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BBC engineers see PLT knocking out DAB

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Recent experiments carried out by the BBC demonstrate how power-line networking can interfere with FM radio and knock out DAB entirely, but only for those who get a decent data rate.

The new study was commissioned by the BBC and authored by one current and one former BBC engineer. The study examines transmissions coming off PLT kit, but while they were measuring the signal strengths and monitoring the frequencies, the engineers turned on a portable radio to discover if it still worked. They found that it did not. That is a critical issue as the only technical requirement for PLT kit states that it must not prevent other devices "operat[ing] as intended". PLT kit is required to conform to the Electromagnetic Compatibility Regulations 2006, which state that such interference can't be allowed.

Power-line telecommunication (PLT) involves sending radio signals over mains electrical wiring. Generally the kit consists of two or more oversized plugs with an Ethernet socket in the back. PLT is incredibly easy to use and penetrates walls in a way that Wi-Fi can't, but those radio signals also leak out of the wiring to fill the house, and neighbourhood, with unwanted interference.

The first PLT systems used frequencies between 2 and 30MHz (confusingly known as High Frequency, HF, despite being way down the dial by today's standards), and thus only interfered with the kit of radio hams and the like. But the need for speed has pushed some devices into the 50-305MHz band (Very High Frequency, VHF) where FM and DAB like to play, which is when the BBC got interested.

In their tests (33-page PDF/1.9 MB, easier to read than it looks) the two engineers started in a screened room, but then tried the same thing in two typical houses to see if using a pair of PLT devices would interfere with FM and/or DAB reception, and discovered that it did.

The study states: "A distinctive popping or ticking could be heard when the PLT was idling. Once it was busy, there was a continuous 'tearing' sound which was at best annoying and at worst made comprehension impossible." This clearly shows that when PLT was in use portable radio equipment "cannot operate as intended".

The BBC engineers did find that when the PLT equipment had trouble making a connection – if it were, for example, on a separate ring main or used in the presence of a compact fluorescent lamp – it would fall back to the HF band and thus only bother the hams and their ilk. But the engineers noted that when the kit was running at top speed it was able to knock out the DAB reception entirely at one of the houses tested.

DAB is particularly vulnerable to interference as it either works or it doesn't, with just a small change in signal strength flipping it between the two. FM radio can scale back from stereo to mono when necessary, and can cope with quite a bit of interference before becoming unintelligible – though listeners may decide to tune out before that happens.

Ofcom still maintains that all the complaints about PLT come from one lobby group, and the problem is only preventing "one man" from pursuing his "hobby". But if your FM radio dropped back to mono, or acquired a background hiss, would you really think to complain to Ofcom? The engineers discovered both things happening when PLT was in operation, even when an external aerial was being used.

We asked Ofcom if the BBC's research constituted proof that normal operation was being prevented, but the regulator needs a great deal of confidence before it can take on a criminal case against the manufacturers (as it would be required to do). So, perhaps wisely, Ofcom is still digesting the research and will let us know when it has done so. ®

Bootnote

Thanks to Brian Morrison for pointing us at the research, which was posted by the BBC late last month

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