SSTV (slow-scan television)

The SSTV screen (the Digital tab, then SSTV in its switch; on the all-in-one desktop layout the SSTV button in the bar under the title) receives pictures from the radio's audio and transmits a picture of your own. The Pi does the decoding and the encoding; the page only shows the picture as it arrives and prepares the one to send.

SSTV sends a still picture as a sound: the pitch of the tone is the brightness of the picture (1500 Hz black, 2300 Hz white, 1200 Hz the sync at the start of each line). A transmission starts with a short calibration header that names the mode, so the receiver knows by itself what is coming. The software is written from the published mode descriptions; nothing from other SSTV programs is used.

Modes

Robot 36 and Robot 72, Martin M1 and M2, Scottie S1, S2 and DX, PD50, PD90, PD120, PD160, PD180 and PD240, for receiving and for sending. Checked against two independent SSTV programs in both directions (their pictures decode here, and ours decode there). The time a mode takes is shown in the mode list; the usual ones are Robot 36 (37 s), Martin M1 (115 s), Scottie S1 (111 s) and PD120 (127 s).

Receiving

  1. Open the screen. The radio is put on the frequency and mode of your last visit (the first time, 20 m 14.230 in data-USB) and reception is switched on. Choose a preset from the Band list or tune freely (see Tuning below). The presets are the usual calling frequencies: 80 m 3.845 LSB, 40 m 7.171 LSB, 20 m 14.230, 15 m 21.340, 10 m 28.680 USB, and the ISS SSTV events on 2 m 145.800 FM and 70 cm 437.550 FM.
  2. When a header is heard the picture area starts and fills line by line, with the mode, the line count and the time left. A finished picture is saved on the Pi and appears under Received pictures: tap it to see it large, download it as a PNG, or delete it. A picture that is cut short is kept as partial when it got at least 10 % of the way.
  3. Leaving the screen switches reception off and puts the radio back on the mode and frequency it had; Clear only empties the picture area. The next visit returns to the frequency, mode and step you used last.

The receiver follows the transmitter's timing line by line, so a sound card that runs a little fast or slow does no harm, and a picture right behind another one is picked up too. Signals down to about 12 dB signal-to-noise ratio (in 3 kHz) give a picture (noisy at that level); weaker ones are not recognised. The radio's filter must pass 1100 to 2300 Hz: if it is narrower, the Pi opens it (the same automatic widening as FreeDV, freedv.auto_width).

Tuning

Under the frequency display there is a Frequency box (type 437.550 for MHz, 437550 for kHz or 437550000 for Hz, then Set or Enter), - and + buttons with a step (100 Hz, 500 Hz, 1, 2.5, 5 or 10 kHz) and a Mode list (USB, LSB, FM, FM-N, AM, AM-N and the data modes the radio has). The buttons use the same tuner as the Radio tab: taps in a row add up while the radio catches up, and the arrow keys of the keyboard tune by the same step. A Squelch slider appears in the FM modes. SSTV is received in any of these modes: the presets use a data mode on the HF bands (it also skips the microphone processing when you send) and plain FM for the ISS, but nothing forces that: if you change the mode on the radio, SSTV keeps decoding.

The ISS SSTV events (437.550 MHz FM)

The ISS signal is Doppler shifted by up to about 10 kHz during a pass: it arrives high at the start (about 437.560), at 437.550 at the closest point and low at the end (about 437.540). The program does not calculate the shift: you follow it: choose the 70cm ISS preset (FM), set the step to 2.5 or 5 kHz and tap - as the pass goes on, watching the picture. Before the pass: use the wide FM mode, open the squelch (slider at open), and check that the radio sends FM audio over USB (menu 077 FM PORT SELECT = USB, see Radio connection). The Robot 36 / PD120 pictures of such events are received only: do not transmit.

Sending

  1. Press Choose a picture and pick a photo or drawing from the device (any format the browser can read). Choose the mode; the preview shows exactly what will be sent, at the mode's size. Fit decides whether the picture fills the frame (and is cropped) or is shown whole with bars.
  2. My callsign prints your callsign on the picture (the callsign set under FT settings in the FT8/FT4 screen), and Message adds a line of text (a report, your name, "CQ SSTV"). Text at puts them at the bottom or the top.
  3. Send picture asks for confirmation and sends the picture to the Pi, which transmits it. A bar shows the progress; Halt TX stops it at once.

Sending goes through the same protection as every transmission: the transmit permission in the administration page, the control of the radio (only the page that has control can send), the heartbeat (the Pi stops at once if the page disconnects) and the transmit time limit. A mode that takes longer than the time limit is refused before anything is keyed, and the page says so (an administrator can raise safety.tx_timeout_s in Admin > Config: the default of 120 s allows Robot, Martin and Scottie S1/S2 and PD50/PD90, but not PD120 and the longer ones). The radio's ALC must stay at zero: the tones are sent at the TX level under SSTV settings (default -6 dB), and the radio's speech processor must be off. SSTV and FreeDV cannot share the radio: opening the SSTV screen switches FreeDV off.

Sound

This device's sound is off while the screen is open (the decoder works on the Pi's own audio and the tones are not nice to listen to); SSTV settings > Sound here can make it Low instead. It comes back when you leave the screen if it was on.

If no picture appears