synthc

A polyphonic subtractive synthesizer that lives in your terminal. Written in C, ncurses front end, ALSA output.

  synthc 1.0    Fat Bass        oct C3   voices 3/4   cpu 6%   LATCH
  default  48000 Hz  1024 frames (21.3 ms)              L ▓▓▓░░░  R ▓▓░░░░
 ╭─ patch ──────────────────────────╮╭─ output ─────────────────────────────╮
 │  OSC  FILTER  ENV  LFO  FX  SEQ  ││ scope                        x4.1     │
 │ ▸Cutoff         620 Hz ▓▓▓▓▓░░░░ ││       │────     │──│     │────│       │
 │  Resonance        0.32 ▓▓▓░░░░░░ ││ ─│─│──│────────││─────────────│─│──── │
 │  Mode            LP 24 ░░░░░░░░░ ││  ││       ││  │        ─     │    ││  │
 │  Env Amount       0.55 ▓▓▓▓▓▓▓░░ ││ spectrum                              │
 │  Key Track        0.30 ▓▓▓░░░░░░ ││ ███████████████████▆████▁             │
 │  Drive           3.0 x ▓▓▓▓▓░░░░ ││ ██████████████████████████▃▄▅         │
 ╰──────────────────────────────────╯╰──────────────────────────────────────╯
      ▄  ▄     █  ▄  ▄     ▄  ▄     ▄  ▄  ▄
    ██▀▓▓▀▓▓ ▓▓█▓▓▀██▀▓▓ ▓▓▀▓▓▀▓▓ ▓▓▀▓▓▀▓▓▀▓▓
    C3                   C4
    seq C3   D3   E3   F#3  A3    ·   C4    ·

Build

Needs gcc (or clang), ALSA and ncursesw development packages.

make
./synthc

Arch: pacman -S alsa-lib ncurses · Debian/Ubuntu: apt install libasound2-dev libncursesw5-dev

make install puts it in /usr/local/bin (override with PREFIX=).

Playing it

Your computer keyboard is the piano. Two rows, one octave each:

lower   z s x d c v g b h n j m , l . ; /     C  D  E  F  G  A  B  and a bit more
upper   q 2 w 3 e r 5 t 6 y 7 u i 9 o 0 p     the octave above

The black keys sit where they do on a real keyboard: s is C#, d is D#, and so on. [ and ] move the whole map up and down an octave.

A terminal never tells a program when a key is released. synthc works around this by watching your keyboard's auto-repeat: a key that stops repeating is treated as released. It plays well, but if notes cut out and retrigger while you hold them, your repeat delay is longer than the default window — either shorten the repeat delay (xset r rate 250 40 on X11, or the keyboard panel on Wayland/GNOME/KDE) or raise the window with --keyhold 900. For sustained chords, Enter toggles latch, which holds notes until you press them again.

Controls

Tab / Shift-Tab	next / previous parameter page
↑ ↓	select a parameter
← →	change it (Shift+←/→ for fine steps)
[ ]	octave down / up
- =	master volume
Enter	latch held notes
Space	start / stop the sequencer transport
8, F4 or ^T	step record — every note you play writes a step and advances
Backspace	write a rest (while step recording)
Delete	clear the sequence
4, F3 or ^A	arpeggiator on / off
1, F2 or ^P	panic, all notes off
PgUp / PgDn	previous / next factory preset
\	randomize the patch
'	back to the Init patch
F9 / F10	save / load a .patch file
F5 or ^R	start / stop writing a .wav of everything you play
F1 or ?	key map
F12 or Ctrl-C	quit

Shortcuts avoid the digits 2 3 5 6 7 9 0 because those are black keys.

The synth

Sixteen voices, each with:

• Two oscillators — sine, triangle, saw, square, pulse, noise. Saw, square and pulse are PolyBLEP band-limited, so they stay clean up the keyboard (measured: no aliasing product above −42 dBc with the filter wide open). Plus a sub oscillator an octave down and a noise source.
• A 4-pole ladder filter in the zero-delay-feedback topology, with a saturating nonlinearity in the loop and Oberheim-style stage mixing for 24/12 dB lowpass, 12/24 dB bandpass and 24 dB highpass. Self-oscillates.
• Two ADSR envelopes — one for amplitude, one for the filter, with a bipolar envelope amount and keyboard tracking.
• An LFO (sine/tri/saw/ramp/square/sample-and-hold) routable to pitch, cutoff, pulse width and amplitude.

Then, on the master bus: saturation, stereo chorus, a tempo-syncable ping-pong delay with damping, and a Freeverb-style reverb. A soft limiter on the end means nothing ever clips hard.

The arpeggiator runs up, down, up/down, down/up, random or as-played, over one to four octaves, at any note division from 1/1 to 1/32 including triplets. The 16-step sequencer shares the same clock and has swing. Both are timed in the audio thread, so steps land on the sample.

Nine factory presets (--list-presets), and \ rolls a new random patch that is usually musical and occasionally better than anything you'd have dialed in.

Files

Patches are plain text — key value per line — so you can edit them by hand:

name Fat Bass
cut 620
res 0.32
...

F5 records to synthc-YYYYMMDD-HHMMSS.wav in the working directory (16-bit stereo). The audio thread writes into a lock-free ring that the UI thread drains, so recording never disturbs playback.

Without a sound card

./synthc --render out.wav 12        # renders a demo chord progression
./synthc -P 4 --render acid.wav 8   # ...using preset 4

Useful on a headless box, and it's how the DSP gets tested.

Latency

synthc asks for a 256-frame period (~5 ms). PipeWire and JACK pick their own period and will usually hand back 1024 frames (~21 ms), which is fine to play but not tight. To go lower:

PIPEWIRE_LATENCY=256/48000 ./synthc     # per-application hint
./synthc -d plughw:0,0                  # talk to the card directly

The header line always shows what you actually got.

Layout

src/synth.h    types, the Patch struct, declarations
src/dsp.c      envelopes, the ladder filter, delay lines
src/engine.c   oscillators, voice allocation, arp/sequencer clock, effects
src/params.c   the parameter table, factory presets, patch file I/O
src/audio.c    ALSA playback thread, WAV writer, offline rendering
src/ui.c       ncurses front end

Adding a parameter means adding one line to the table in params.c and one field to Patch — the UI page, the value bar, the formatting and the preset file format all follow from that.