cassini-format

A Cassini portable meeting is an ordinary .opus file. Any player plays it, because that is all it is: Ogg Opus, 48 kHz. It also carries its own word-timestamped transcript, the speakers, and a record of what produced the text, in the OpusTags header, where a player that does not care ignores them.

The website is https://format.gocassini.com/. Everything on it is built from this repository.

• SPEC.md: the specification. Version 1, published 2026-09-02.
• spec/: the two JSON Schemas; the transcript body (cassini-words-v1.md); the audio digest (cassini-opus-audio-integrity-v1.md); the conformance suite; and the private drafts that came before version 1.
• tools/: small readers and a producer in Python and JavaScript, standard library only, meant to be copied.
• site/: the website, and under site/static/demo/ a real file to check against.
• design/: the measurements and arguments behind the non-obvious parts. Some files are historical and not published on the site.
• LICENSE.md: CC-BY-4.0 prose, CC0 everything an implementer copies.

The site lists every tool, with its language and licence, at https://format.gocassini.com/build/, including a browser reader that needs no install. From this repository:

tools/cassini-extract.py meeting.opus --tags   # the descriptor tags
tools/cassini-extract.py meeting.opus --list   # which transcripts are inside
tools/cassini-extract.py meeting.opus          # the manifest as JSON
tools/cassini-read-pure.py meeting.opus        # no ffprobe: parses the container itself

Version 1, published 2026-09-02, simplified 2026-09-07. The change history is in CHANGELOG.md.

Two layers of metadata. The first is plain comments any tool shows: TITLE, CASSINI_SPEAKER_COUNT, CASSINI_AUDIO_OPUS_SHA256. The second is a JSON manifest, gzipped, base64url-encoded and split across numbered comments. Each transcript body is a second such payload under its own prefix.

CASSINI_FORMAT=org.cassini.portable-meeting/1
CASSINI_PAYLOAD_ENCODING=base64url+gzip+utf8json
CASSINI_PAYLOAD_CHUNK_COUNT=1
CASSINI_PAYLOAD_SHA256=a4d048386f81bd4b...
CASSINI_PAYLOAD_000=H4sIAAAAAAAC_61W227jNhD9...
CASSINI_TX_SCRIPT_PAYLOAD_CHUNK_COUNT=3
CASSINI_TX_SCRIPT_PAYLOAD_000=H4sIAAAAAAAC_...
CASSINI_DECODE_HINT=Concatenate CASSINI_PAYLOAD_000..N for the manifest; ...

CASSINI_DECODE_HINT is there so that someone with ffprobe and no documentation still gets the transcript out. A message in a bottle.

Three commitments run through it:

• Progressive enhancement. A player that understands nothing plays the audio. A reader that understands the tags gets everything.
• Keep what a better model could use later. The words a recogniser produced survive next to derived display text, with provenance saying what made each.
• The digest is a join key, not a seal. A SHA-256 over the Opus packets says whether this transcript describes this recording. It catches accidents, not adversaries.

gocassini is the reference implementation: it records a meeting, transcribes it and packs the result into one of these files. It is open source and ships as a Nextcloud app. The readers in tools/ and on the site are the others. If you build one, say so: a format that only one program reads is not really a format.

Issues and pull requests welcome.