TouchSynth

TouchSynth

A small MCU-controlled analog monophonic synthesizer on a 100 × 100 mm, 4-layer PCB. It has no potentiometers and no mechanical switches: 13 capacitive touch keys plus SHIFT / − / + play and edit it, and 16 LEDs show the settings. A CH32V003 (or CH32V002 / V006) generates the oscillator timing and four PWM control voltages for a discrete analog voice:

• DCO: a TL072 integrator reset by the MCU timer. The charge current tracks pitch, so the saw amplitude stays flat. Saw, variable-width pulse, sub-oscillator and noise.
• VCF: a 24 dB/oct transistor ladder (BC847BS) with an expo converter and resonance up to self-oscillation.
• VCA: a differential-pair VCA. An 8-bit R-2R trim DAC nulls the CV feedthrough ("thump").
• Line out, an audio loopback into the MCU ADC (for calibration and tests), and an optional MIDI in.

All assembled parts are JLCPCB Basic or Promotional Extended parts: 40 BOM lines, 196 placements, about USD 3.65 of parts per board at list price. The MCU, headers and MIDI parts are hand-soldered.

Status (2026-10-18): board 1 (rev 0.1, CH32V006) is built and brought up. It passes the bring-up steps and functional tests T1, T2 and T4–T12 (noise marginal at about −55 dB); T3 (touch scan rate) and T14 (MIDI) have not been measured. The results are in the design report, section 10. Firmware 0.1 runs on the board, including multitrack demo pieces and percussion presets. Calibration storage in flash is still open.

The polyphonic_exp branch experiments with extensions. So far it turns the pulse output into a PWM DAC (CH32V006) for digital voices through the analog filter and VCA, starting with a modal timpani, the kettle (kettle_model.mp3; report, section 10.5).

Design: cpldcpu & Claude Opus 5.5.

Hear it

Recorded from board 1's line out. The synth is monophonic, so the multitrack pieces were played one part at a time and mixed:

The three built-in demo tunes (SHIFT + C' on the board), recorded from board 1: acid, prelude (Bach BWV 846) and ode (Beethoven). Also from the board: a waveform tour and a scale on the self-oscillating filter. The behavioural model renders every single feature (waveforms, filter, envelopes, glide, …) in docs/audio/features/. docs/README.md lists all audio files.

Documents

Repository layout

docs/        report (LaTeX), spec, simulation report, V&V plan, figures, audio examples, result CSVs
sim/         ngspice simulations S1–S7 (one folder per block), shared netlists and models, behavioural model
hw/          design.py (single source of truth) -> KiCad schematic, PCB, routing and fab outputs
  touchsynth/  KiCad 10 project (generated; edit the Python sources, not the KiCad files)
firmware/    touchsynth/ firmware 0.1, ch32fun (git submodule)
parts/       JLCPCB Basic / Promotional Extended part lists
tools/       bench tools: hardware tests, scope recording, multitrack mixing, figures; Freerouting jar (not in git)

Flow

Every stage is generated by scripts from hw/design.py, which holds the parts, values, LCSC numbers and nets.

simulate      sim/s*/s*.py                 ngspice-42 in WSL -> docs/results/*.csv, docs/figures/*.png
model         sim/behav/validate.py        behavioural model vs. ngspice
              sim/behav/examples.py        audio examples + spectrograms
schematic     hw/gen_sch2.py               -> hw/touchsynth/*.kicad_sch
              hw/check_netlist.py          KiCad netlist == design.py
              hw/check_sim.py              design.py == simulation netlists
PCB           hw/gen_pcb.py [--route]      placement, silkscreen art, planes, Freerouting + GND stitching
              hw/gen_pcb.py --finish       post-route clean-up (islands, leftovers via the A* router)
fab           hw/fab.py                    gerbers, drill, BOM and CPL -> hw/touchsynth/production/
report        docs/report/gen_tables.py    then: latexmk -pdf touchsynth_report.tex (WSL)
firmware      firmware/touchsynth/gen_tables.py, check_pins.py, make
tunes         sim/behav/tunes.py, pieces.py    demo tunes, multitrack pieces and presets (-> firmware tables)
bench         tools/hwtest.py              functional tests over the SWIO console -> docs/results/hw_board1*
              tools/hw_figures.py          board 1 figures for the report
              tools/record_parts.sh        record one multitrack part per take with the scope (on the lab PC)
              tools/mix_parts.py, suite.py align, clock-correct and mix the takes -> docs/audio/

The bench tools need the lab setup (the PC with the programmer, the scope address) in tools/lab.conf: copy tools/lab.conf.example and edit it; lab.conf is not in git.

gen_pcb.py needs KiCad's bundled Python (pcbnew) and Java 17 for Freerouting 1.9.0. The hw scripts find KiCad 10 in its standard install folder, or in KICAD_DIR if set. The other Python scripts use a normal Python 3 with numpy and matplotlib. ngspice, LaTeX and the RISC-V toolchain run in WSL.

Key numbers