(Araucaria) PLT

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 BBC engineers see PLT knocking out DAB

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Radios go silent as the data flows

By Bill Ray<http://forms.theregister.co.uk/mail_author/?story_url=/2011/04/13/bbc_plt/>
 • Get more from this author<http://search.theregister.co.uk/?author=Bill%20Ray>

Posted in Wireless <http://www.theregister.co.uk/networks/wireless/>, 13th
April 2011 12:25 GMT <http://www.theregister.co.uk/2011/04/13/>

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<http://downloads.bbc.co.uk/rd/pubs/whp/whp-pdf-files/WHP195.pdf>)
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 <http://www.theregister.co.uk/2011/04/05/ofcom_plt_response/>, 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
Fonte : http://www.theregister.co.uk/2011/04/13/bbc_plt/
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