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Protocol compatibility #

Compatibility contract: the pinned fldigi implementation (v4.1.23, 61b97f41…) and the published FSQ documentation by ZL1BPU/ZL2AFP.

Key reference facts (from the Gate 0 audit):

Internal sample rate 12000 Hz; symbol length 4096 samples at 3 baud; speeds 1.5/2/3/4.5/6 baud via symlen scaling; 33 tones, 3-bin spacing, ≈290 Hz bandwidth at 3 baud.
Callsign CRC: CRC-8 poly 0x07, init 0x00, two lowercase hex digits.
Callsign validity, as OpenJSQ's transport enforces it (transports/fsq_legacy/src/fsq_legacy.cpp, is_valid_callsign and is_valid_station_callsign): a callsign is 1–15 characters drawn from letters, digits, - and /, with at least one letter or digit; a STATION callsign (a directed destination) must additionally contain at least one letter and at least one digit. A group destination is @ followed by 1–30 letters, digits or /. The special addresses allcall and cqcqcq are reserved broadcast destinations. (An earlier revision of this note said "length 3–20"; that was never what the code checked.)
Directed-message triggers and parse_* dispatch: see audit §8. The exact on-air frame — BOT, sender, CRC-8, destination, trigger, body, optional CRC-16 suffix, EOT — is specified in FSQ_FAST_PUBLIC_SPEC.md Part X–XI, and the bodies OpenJSQ's automatic replies put on the air (snr=-3 db, Heard: lines, ack, and the rest) in JSQ_PUBLIC_PROTOCOL_SPEC.md §56.3.

Directed-command reference (from the pinned fldigi implementation) #

Semantics verified against the parse_* bodies in the vendored fsq.cxx. Wire form: DEST<trigger>[args], sent inside a directed frame.

Trigger	Wire example	Meaning	Remote behavior (fldigi)
(space)	`w4ijg hello`	Chat text	Displays the message
`?`	`w4ijg?`	Signal report	Replies `snr=<estimate>`
`$`	`w4ijg$`	Heard-list query	Replies `Heard:` + its heard list (multi-line)
`@`	`w4ijg@`	Location query	Replies with its QTH text
`&`	`w4ijg&`	Station-message query	Replies with its QTC text
`^`	`w4ijg^`	Version query	Replies `fldigi <version>`
`*`	`w4ijg*`	Wake station	Enables its monitor (SELCAL) and acks; works even when "asleep"
`!`	`w4ijg! text`	Repeat	Echoes the message back immediately (OpenJSQ differs by design: it repeats its own last transmission to the requester, not the supplied text — `JSQ_PUBLIC_PROTOCOL_SPEC.md` §79)
`~`	`w4ijg~ text`	Delayed repeat	Echoes back after 15 s (OpenJSQ: repeats its last transmission after a bounded delay, §79)
`#`	`w4ijg#[name.txt]text`	Store message/file	Writes text to `name.txt` (default `<sender>.txt`), acks
`+`	`w4ijg+[name.txt]`	Read message/file	Sends the stored file's contents back
`\|`	`w4ijg\|text`	Alert	Pops an alert dialog with the text
`>`	`w4ijg>`	Faster	Steps its FSQ speed up one notch, confirms new baud
`<`	`w4ijg<`	Slower	Steps its FSQ speed down one notch, confirms
`;`	`via;dest text`	Relay	`via` retransmits to `dest`, inserting `[origin]`
`[`	`dest[origin] text`	Relayed marker	Marks traffic relayed from `origin`; relayed queries answered via the relay path
`%`	`w4ijg%S`	Image transfer	Starts image RX (mode letter selects size) — OpenJSQ parses the trigger and answers nothing; no image service is transmitted by this release
`-`	—	Unsupported	fldigi replies "not supported"

allcall frames only honor space, *, #, %, [ (no query amplification storms). OpenJSQ mirrors this table in DirectedCommand and the session's compose logic; every mapping is unit-tested.

Forms (ICS-213 and similar) #

Forms are structured text riding the existing transports: transmitted as a directed message or stored at the remote station via #[formfile]. The serialized layout (field labels + values, flmsg-compatible where feasible) is a UX/application-layer concern — see DESKTOP_UX.md. No modem or protocol changes are required to carry forms.

This document grows into the full field-by-field compatibility spec as the fixture corpus expands. Fixture expectations are the ground truth; do not silently update them (see CONTRIBUTING.md).