La revolució del nou mode digital FT8

 

El passat mes de juliol va aparèixer un nou mode digital anomenat FT8. Aquest mode té una seqüència de transmissió i recepció de 15 segons a diferència dels modes JT65 i JT9 que és de 48 segons i és totalment automàtic. Això ha estat una revolució i molts radioaficionats prefereixen l’FT8 en comptes dels altres modes que són molt més lents. El programari WSJT-X inclou diversos modes: JT65, JT9, FT8, JT4, QRA64, ISCAT, MSK144 i WSPR.

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Implementation of FT8 and its auto-sequencing feature is now more capable and more polished. The decoder is faster and better: it now includes signal subtraction, multi-pass decoding, and the use of accumulated “a priori” information as a QSO progresses. Sensitivity extends downward as far as -24 dB in some circumstances.Overlapping signals 2 and 3 deep are frequently decoded at essentially the same frequency. On a crowded band we sometimes see more than 30 decodes in a single 15-second interval, over a 2 kHz window. The North American VHF Contesting Mode has been extended to include both FT8 and MSK144 modes. The “RC2” release also includes many minor bug fixes and an extensively updated WSJT-X User Guide. Depending on what code revision you upgrade from, it may be necessary to do a one-time reset of the default list of suggested operating frequencies. Go to *File->Settings->Frequencies*, right click on the table and select *Reset*.

  • New mode called FT8: sensitivity down to -20 dB on the AWGN channel; QSOs 4 times faster than JT65 or JT9; auto-sequencing includes an option to respond automatically to first decoded reply to your CQ.
  • New mode for accurate Frequency Calibration of your radio.
  • Improved performance of decoders for JT65, QRA64, and MSK144. MSK144 includes facilities for amplitide and phase equalization and an “SWL” mode for short-format messages.
  • Options to minimize screen space used by Main and Wide Graph windows.
  • Enhanced management scheme for table of operating frequencies, and a new set of default frequencies specific to the three IARU Regions.
  • Improved CAT control for many rigs, including those controlled through Commander or OmniRig.
  • New keyboard shortcuts to set “Tx even/1st” ON or OFF.
  • A number of (mostly minor) bug fixes and tweaks to the user interface. For example: new behavior for the audio level slider; correctly logged QSO start times in certain situations; correct control of FT-891/991 and some other radios via rigctld.

Brief description of the FT8 protocol

WSJT-X Version 1.8.0 includes a new mode called FT8, developed by K9AN and K1JT. The mode name “FT8” stands for “Franke and Taylor, 8-FSK modulation”. FT8 uses 15-second T/R sequences and provides 50% or better decoding probability down to -20 dB on an AWGN channel. An auto-sequencing facility includes an option to respond automatically to the first decoded reply to your CQ. FT8 QSOs are 4 times faster than those made with JT65 or JT9. FT8 is an excellent mode for HF DXing and for situations like multi-hop E_s on 6 meters, where deep QSB may make fast and reliable completion of QSOs desirable.

Some important characteristics of FT8

T/R sequence length: 15 s
Message length: 75 bits + 12-bit CRC
FEC code: LDPC(174,87)
Modulation: 8-FSK, tone spacing 6.25 Hz
Constant-envelope waveform
Occupied bandwidth: 50 Hz
Synchronization: 7×7 Costas arrays at start, middle, and end
Transmission duration: 79*1920/12000 = 12.64 s
Decoding threshold: -20 dB; several dB lower with AP decoding
Multi-decoder finds and decodes all FT8 signals in passband
Optional auto-sequencing and auto-reply to a CQ response
Operational behavior similar to JT9, JT65

We plan to implement signal subtraction, two-pass decoding, and use of a priori (AP) information in the decoder. These features are not yet activated in v1.8.0.