No British prosecution has ever been secured on the basis of a TV detector van alone. Not in 1952, when the GPO first sent the vans out on behalf of the BBC. Not in the 1970s, when the public information films were at their most lurid. Not since. The National Audit Office's 2015 review of licence fee collection describes the vans as the BBC's final "detection and enforcement" option, without ever quantifying a contribution to revenue beyond the threat they imply. Courts have not admitted detection-van evidence as sufficient for conviction, and the BBC has not tried to make them.

The physics was real, though. An analogue television set's tuner needed a local oscillator running a few megahertz above whatever channel you were watching. The oscillator leaked a faint RF signature in the 45 to 75 megahertz range, and a sensitive receiver in a van outside could pick it up and, in theory, work out which channel a given house was tuned to. The vans had directional antennas on the roof and the operators had headsets, and the whole apparatus was sufficiently plausible that the public information film could show one rolling slowly down a suburban street while a stern narrator read out your postcode.

What the films left out is that knowing somebody is watching is not the same as proving it in a magistrate's court. You also have to convince the magistrate that the leakage signature is yours, not your neighbour's. You have to convince them that the operator's headset reading is a valid form of evidence. You have to convince them that the van was where the operator said it was. The BBC's legal team understood this from the start, which is why they preferred the cheaper route: a letter through the door saying we know.

The letter is the part that survives. The vans are mostly gone. The serious residential sweeps appear to have wound down around the digital switchover, by which point a digital tuner did not leak the same RF signature, and a tablet or laptop did not have a tuner at all. But the threat-letter design has been preserved with care: red and black, the same severe typography, the same paragraph structure that opens with your address and closes with the words officer will visit. The letters cost very little. The deterrent effect is the entire product.

There is a strange honesty in admitting that. Most enforcement regimes pretend the equipment works. The TV licence regime, read through the National Audit Office report, more or less concedes that the vans are theatre. A 2020 LessWrong post that pulled actual warrant applications found one where the "detection equipment", on inspection, was just a camera pointed through the front window. The author noted dryly that the warrant itself called it a camera further down.

What I find hauntological about this is the time-shift. The vans were calibrated to a country where everyone watched terrestrial television at predictable hours on a set built around a 45 MHz local oscillator. That country has been gone for fifteen years. The infrastructure that enforced it is mostly gone too. The letters keep arriving on the same schedule because the deterrent was never the technology. It was the willingness to keep sending the letter. The country changed; the envelope didn't.

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