WSJT-X 2.3.0 introduces FST4 and FST4W, new digital protocols designed particularly for the LF and MF bands. Decoders for these modes can take advantage of the very small Doppler spreads present at these frequencies, even over intercontinental distances.

As a consequence, fundamental sensitivities of FST4 and FST4W are better than other WSJT-X modes with the same sequence lengths, approaching the theoretical limits for their rates of information throughput. The FST4 protocol is optimized for two-way QSOs, while FST4W is for quasi-beacon transmissions of WSPR-style messages. FST4 and FST4W do not require the strict, independent phase locking and time synchronization of modes like EbNaut.

The new modes use 4-GFSK modulation and share common software for encoding and decoding messages. FST4 offers T/R sequence lengths of 15, 30, 60, 120, 300, 900, and 1800 seconds, while FST4W omits the lengths shorter than 120 s. Submodes are given names like FST4-60, FST4W-300, etc., the appended numbers indicating sequence length in seconds. Message payloads contain either 77 bits, as in FT4, FT8, and MSK144, or 50 bits for the WSPR-like messages of FST4W. Message formats displayed to the user are like those in the other 77-bit and 50-bit modes in WSJT-X. Forward error correction uses a low density parity check (LDPC) code with 240 information and parity bits. Transmissions consist of 160 symbols: 120 information-carrying symbols of two bits each, interspersed with five groups of eight predefined synchronization symbols.

We recommend that on the 2200 and 630 m bands FST4 should replace JT9 for making 2-way QSOs, and FST4W should replace WSPR for propagation tests. Operating conventions on these LF and MF bands will eventually determine the most useful T/R sequence lengths for each type of operation.

Further details and operating hints can be found in the “Quick-Start Guide to FST4 and FST4W”, posted on the WSJT web site:

WSJT-X 2.3.0-rc1 is a beta-quality release candidate for a program upgrade that provides a number of new features, capabilities, and
defect repairs. These include:

• New modes FST4 and FST4W targeting LF and MF bands.
• Improved noise baseline discovery for more reliable SNR estimates.
• On the waterfall and 2D spectrum a tool-tip shows the frequency offset under the mouse pointer.
• The *On Dx Echo* Doppler compensation method has been modified in response to feedback from Users. Basic functionality is unchanged. See the User Guide (Section 8.1) for more information.
• Improved user_hardware script or program initiation for WSPR band-hopping mode.
• Decoded QSO mode message display narrowed to make appended information easier to view without scrolling the window.
• Option to record the propagation mode in logged QSO records.
• ADIF v3.1.1 compliance.
• Option to connect to PSKReporter using TCP/IP for those with very poor Internet connections.
• Major rewrite of the PSKReporter interface to improve efficiency and reduce traffic levels.
• Removal of the Tab 2 generated messages.
• Accessibility improvements to the UI.
• Tweaked decode speed options for a better user experience with lower powered single-board computers like the Raspberry Pi.
• Updates to UI translations in Spanish, Italian, Catalan, Chinese, Hong Kong Chinese, Danish, and Japanese.
• Audio devices only enumerated when starting up and opening the “Settings->Audio” device lists.
• Option to select the default audio device removed to minimize the likelihood of system sounds being transmitted.
• Better handling of missing audio devices.
• Improved and enhanced meta-data saved to .WAV files.
• More reliable multi-instance support.
• Included CTY.DAT file moved to installation share directory.
• The bundled Hamlib library is updated to the latest available which fixes several regressions, defects, and adds new rig support.
• Fixed some edge-case message packing and unpacking defects and ambiguities.
• Fix a defect that allowed non-CQ messages to be replied to via the UDP Message Protocol.
• Fix a long-standing defect with Tx start timing.
• Repair a defect with style sheets when switching configurations.
• Repair defects that made the astronomical data window an several main window controls unreadable when using the dark style sheet.
• Repair a regression with setting WSPR transmitted power levels.
• Repair a regression with newly created ADIF log file’s header.
• Many other defects repaired.