Misapprehensions about Analogue Switch-off
"I'm very sad that I won't be able to watch my films soon."
"How do you mean?"
"All my tapes, I love them. I've got all the old films. But they're switching it off. It's really not fair."
I didn't speak. I thought hard... |
Regarding the misreporting of symptoms
in a recent instance it turned out that the report was 100% accurate, although I was highly sceptical...
In this instance the report was that the TV would only work when a person was sitting in a particular armchair. |
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Interference
from DTT
The arrival of Channel Five caused great congestion on the UHF TV
bands. It suddenly became difficult to find a clear channel for
VCR and satellite receiver outputs. I didn't think things could
get any worse - then
digital terrestrial transmissions started! |
Analogue interference to DTT
I had a seemingly inexplicable problem with the ch68 mux from Belmont. This was in a place where Belmont reception was reasonably good, but the ch68 mux had a very poor BER, despite the signal strength being quite decent. I suspected local interference... |
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Channel
Five: the Aftermath
When
the transmission details for Channel Five were announced in the
early nineties, many of us in this trade were appalled that the
four-channel transmission plan which had served us so well in
the UK was to be so seriously compromised. The four-channel system
was well thought out and carefully engineered, and at the vast
majority of locations it allowed good reception of four TV services
on one small aerial. |
Coaxial
Cable screening
When UHF TV transmissions started we all changed
over to 'low loss'coax, and very good cable it was, with a dense
braid that provided virtually 100% screening. Although good quality
low loss is still available, the vast majority of the cable now
on the market has quite sparse braiding, with a coverage of only
20% or so. |
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Wideband
rubbish
I’ve
noticed these odd looking aerials before, but today I finally
got my hands on one. The thing is obviously supposed to be a
wideband UHF aerial. The design is terrible, though. The 16
directors are about the correct length for Group C/D. In fact
there are just two lengths of director, 140mm and 145mm. The
folded dipole is just that, a folded piece of tube with no balun
or attempt at impedance matching. It is 255mm across, about
right for Gp A. |
Red
box scam
I recently visited a new customer to discuss
the positioning of her proposed satellite dish. Noticing a shiny
red plastic box fitted to the aerial mast, I assumed that it would
be a splitter. Guessing from this that the house must have two
or more TV sets, despite the fact that the customer was a fairly
elderly lady living alone, I asked if Sky reception would be required
in other rooms. The customer replied that she only had one set,
so I had to explain why I’d thought there were others. |
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Quick
and easy TV tuning
Campers intending to take a TV set on holiday
should contact BBC Engineering Information for details of transmitter
locations, channels, and polarity. Thus armed, it becomes much
easier to adjust the aerial for good reception. From the fun and
games that I’ve seen on campsites, though, for many people
this is only half the battle. They also have to tune in the correct
channels on the TV set. If the TV set does not display the ‘real
channel’ number, but simply scan tunes along an uncalibrated
green line on the screen, as many do, it is impossible to know
just what channel you are tuned in to. |
Stacking
Aerials to improve Bit Error Rate?
This item is concerned with large TV distribution
systems and their conversion for DTT. Since c/n inevitably degrades
with every successive stage of amplification, it seems a good
idea to start off with the best c/n ratio possible. This is especially
the case when there is a lot of amplification in the system, for
instance 45dB at the head-end and 40dB in repeaters. I should
mention that I'm thinking of a broadband rather than channelised
system. |
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Channel planning for distribution systems - the n+9 and n+5 conundrums explained
When planning a distribution system you should, if possible, avoid having signals on channels five or nine channels apart. Thanks to modern TV receiver design the problems caused by these channel combinations are far less important than they used to be, but nevertheless it’s worth avoiding these channel spacings if you can, especially if the system will feed any older TV sets. The channel plan for systems that carry both analogue and digital signals should also avoid these channel relationships where possible, treating both analogue and digital signals as potential interferers. |
The
Lesser Spotted Decibel
Unfortunately
we use one word, ‘decibel’ for several units that
have different uses.
The
decibel is a ratio – nothing more, nothing less. In practice
it is usually accepted as a ratio of powers measured logarithmically.
As
it is a ratio it must have a reference, but this is where usage
falls down. Sound is measured in dB relative to 'the threshold
of hearing' which I believe actually has been defined as an
absolute sound pressure level. It has become practice to measure
sound levels in dB relative to that threshold but just to give
the reading in 'dB' without nominating the reference. |
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Terrestrial
TV - Do we need it?
I've been on holiday in the Scottish Highlands.
Like many other areas of the UK, the terrestrial television network
there has countless low-powered transmitters, some serving only
a few hundred houses. Some of these transmitters are at the end
of a very long retransmission chain, and it shows. |
Vandalism
by alarm contractor
Today a Housing Association rung up. "Can
you go to Sunset Villas urgently? There's two things wrong. First,
the alarm company say there's a serious fault with the aerial
system, and secondly some of the residents have really poor reception
since Tuesday." |
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Playing
about with a DAB aerial
Digital
Audio Broadcasting uses roughly the same frequencies as some
of the old 405-line VHF TV transmissions.
For some time I have been on the lookout for an old Band III
TV aerial so that I could experiment with DAB. Yesterday I found
one. It was a 9-element yagi and it had spent the last forty
years in a loft, so it was in good condition. I connected it
to a length of coax, and believe me it seemed funny working
on a Belling |
Slapdappery
Paul
came in the other day, aghast. He had been approached on the
street by another aerial installer, an older man, who wanted
to buy a few bits and pieces having been 'caught short'. Since
Paul was just in the process of lifting his analyzer out of
the van the conversation had turned to test equipment. The experienced
rigger advised Paul that the analyzer was 'only a telly really,
ain't it?' and that even an ordinary meter was unnecessary.
Paul learnt from this meeting that the best way to do the job
is to point the aerial in approximately the same direction as
all the others, and then lie through your teeth about the poor
reception |
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Strange
cable fault
It was a very large private
house set in an ancient quarry. The owner was on holiday and
he had left instructions that the four large TV aerials were
to be removed. Accordingly a steel mast 7m high had been fixed
to the cliff face about 80m from the house, and out of sight.
The estate workers had installed a 54mm plastic duct all the
way from the mast to the house, via a river bridge. I was asked
what type of cable to pull in and I suggested that they put
at least six lengths of CT125DB (direct burial) in the duct.
One for UHF, |
Ten
taped ‘T’ junctions and a wailing woman
Suppose you had to connect ten TV sets to an aerial. The tellys
are in a ten-storey building, one on each floor directly above
each other, so they are about 3 metres apart. You could get
a 30 metre length of coax and run it down the building, then
cut into it near each telly and splice in a short length of
coax to run to that TV set. By ‘splice in’ I mean
just that – make a ‘T’ junction by twisting
the cables together.
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The
other side
Due,
I suppose, to the way television has developed, the word 'channel'
has ended up with two entirely different meanings. Actually,
they aren't entirely different — it would be better if
they were — they are related in a way that causes endless
confusion. By now you're ahead of me of course. We have ‘channel’
as in ‘8MHz wide slot somewhere between 470 and 860MHz’
and ‘channel’ as in ‘Channel Four’.
(Let’s not mention ‘S’ channels or satellite
channels or VHF FM channels or DAB channels!). Understandably
the public thinks that ‘channel 1’ is the same thing
as the button on the remote that has a ‘1’ on it,
and is synonymous with BBC-1. By an extension of this logic
channel 47 must be what |
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