave generator with programmable frequency, ADSR amplitude modulation with adjustable ADSR parameters and a simple one-pole IIR filter with programmable cutoff frequency. Digital audio output is generated using a simple first-order delta-sigma modulator. The lowpass filter for the reconstruction of this signal must be realized externally.

All parameters can be programmed using a simple SPI slave interface. Sound generation for a note starts when asserting the trigger signal and stops when the trigger is de-asserted again. Triggering can happen both via a dedicated input or via SPI, which enables fully customizeable operation using the SPI port. A VST/CLAP plugin is provided to generate the SPI commands from DAWs.

This project is educational and therefore makes some decisions that might not lead to an optimal design. For example, the structural presentation of the signal processing pipeline is directly realized in hardware. Some parts of the design are therefore clocked with a frequency as low as the sample rate and some even lower. This wastes a lot of possible performance, but it is easier for students to map the audio application to the circuit mentally, compared to introducing a more complex microcontroller-based system (which might be a more efficient design). The design shows how to realize a serial-parallel multiplier, use negative edge clocking, use simple small clock dividers, use multiple clocks etc.

More detailed information on all these topics will be provided later on.

How to test

As the design generates a lot of data on the single serial output pin, testing generates a lot of data. The cocotb testbench simulates and external lowpass and stores the audio data to a .s16 file which you can convert to .wav using ffmpeg:

Play the output file to assess whether the output is reasonable.

In addition, the testbench also compares to a golden reference output datastream, that was generated from behavioral simulation. This test is used to determine if there are any differences for the final implemented designs.

External hardware

Button PMOD
Audio PMOD
SPI Master (No PMOD available. Use Adafruit board)

TODO: More detailed information about these things.

#	Input	Output
0	Trigger	Audio Data
1	SPI CLK
2	SPI MOSI
3	SPI nSS
4
5
6
7

391 KCH CD101 Saw Synth