SSTV on 20 metres, 14.230 MHz: pictures sent as sound

What you hear

A warbling whistle that goes on for two or three minutes, like a slowed-down modem. It opens with a short header, a calibration tone and a few pulses, then settles into a rhythm: a sharp click at 1200 Hz at the start of every line, then the line itself as a tone that wanders between 1500 and 2300 Hz, dark pixels low, bright pixels high. A picture of a blue sky sounds different from a picture of a face, and after a while you can hear the horizon go by. Between pictures the frequency carries voice: the operators announce themselves, exchange reports and say which picture is coming next.

Decoded, the pictures are what amateurs have always sent each other: the station and its antennas, landscapes, pets, holiday snaps, cards with the callsign in large letters, a CQ slide, a 73 slide with the signal report written in. The common modes are Martin 1 and Scottie 1 at just under two minutes a picture, PD120 at a little over two, and Robot 36 when someone wants a quick low-resolution one. The report is in three figures for readability, strength and video quality.

Who transmits and why

Radio amateurs. Slow-scan television goes back to the late 1950s, when pictures were eight seconds long, black and white and viewed on a long-persistence radar tube; the colour modes used today date from the 1980s, and since the 1990s a computer with a sound card does the work. 14.230 MHz is the agreed SSTV frequency of 20 metres across the world, with 14.233 used when it is busy. People send pictures for the same reason they ever did: it is a contact you can see, and a picture that has crossed an ocean on a few tens of watts is worth keeping.

When and where to listen

Daytime on 20 metres, and the frequency is rarely idle: around midday in Europe, in the afternoon from across the Atlantic, more at weekends and during special events. If nothing is happening, wait a few minutes; a CQ in voice usually precedes the first picture. Any receiver with 20 metres works, and the closer the receiver is to the sender the cleaner the picture: fading puts streaks across the lines, noise puts speckle on them, and a weak signal gives a picture that looks like a wet watercolour. The signal is upper sideband, and the tuning has to be right within a few tens of hertz or the picture comes out with its colours shifted.

In ShackDeck

Station catalogue → Fax and digital signals → SSTV 20 m (14.230). The program picks a public receiver with 20 metres, tunes it to 14.230 MHz in USB and sets the SSTV mode; no receiver, antenna or account of your own is needed. The decoder waits for the header, reads the mode from it, Martin, Scottie, Robot or PD, and draws the picture on screen line by line as the sound arrives, resynchronising on the line pulses so that a dropout does not shear the rest of the image. If you arrive in the middle of a transmission you get the lower part only; the next picture starts clean. Finished pictures go into a gallery below the decoder and can be saved as PNG files.