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://cassini-format.codemyriad.io/. Everything on it is built from this repository.

What is here

• 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.

Try it on a file

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

The shape of it

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 any cleanup, 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.

Implementations

The reference producer is gocassini, which records, transcribes and packs these files. 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.