ShowReel · Feature

The chiptune that outgrew its blog post

The AsciiWorldEngine lift ride needed a soundtrack, and the instruction was simple: source it legally or ship it silent. Rather than gamble on a "free to use" file, a 114-line Python script wrote its own chiptune. The captain looked at it and asked why a generator that times itself to a video lived beside a blog asset instead of inside the renderer that times things to video for a living. It doesn't any more.

ShowReel Audio · Rust 28 Aug 2026

Samples licensed — every note is synthesised

809

Lines of Rust doing the synthesis, src/music.rs

ms from a hard cut to the nearest bar line

0.9997

Envelope correlation vs. the Python original (spectrum: 0.9999)

A problem worth avoiding

The lift ride is a demo video for this blog — a walk through AsciiWorldEngine's city and up in the tower's glass lift, cut as a short film in ShowReel. It needed music, and this is a public site, so the rule was blunt: source audio legally and keep the licence, or ship the clip silent. Nobody wanted to be the one explaining a licensing question later.

Instead of hunting for something labelled free-to-use and hoping the label held up, the answer was to not need one. chiptune.py — a 114-line Python script, committed alongside the video assets — synthesises three voices in the NES/SID idiom: a duty-cycled pulse bass, a pulse-wave arpeggio lead with a little vibrato, and noise bursts standing in for hats and kick. Its own docstring says what it is: self-authored, nothing sampled, nothing pulled off the internet, public domain.

The obvious question

The captain saw it and asked the question that actually mattered: if a generator times its own output to a video and plays inside that video, why does it live next to a blog asset instead of inside the renderer built to time things to pictures? Should it not be a feature, in Rust?

It now is. ShowReel's own commit for it calls the Python file what it was — "the devlog's throwaway chiptune.py" — and ports the idiom faithfully rather than treating it as a spec to improve on: the same pulse bass, the same arpeggio lead, the same noise drums, none of it reinvented.

What shipped

A track's source can now be generated music instead of a file. A Music spec resolves to a synthesised WAV at exactly the point a file track's asset resolves to a path, so from there it is an ordinary audio input — placement, gain, fades, multi-track mixing and the mobile cut all compose for free rather than through a second audio path built to match the first.

Authoring is one word in the film file, mirroring the shorthand the colour grade already uses:

"audio": [
  { "music": "funk" },
  { "music": { "mood": "dreamy", "key": "C", "bpm": 96 }, "fade_out": 2.0 }
]

A bare mood word for the reference tune, sized to the film, or an object for key, bpm and tempo fit — examples/music_demo.film.jsonc in the ShowReel repository.

The timing is the point

With no duration given, the tune sizes itself to exactly the film's length, so it never needs trimming and never hard-cuts to silence. fit: "film" goes further: the authored tempo is treated as a target and nudged so a whole number of bars spans the film exactly, landing the final downbeat on the last frame. Space scene cuts a whole number of bars apart and every cut becomes a chord change, generated to fit rather than hand-aligned after the fact.

The self-contained demo film proves it: four bar-aligned scenes at 128 BPM, and its three hard cuts land within about 7 ms of a downbeat in the rendered audio.

ShowReel's own proof file, rendered end to end with no assets — every background is a colour, the only source is the chiptune the one music track synthesises. Each hard cut lands on the downbeat of bar 5, 9 or 13; the progress bar under the second scene's title is that beat grid drawn on screen.

Checked against the thing it replaced

The Rust port was checked honestly against the Python original rather than assumed faithful: envelope and spectrum correlation of roughly 0.9997 and 0.9999. The one deliberate difference is a seeded random generator in place of the Python script's global one, which means renders now reproduce — same film, same audio, every time. That's a real improvement on the thing it replaced, not just a port of it.

“It was in the wrong repo, as a throwaway script, until someone asked the obvious question.”

Why this one is worth telling

Back where it started

The feature went straight back into the video that caused it. The AsciiWorldEngine lift film's soundtrack is arranged to its own cuts, not laid under them: sparse under the opening terminal, a riser through the bar before the reveal, the full band dropping in with a crash on the cut to the city, driving through the street and the ride, and a one-bar resolve to the tonic at the end so it lands rather than getting chopped. The ShowReel feature landed at 19:03; the recut using it landed at 19:21. Eighteen minutes.

Figures on this page were checked against the ShowReel and devlog repositories directly: chiptune.py's original line count from the commit that added it, src/music.rs and its own module docs read directly rather than quoted from the commit message, the 7 ms and correlation figures from ShowReel's own README.md, and the two commit timestamps from each repository's own log.

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