Also known as: JS8Call, JS8
JS8Call is an amateur-radio keyboard-to-keyboard chat mode that takes the weak-signal waveform of FT8 and turns it into a real conversational system. Where FT8 exchanges only rigid, pre-formatted station reports, JS8Call layers free-form text messaging on top of the same 8-FSK tones — adding relaying, group calls, and store-and-forward so operators can hold slow chats, pass messages, and reach stations they can’t hear directly, all at signal levels far below the noise floor.1 It is best understood not as a new waveform but as an application built around a proven one.
How it works
At the radio layer JS8Call sends the same Gaussian 8-tone FSK symbols as FT8 and protects them with the same LDPC forward error correction, so its sub-noise sensitivity is essentially inherited. The key changes are above the waveform:
- Slower, selectable speeds. JS8Call offers Normal, Fast, Turbo, and Slow sub-modes (roughly 10-, 6-, 4-, and 30-second frame times), trading throughput against sensitivity.
- Free-form text. Instead of FT8’s fixed 77-bit report, JS8Call packs arbitrary characters into a stream of frames, so operators type real sentences that assemble across multiple transmissions.
- Directed protocol. Messages can be addressed to a specific callsign or group, can query a station’s status or SNR, and can be relayed hop-by-hop through intermediate stations.
- Store-and-forward. A station can leave a message for another that isn’t currently on; it is held and delivered when that station appears.
Because transmissions are not locked to strict UTC slots the way FT8’s are, JS8Call is more forgiving of timing, though an accurate clock still helps decoding.
Relevance to SDR
JS8Call occupies HF amateur allocations (with common calling frequencies such as 7.078 and 14.078 MHz) and is popular for low-power, emergency, and off-grid messaging where its weak-signal reach beats voice. Any receiver that can deliver clean SSB audio works as a front end: a traditional HF transceiver, or an SDR such as an RTL-SDR with an upconverter, an Airspy HF+, or an SDRplay piped into the JS8Call application.
GopherTrunk does not decode JS8Call — it is a trunked land-mobile scanner (P25, DMR, NXDN, TETRA and similar), not an HF weak-signal messaging client. JS8Call is decoded by its own free software of the same name, which descends directly from the WSJT-X codebase.2