This plugin can be used to demodulate and decode FT8 signals. FT8 is used by amateur radio to perform contacts (QSOs) with very weak signals. It is used mostly but not limited to HF bands. The protocol and modulation details are described in [QEX July/August 2020 article](https://www.iz3mez.it/wp-content/library/appunti/Protocols%20FT4%20FT8%20QEX.pdf)
The decoder code is based on [ft8mon](https://github.com/rtmrtmrtmrtm/ft8mon) code written by Robert Morris, AB1HL. The core of the decoder is stored in a separate `libft8` library in the `ft8` folder of this repository and is placed under the `FT8` namespace.
The excellent work of Robert Morris makes the decoder on par or even slightly better than the original [WSJT-X](https://wsjt.sourceforge.io/wsjtx.html) code.
Use the wheels to adjust the frequency shift in Hz from the center frequency of reception. Left click on a digit sets the cursor position at this digit. Right click on a digit sets all digits on the right to zero. This effectively floors value at the digit position. Wheels are moved with the mouse wheel while pointing at the wheel or by selecting the wheel with the left mouse click and using the keyboard arrows. Pressing shift simultaneously moves digit by 5 and pressing control moves it by 2.
The FT8 decoding sample rate of 12 kS/s is further decimated by powers of two for the spectrum display (B) and in RF filter limits. This effectively sets the total available bandwidth depending on the decimation:
There are 10 filters in the filter bank with indexes 0 to 9. This selects the current filter in the bank the filter index is displayed at the right of the button. The following controls are covered by the filter settings:
This AGC is based on the calculated magnitude (square root of power of the filtered signal as I² + Q²) and will try to adjust audio volume as if a -20dB power signal was received.
As AGC compresses the signals you have to experiment whether AGC or no AGC give the best result in decoding signals. With AGC on it is easier to adjust the volume (A.10) as the audio signal will be more stable.
This is the volume of the audio signal in dB from -10 (0.1) to 40 (10000). It can be varied continuously in 1 dB steps using the dial button. When AGC is engaged it is recommended to set a low value in dB not exceeding 3 db (gain 2). When AGC is not engaged the volume entirely depends on the RF power and can vary in large proportions. Hence setting the value in dB is more convenient to accommodate large differences.
This shows the level of the signal entering the FT8 demodulator and decoder and peaks (shown by the tiny red vertical bar) should never exceed 100%. In fact there is a 10% guard so 100% is actually 90% of the signal volume. Note that the decoder will work well even with a few % volume however you should try to set the volume (A.10) so that big signals reach at least ~20% to have the best dynamic range.
This is the spectrum display of the demodulated signal (SSB) or translated signal (DSB). Controls on the bottom of the panel are identical to the ones of the main spectrum display. Details on the spectrum view and controls can be found [here](../../../sdrgui/gui/spectrum.md)
- **6**: First callsign area. May contain spaces (ex: "CQ DX"). Note that for messages types 0.1 and 5 it is slightly different from the standard (See C.10)
- **7**: Second callsign area and is always a callsign. This might be slightly different from the standard (see above)
The baseband (audio) may be recorded as a 12 kHz mono PCM .wav file similarly to what can be done with the original WSJT-X program. Each record is the 15s FT8 period (180000 16 bit signed samples or 360000 data bytes). Files are stored in the system application data location (see above for details) in the `ft8/save` folder and have the following file name format:
<date>_<time>.wav
Where date is the slot date in YYYYMMDD format (in WSJT-X this is YYMMDD) and time is the slot time in HHmmss format.
<h3>C.10: Messages table</h3>
Displays the received messages in a table which columns are the following:
- **UTC**: UTC time in HHmmss format of the FT8 slot
- **0.0**: Free text. The first call area is populated with the free text. second call and locator areas are left blank.
- **0.1**: DXpedition. Message carries two compacted messages (one for RR73 and one for report to two different callees). They will be de-compacted in two separate messages for display. This is different from the standard that yields a single message.
- **5**: European VHF (6 char locator). The report + QSO number is appended to first call area not to pollute the second call area which contains only a callsign, This is slightly different from the standard.
- **Call1**: This is the first call area and may contain the caller callsign, a CQ or a custom 13 character message in which case the second call and locator areas are empty. It may be slightly different from the standard for message type 5 (see above).
- **Call2**: This is the second call area and will contain the callsign of the responding station. This is always a callsign and this may differ slightly from the standard for messages type 5 (see above).
- **Info**: FT8 decoder information if any. If OSD is active (see C.1.3) and OSD was activated it reports the OSD decoder status as `OSD-N-MM` where N is the OSD depth reached and MM is the number of correct LDPC bits.
When processing the audio baseband several instances of the core decoder will be spawned in separate threads. This adjusts the number of threads. You have to experiment with it to find which is the best number for you. The default of 3 will normally yield good results comparable if no better to what is obtained with the original WSJT-X software.
<h4>C.1.2: Decoder time budget</h4>
This is the time in seconds after which the decoder threads will be prompted to stop. It can be varied from 0.1 to 5 seconds. You can imagine that the longer the decoder runs the more messages it will harvest however the default of 0.5s will be enough in most cases. You can still experiment with it to find what value is the best in your own case.
This toggles the Ordered Statistics Decoding (OSD). OSD is used if the CRC check fails after LDPC processing. Be careful with OSD as it will try to find solutions that validate the CRC but these solutions can be wrong and thus yield false messages. To mitigate this you may engage the callsign verification function (C.1.6). Ultimately you may post process the results (logs) through a list of valid country prefixes and correlate country and locator.
OSD search may find invalid solutions as mentioned above. When checking this option the callsigns in messages not passed through OSD (thus very certainly valid) are stored for the life of the plugin. When OSD is engaged the second callsign field which is always a callsign is checked against this list and if the callsign is not found the message is rejected. This is quite efficient in removing false messages when OSD is engaged although some valid messages may be removed.
Use this button to create a new preset. It will take the values from the row of the selected cell in the table (if selected) and put the new preset at the bottom of the table
