Pantograph
Trace and record CV — record knob movements, loop them at bipolar speed. A program card for the Music Thing Workshop System Computer.
Pantograph lets you trace a performance on the X and Y knobs (e.g. oscillator pitch and filter cutoff), record it, then loop it back out the two CV outputs at variable speeds.
Switch UP auditions live; hold DOWN to record (monitored, so you hear it as you play); release to MIDDLE and it loops exactly that length. The Main knob sets playback speed: centre = freeze, left = reverse, right = faster.
Patch a gate into Pulse In 1 and MIDDLE becomes a triggered envelope; unpatched it free-loops. Pulse Out 1 fires a trigger at the end of a recording. Pulse Out 2 runs a gate from the shape of the recording.
Controls
Switch
| Position | Mode | What it does |
|---|---|---|
| Up | Live | Audition — X/Y play live to the CV outs, CV-in summed on top |
| Middle | Play | Loop the recorded trace at Main-knob speed; X/Y knobs attenuate each trace |
| Down (hold) | Record | Hold to record a fresh trace; release to Middle to loop it |
Knobs
| Knob | Name | What it does |
|---|---|---|
| Main | Speed | Playback speed, bipolar — centre = freeze, left = reverse, right = faster |
| X | Trace X | Performance value in Live/Record; playback depth (attenuation) in Play |
| Y | Trace Y | Performance value in Live/Record; playback depth (attenuation) in Play |
Inputs & outputs
Inputs
| Jack | Name | What it does |
|---|---|---|
| CV In 1 | X Mod | Live CV summed onto the CV 1 output |
| CV In 2 | Y Mod | Live CV summed onto the CV 2 output |
| Pulse In 1 | Trigger | When patched, Play (Middle) becomes a one-shot: each rising edge retriggers. Unpatched = loop |
(Pulse In 2 and both Audio Inputs are unused.)
Outputs
| Jack | Name | What it does |
|---|---|---|
| CV Out 1 | Trace X CV | Trace X playback |
| CV Out 2 | Trace Y CV | Trace Y playback |
| Pulse Out 1 | End of Cycle | Trigger on every trace wraparound |
| Pulse Out 2 | Contour Gate | Gates high on Trace X's recorded shape |
(Both Audio Outputs are unused.)
LEDs
- Live: two 3-LED level bars (brightness = |output| per X/Y).
- Record: buffer-fill gauge (LEDs fill 1→6 as the recording approaches the ~75 s cap).
- Play: a moving position dot — reverses and freezes with speed, and holds on the last LED when a trigger finishes.
Repository layout
| File | Purpose |
|---|---|
pantograph.h | Pure signal logic — integer/fixed-point math, no Pico headers, host-testable |
pantograph.cpp | ComputerCard glue — the RP2040 firmware entry point |
tests/ | doctest host tests for the pure logic (fast, no hardware) |
CMakeLists.txt, pico_sdk_import.cmake | Firmware build |
ComputerCard.h | Vendored ComputerCard framework header |
.clang-format | Formatting config (enforced by a pre-commit hook) |
The design keeps everything the card computes in pantograph.h as plain integer
math (the RP2040 has no FPU), so it can be developed and tested entirely on the host;
pantograph.cpp is a thin layer that wires that logic to the ComputerCard I/O.
Development
Build the firmware (Raspberry Pi Pico SDK, C++17):
cmake -S . -B build -DPICO_SDK_PATH=/path/to/pico-sdk
cmake --build build --target pantograph -j4
Flash the resulting build/pantograph.uf2 by holding the Computer's boot button while
connecting USB (it mounts as RPI-RP2), then copy the .uf2 onto it.
Run the host tests:
cd tests && make
Formatting is enforced by a clang-format pre-commit hook (skips the vendored
ComputerCard.h and doctest.h): if a staged source isn't clean it's reformatted in
place and the commit aborts so you can review and re-stage.
Releasing
The card ships in the Workshop System program-card catalogue at
TomWhitwell/Workshop_Computer
under releases/90_Pantograph/ (with info.yaml metadata; the tests/ are dev-only
and not part of the release).
Thanks
Written by Kenny Shen (@qoelet).
- Tom Whitwell for the Music Thing Workshop System.
- Chris Johnson for ComputerCard.