780 Mini Synth :: Quicker, easier and cheaper to make your own chip!

How it works

A monophonic synthesizer with three oscillators and an ADSR envelope.

Signal chain

Phase accumulator — 16-bit register incremented every sample tick (50 kHz) by a note-dependent value. Wraps freely.
Triangle wave — the phase is folded at the midpoint (phase[15] ? ~phase : phase) and shifted to 11 bits.
Square wave — derived from phase[13], giving a square at 4× the fundamental frequency.
LFO-detuned triangle — a second 16-bit phase accumulator runs at almost the same rate, offset by a ~4 Hz triangle LFO, producing a slow chorus/detune effect.
Mix — the three oscillators are summed into a 12-bit audio sample.
ADSR envelope — pulse-density amplitude control: a 7-bit counter (period 125) runs at the full 50 MHz clock. When the counter value is below the current volume level the raw audio is passed; otherwise the output is held at the midpoint (12'h800, silence). This gives a chopping frequency of 50 MHz / 125 = 400 kHz, well above the audio pmod RC filter cutoff, with no aliasing into the audio band.
Sigma-delta modulator — a 12-bit first-order sigma-delta accumulator converts the envelope-controlled sample to a 1-bit PWM stream on uo[7] at the full 50 MHz clock rate.

Note selection

The 7 input bits (ui[6:0]) select any of the 128 standard MIDI notes (MIDI 0–127, C-1 to G9, roughly 8 Hz–12.5 kHz).

Pitch lookup is compressed using the octave-doubling property of equal temperament: only 12 base increments (one octave, anchored at C2 for precision) are stored. The octave is determined by a comparison chain; for notes at or above C2 the base increment is left-shifted, for notes below C2 it is right-shifted.

ADSR envelope

ui[6] is the gate input. A rising edge triggers the attack phase; releasing the gate (falling edge) triggers release.

Phase	Rate	Duration
Attack	vol++ every 8 samples	~20 ms (0 → full)
Decay	vol-- every 32 samples	~30 ms (full → sustain)
Sustain	held at ~62% volume	until gate released
Release	vol-- every 64 samples	~100 ms (sustain → 0)

Parameters

Parameter	Value
Clock	50 MHz
Sample rate	50 kHz (÷1000)
Phase accumulator	16-bit
Audio sample	12-bit unsigned
Chop counter period	125 (f_chop = 400 kHz)
Note range	MIDI 0–127 (C-1–G9)
Note resolution	1 semitone (128 notes, 7 bits)

How to test

Connect the Tiny Tapeout Audio Pmod to the output pin header. Drive ui[6:0] with a 7-bit MIDI note number (0 = C-1, 69 = A4, 127 = G9) and set ui[7] high to open the gate. The ADSR envelope will attack, decay, and sustain while the gate is held; releasing the gate triggers the release phase.

A Verilator simulation is included under verilator/. Build and run it with:

cd verilator
make
./synth
# Output written to audio.raw — play back with:
ffplay -f u16le -ar 50000 audio.raw
# Or convert using ffmpeg:
ffmpeg -f u16le -ar 50000 -i audio.raw audio.wav

This renders a chromatic sweep of 64 notes (MIDI 36–99, C2–Eb7), 0.8 s gate-on + 0.2 s release per note, to audio.raw as 16-bit unsigned PCM at 50 kHz mono.

External hardware

Tiny Tapeout Audio Pmod connected to the uo output header.

#	Input	Output
0	MIDI note bit 0
1	MIDI note bit 1
2	MIDI note bit 2
3	MIDI note bit 3
4	MIDI note bit 4
5	MIDI note bit 5
6	MIDI note bit 6
7	Gate (note on)	AudioPWM

780 Mini Synth

780 : Mini Synth