Overthrow the state with a ZX Spectrum

Just a quicky to round off the weekend: I thoroughly enjoyed this post -- via the ever-entertaining MetaFilter group blog.

It describes, in the words of participant Eugeniusz Pazderski, how a group of radio astronomers working in Poland during the early days of the Solidarity resistance movement managed to subvert the state TV station by putting their own message on top of the official programming. They did this with virtually no resources - a modified portable black and white TV set, a ZX Spectrum, and a homebrew radio transmitter that put out not much more power than a walkie-talkie.

In the UK at the time, such an assembly could be put together by a reasonably resourceful sort for under a hundred pounds - yet even that took the rebel astronomers virtually all their personal resources, enormous amounts of time, and ended up with some very unpleasant experiences in the hands of the security service. Mostly, though, it took lateral thinking of a very high order.

Normally, if you want to take over a TV frequency you need to beat the target transmitter at its own game - lots of power to overwhelm it, or by breaking into one of the secure feeds using sophisticated equipment. If you want to cause interference, though, it's a lot easier, especially if you pick an area with a weak signal in the first place. You can just generate some electrical noise, and the viewer will see a bit of snow on the picture, some patterning or other disturbances - but the picture itself remains largely unchanged.

Analogue TV works by the transmitter sending out a pulse every 25th of a second that says "Picture starting", which makes the TV receiver move the electron beam that paints out the picture on the tube to the top left of its screen. After that pulse, the transmitter sends out a series of much faster pulses that say "Line starting!", which makes the TV move the beam across the screen. During that time, the transmitter varies the signal to make the beam lighter or darker; at the end of the line, the receiver waits for the next line pulse before moving the beam back to the left hand side of the screen, slightly below where it was before, ready to paint out the next line.

What the very bright astronomers realised, because they'd been using a similar technique to generate the precise timings for their experiments, is that if you generate that interference at just the right time during each line of a TV picture, it can spell out words, a bit like a dot matrix printer. You can get that precise timing information from a TV set tuned to the programme you're after, as the set needs to recover the timings from the signal to synchronise itself to the transmitter. I could make the necessary modifications to a telly in an hour or so.

The next bit is to feed the timing info to the Spectrum, which has a small machine code program that waits for the start of the picture, then counts the number of lines until it gets to the right bit of the picture. It then sends out a series of pulses to the transmitter - which can be very simple, as all it needs to do is radiate a pulse at the right frequency - that mark out the first row of pixels in the message you're sending. Then wait until the next line starts, send out the next series of pulses, and so on.

Eight lines of this, and you've got a row of text that can say whatever you like - displayed on all the TVs that are within the range of your pulses.

Again, once you've got the idea, the mechanics of the software are simple and well within the reach of anyone who's competent with an assembler. The transmitter is no more complex than the sort of thing hams were using to send Morse in the 1920s.

This is the same principle as a bit of technology called a genlock, which TV companies and video editors use to place their graphics onscreen and mix pictures together. Today it's a commonplace function; at the time in the West it was a complicated and expensive piece of equipment.

By moving the genlock to the very end of the chain, to within a mile or so of the receivers, the astronomers bypassed all the security and expense. Nobody had done it that way before, but by seeing the system as a whole and tapping in at just the right place, they bypassed the protection and used the transmission against itself.

Not so simple today. Interference to digital TV signals will just result in the receiver dropping an entire block of information or silently fixing the holes in the signal by making a best guess as to what's there.

But it's a great story, and I wish I'd been that clever and naughty in the days of Spectrums and analogue telly. All I could do was wipe out the village's reception with a badly-tuned World War II vintage tank transmitter I'd borrowed from the cadets...

Eugeniusz Pazderski, you are well named.