JG25 · The design story · Overview
A signal for voice radios.
A packet built for JSQ.
JG25 is OpenJSQ’s experimental native FM packet modem. Its development joined two efforts: finding the tones that worked through ordinary voice-radio audio, and stripping the packet format down to the information JSQ services needed.
Part one · The tone story
Start with the radio’s microphone and speaker path.
The modem had to work through ordinary FM voice audio, with pre-emphasis, de-emphasis, and the radio’s filters. A flat high-speed data port could not be a requirement.
Two-tone, 1,200-bit/s packet provided the starting point. Four tones could carry two bits per symbol, offering more data without raising the symbol rate as far. But faster symbols made more mistakes, and squeezing the tones closer together hurt reception. Speed, spacing, and tone placement had to be chosen together.
The receiver became part of the answer.
Radio recordings showed that some tones arrived weaker and that audio filtering blurred their timing. Correcting the levels helped, but changing how the receive filter handled timing made a much larger difference. The same 36 recorded frames produced this result:
The same 36 recorded frames, two filter designs
More decoded frames is better
Saved-radio replay. Both filters used +6 dB/octave conditioning with nearly identical magnitude responses; their phase behavior differed.
A phase-aware detector later recovered 18 previously failed JG2400 bodies while retaining 21 successful controls. Those were replays of saved audio, not new over-air trials.
Once the receiver was repaired, most tested tone changes lost their advantage. A JG2000 candidate still needed 1.7–2.4 dB less signal in simulation, so it remains worth radio testing. The existing transmitted format was kept while the receiver improvements went ahead.
Part two · The packet and FEC story
Keep the information the services need.
AX1200 already carried JSQ inside an AX.25 packet. That kept compatibility with existing packet equipment and digipeaters, but put a service envelope inside another frame.
The format work examined what was actually needed from X.25-derived AX.25 framing and the service envelope. JG25 gave the essential addressing, packet type, identifiers, and fragment information one native home, with type-specific fields and room for extensions.
Compare the packet structuresAX1200: JSQ inside AX.25 → JG25: one native envelopeProtection was part of the format.
The body could be lean, but the receiver still had to find the packet and learn its speed, FEC, and length. A slower, always-protected header made that introduction more dependable without slowing the whole body.
For the body, F0 favors low overhead; F1–F3 add error-correcting redundancy. Interleaving helps spread burst damage, and CRC-32C checks the decoded result. In the reference simulations, F3 needed the least signal at each speed. F1 and F2 did not consistently improve on F0, so extra protection has to justify its airtime.
The two stories come together
A faster strong-path option, with protection choices.
JG2400/F0 became the speed-first default. With a 256-byte payload, its calculated rate over a complete waveform was about 1,881 bit/s, versus 926 bit/s for AX1200. That reflects the waveform and packet format together, and excludes turnarounds, acknowledgements, and retries.
The faster option needed more signal: about 3.4–3.9 dB more than AX1200 at 90% delivery on three fitted voice-radio models. F3 reduced much of that gap at the cost of airtime. The result was a choice of seven speeds and four body-protection settings for different paths.
Operators choose the transmit speed and protection. Each frame carries its own opening and header, while JSQ services handle acknowledgements and retries above the modem.
The rate and signal comparisons are software-test results, not measured on-air transfer speeds or calibrated RF sensitivity readings. Most fitted models draw on one radio pair. This account covers FM development and does not establish HF performance.
Read the full development story.
Follow the tone search, the native packet format, and the tests that shaped their protection.
The development story →Illustrated report · PDF