Digital set-top boxes (slightly O/T) - weak signal area.

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FSVO "never" (such as "never, ever, ever") perhaps.

A couple of points here: firstly, many contributors to this thread have said that they can get DTT reception, despite the postcode database (which does err slightly on the cautious side) predicting otherwise. Be aware that signal strength alone is not the only issue. Every UHF channel is re-used hundreds of times over and coverage is mostly limited by co-channel and adjacent channel interference from other TV transmitters, rather than by thermal noise. Now, as many will be aware, UHF propagation is seriously affected by certain weather conditions which give rise to 'tropospheric ducting', increasing the signal levels from remote transmitters by large amounts and causing severe interference[1]. The statistics of this effect are well-studied and DTT coverage is planned to work for 99% of the time (c.f. 95% of time for analogue TV). Some of those receiving in areas which are not officially classed as 'covered' can expect to experience tropospheric interference for more than 1% of the time.
Secondly, in the absence of any test equipment, a simple test of signal margin is to insert a 3 dB attenuator pad at the receiver input. If this leads to reception failure then your signal is somewhat marginal.

Oh yes there are: look at Belmont, Sandy Heath, Wenvoe and Waltham, to pick four examples of main transmitters which have some of the mux's well outside the original analogue group.

And has concentrated on population served: thus Crystal Palace, Sutton Coldfield and Winter Hill get more-or-less in-group DTT channel allocations, but for the rest of the country it's much more patchy.

I think you're just introducing a red herring in suggsting that DTT reception will be correlated with analogue C5 (which is not even radiated from many TX sites). In any case very few people upgraded their aerials for C5, so the existing one will most likely match the original 4-channel plan group.

No, it's not a con. What Andy L was talking about there is 'impulsive interference' - interference from sparking contacts - to which DTT is quite sensitive. Impulses propagate on, and are radiated from, mains wiring, and thus are much stronger in the house and loft than above the roof (which provides some screening). A well-screened coax downlead from antenna to receiver will largely prevent impulses entering the signal path. With poor screening, a coupling mechanism exits to allow impulse energy into the receiver, resulting in the familiar momentary 'freezes' and clicks.
Using well screened coax is necessary, but not sufficient. Any gaps in the screening will allow the interference to get in, and poorly screened outlet plates and flyleads are the other betes noir. At the other end of the coax, the lack of a balun on the aerial will also provide a coupling mechnanism. This is all sound theory, and has been verified experimentally in tests done under controlled conditions. Oh, and an aerial in the loft is likely to be close to mains wiring, so is asking for trouble.
With a really well-screened system and a decent receiver you can get solid reception with signal levels down to 35 dBuV, even on the 64-QAM muxes (ITV/C4 and SDN). With grotty brown open-weave coax and a typical outlet and flylead, you may need 45 or even 50 dBuV before the signal is strong enough to swamp the impulse interference.
[1] http://www.bbc.co.uk/reception/factsheets/docs/reception_weather.pdf
Also relevant: http://www.dtg.org.uk/publications/books/r_book2.pdf http://www.dtg.org.uk/publications/books/cai_dtg_bmrk_aerials.pdf
--
Andy



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The argument applies well for a DIYer even if the picture *does* break up six months hence. It's no big problem to *then* decide to go up in the loft (wherever) and improve the aerial, downlead or whatever. As I said I agree that for a commercial installer this isn't a sensible approach.

Yes, I'm among them.

Yes, so how does that affect my suggestion/comment that 'seeing is believing'? All I was saying was that for someone who is installing FreeView (and maybe an aerial) themselves observation of the picture is a good enough indicator of whether it's working.
If it becomes marginal occasionally due to atmospheric/tropospheric conditions then one either accepts that it is 'as designed' or one tries some improvements.

A good idea. However the on screen diagnostics available with our Daewoo box are quite good anyway.

I did say 'not so many', I didn't say none! :-)

Which sort of confirms what I said, for most people a wide band (or more than one) aerial probably isnn't needed. As it happens, where we are, served by Sudbury you need a wide[ish] band aerial to get Channel 5 anyway. I think it's Channel 5, I know one of the five analogue frequencies is a fair way from the others. It's not fully 'wide band' either but the frequencies spread over more than one letter band.

... but as we said above for many people this will be what they need. We're talking (or I'm talking) about reasonably competant DIYers here, I'm expecting people to do a little research and check to see what type of aerial they have and will need. The information is all available on the 'net and is pretty easy to find and understand.

I've never seen a balanced feeder aerial input on any domestic TV (or set top box) here in the UK, are you saying that the internal bodge (which is probably what it is) can pick up impulse interference? This is true enough but it's not the sort of thing that most people are willing to do anything about as it would involve digging around inside the FreeView box. Or are you saying something else and I've misundestood?

Thanks for the details re. impulse interference, it suggests that it might be worth improving my downleads though that isn't an easy or cheap option as they're "in the walls" as it were. It might actually be cheaper (for me) to improve the distrubution amplifier in the loft and make very sure that the lead from the aerial to the amplifier is as near perfect as possible.
--
Chris Green

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wrote:>> signal strength alone is not the only issue. Every UHF channel is

A few day's observation under normal conditions isn't necessarily a good indicator of whether it will continue to work 'as planned' all year round...

... OK, but how do you know that you're not suffering more loss of service than necessary? Most people won't have a clue what's happening when the screen goes blank; at least with analogue you see what's happening and most can recognise "interference" in a general sort of way.

Yes, nice features in the SetPal design.
<snip>

That may be true, numerically, but could be construed as misleading to those who do need a wide(er) bandwidth array. IOW you must look at the data for _your_ local TX; what a majority do or don't need, nation-wide, is irrelevant. But having said that, note that the government (in the form of the DTI Digital Action Plan) trying to encourage the widespread adoption of wideband (Group W) aerials in order that people don't have to replace them /again/ when the time for 'digital switchover' (a euphemism for analogue switch-off) comes. Switchover will bring a big re-shuffle of channel allocations as parts of the band are re-assigned. (The most likely scenario, at the moment is that channels 31-40 and 63-68 will be 'lost' - but this is definitely subject to change.)

C1-C4 analogue from Sudbury are on ch. 41-51; C5 is on ch. 35 at -7dB. The existing Group B aerial should fare reasonably well on ch. 35, but the lower ERP will mean noisy pictures for many. The DTT muxes span ch. 39-68 and I think you'd struggle to get the ITV/C4 mux on ch. 68 with a Group B aerial unless you're in a very strong signal area. Remember that the performance of Yagi-type aerials falls off much more rapidly on the HF side of the design bandwidth than on the LF side.
<snip>

I had problems at first finding the former ITC DTT TX pages on the new Ofcom site. The new URL is <http://www.ofcom.org.uk/static/reception_advice/digital_trans_guide/index.a sp-region=show_all.html>.
<snip>

Uh? Where did I mention needing a balanced i/p on the receiver? Some pre-war Band-I 405-line TVs had balanced-twin inputs, OOI, but coax has been univseral since the late 40's (except for VHF TV in the US).

You've misunderstood. The point was that, ideally, you should have a good balun on the antenna - otherwise RF interference picked up on the outside of the coax feeder will find its way into the signal path, even if there are no 'leaks' downstream.

Yes, that's a good strategy.
--
Andy



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No, and *if* problems appear after six months then one can make some small improvements. As I keep saying it's a non-event to have to do this if it's a DIY installation and I have also said that a commercial installer *can't* do it.

If the screen 'goes blank' then (from my point of view anyway) that's worse than acceptable. In reality 'interference' on a digital signal tends to produce artefacts which are recognised by the viewer as deterioration in the signal.

You said "... end of the coax, the lack of a balun on the aerial ...", are you then saying that there should be a balun at the aerial end of the coax? I thought that most aerials have a natural impedance of around 75 ohms which is why 75 ohm co-ax is used. Are you just saying that there should be a 75 ohm to 75 ohm (as opposed to imedance matching) balun at the aerial because the aerial is balanced and the coax isn't? Will this actually provide much improvement? If so it's a fairly simple and cheap thing to do.

Ah, OK, the above *is* what you mean. Can one get ready made 75ohm to 75ohm baluns?
--
Chris Green

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That's true of impulsive interference or other short-term effects, but tropospheric co-channel interference (which was the context) will tend to last for minutes or hours and if its strong enough to push you over the 'digital cliff' then you will get a blank screen (or frozen frame, or BSofD, depending on the decoder).

A half-wave dipole has a 'natural impedance' of 75 ohms and that's what gave rise to 75 ohm coax originally. 75 ohms is also the impedance at which an air-spaced cable of a given o/d has minimum loss (c.f. 50 ohms for a solid polyethylene dielectric cable).
A TV Yagi doesn't have a natural impedance of anything in particular since adding reflector and directors tends to lower the feedpoint impedance; in particular the spacing between the driven element and the first director can have a dramatic effect. The design of TV antennas is still a largely empirical black art (despite some impact from CEM techniques) and involves a juggling act between impedance, gain and bandwidth. Just optimising for maximum gain would give narrow bandwidth and an impractically low feedpoint impedance.

Almost -- I'm saying that there should be a balun at the aerial feedpoint because the aerial is balanced and the coax isn't. The balun's impedance ratio could be 1:1, or 4:1 or something else and that's a matter for the designer that doesn't particulary concern the user.

Yes, it provides a demonstrable improvement in immunity to impulsive interference, especially if the downlead goes through the loft near to mains wiring. To pass the CAI & DTG's 'benchmark' tests an antenna has to pass a 'feeder pickup' test, whcich amounts to a test of the effectiveness of the balun. See http://www.dtg.org.uk/publications/books/cai_dtg_bmrk_aerials.pdf for further details.
--
Andy



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snipped-for-privacy@isbd.co.uk writes

Not necessarily so that. You can have good analogue reception if say your local TX is in A group 21-34 but a digital MUX planted up near the top end of the band say channel 67 like hereabouts off sandy heath then this is stretching things a tad too much!.
An A group aerial is way, way, down on level at that separation and co- ax cables fall in efficiency as the frequency increases, and if they are of the very open loose weave braided types.
Anyway, why is it that people can spend a small fortune on TV's DVD players Home cinema and the like and yet be so penny pinching when it comes to a few tens of pounds for an aerial that will give an adequate signal that will cope with fades and give excellent service over many years, and thousands of hours of good quality service?.
Having worked in the TV industry at both ends for some years it still amazes me as to what crappy and poor reception people will put up with all for the want of a few quid's worth of metal!....
--
Tony Sayer


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I'm in a strong signal area - close to Crystal Palace in SW London where there is probably more RF radiated than anywhere else in the country, and what you've also got to consider is spurious pickup of unwanted signals via the co-ax - normal UHF stuff is poorly screened.
Also, co-ax can deteriorate with age - I've seen it full of water. ;-)
I use radio mics a lot in my job, and the UHF versions of these are extremely sensitive to the type of cable used as they work at tiny powers. And the cable we use for more than very short runs is a great deal larger and more expensive than anything used domestically.
--
*A conscience is what hurts when all your other parts feel so good *

Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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On Thu, 8 Jan 2004 08:53:51 +0000, "Andy Luckman (AJL Electronics)"

I believe that my Freeview installation works *perfectly*. I believe this because I can see it with my own eyes.
You are telling people that they need to replace their aerial and coax before it will work. This is not neccesarily true. It is not true in my case.
I am saying why not try it first? If results are disappointing, yes, you need CT100, gold plated connectors and an enormous outside aerial. But you may not.
I also understand why you will always quote for this when asked for a freeview installation. This is not a criticism. But it's not applicable for advice to this group.
--
Niall

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wrote:

My Freeview set up also works perfectly. I didn't expect it to, as the aerial (roof) cable is rather manky with some holes in it (!). I was quite happy that it might not work but knew I had a strong signal, so I tried it. It has never failed or dropped signal and the quality is always excellent. We also have a TV up in the loft conversion, which I use with a cheap multi element aerial in the loft. The signal here is not very good. Here the Freeview box struggles and the signal drops out occasionally.
I would agree with Niall that signal strength is the most important factor and that, although one should not expect a Freeview box to work with the existing aerial/cable set up, if a strong signal exists, it's worth a try.
--
Bob Mannix
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Bob Mannix wrote:

Signal strength relative to interference is the ONLY important factor actually.
All the aerial does is collect and selectively amplify the wanted signals
whilst not selectively amplifying unwanted signals.

If you have enough of the wnated signals, you don't need an aerial at all :-)
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wrote:

That's true if you count reflected signals as interference.....

.andy
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Andy Hall wrote:

Thats why I mentioned unwanted signals. Those are intereference.

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On Mon, 12 Jan 2004 09:24:53 -0000, "Bob Mannix"

With digital TV, the error rate is the most important thing. If the signal is strong, so can locally reflected signals and these, along with interference pickup, all result in data errors seen at the receiver. Discontinuities in cables and poor connectors can lead to signal reflections in the cable and that will also lead to errors.
Signal strength is also important so that the receiver can differentiate the data correctly, but once the required threshold is exceeded, unlike analogue this does not necessarily result in good reception.
To a point, the transmission system and the receiver can handle and correct the errors, but they can lead to unpredictable results.
This is not to say that trying an existing arrangement is a bad thing for the DIYer, but clearly for a professional it would be stupid to use anything other than good quality cables and antennas. For the DIYer installing a new reception system it also makes sense to use a good quality antenna (not a contract grade one) and good CT100 cable. Like other things, you can either do a job properly or you can bodge it. Typically if you bodge it, you will have to do it again.
.andy
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"Andy Hall" wrote in message

Andy, you need to read up on the basics of COFDM modulation (as used for DVB-T and DAB). Its whole raison d'tre is to provide a multipath reflection tolerant system. It works by dividing up a fast data stream into a large number of much slower ones, so that the likely range of echo delays becomes less than the symbol period. A guard interval is inserted between symbols, to allow the reflections time to catch up, so to speak. Short-delayed echoes, up to the guard interval for the system variant in use, don't lead to errors. In fact they contribute usefully to the received signal power. The guard interval for the UK DTT implementation is 7 microseconds. Longer-delayed echoes will cause intersymbol interference, and degrade the BER, as you'd expect, but tend to be less common, and weaker.
--
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Cool - Better Living Through Mathematics ;-) At those sorts of times (7us), presumably the intention is to provide robustness to "near" reflections, e.g. off big buildings close by - since, as we're taught by the Damestress Grace Hopper herself, a foot is a nanosecond, a microsec is 1000 feet and 7 of them makes 7000 feet which is NADI a mile-and-half, or a couple of them newfangled keel-o-meters. So it's good for multipath suppression for different signal paths from the same transmitter; not designed at all, presumably, for interference between competing transmitters (whose distance difference at nearly all receiving locations would be well over this mile-and-half); and unless you're bizarrely unlucky, the difference in received power would also be considerable; and you presumably monkey with the chosen transmitter freqs (both now and when we do the Great Switch Off) so that you avoid transmitting on the same sort of freq at similar received power anywhere in the intended reception area.
Is that about right, or should I really be asleep by now? ;-)
G'night all - Stefek
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On 13 Jan 2004 01:12:58 GMT, snipped-for-privacy@hp.com wrote:

Only if you use one of those reverse wotsit calculators. Never could get the hang of them.

There's a whole discussion on single frequency networks (or not) and the implications thereof.
What may be nice to do in theory, may not work out in practice so easily because of the installed base of reception equipment etc., especially when coverage targets (a sensitive issue) are considered.

.andy
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I like that. Lots of "throbbing DSPs" in a DTT receiver, to use a phrase coined by an acquaintance of mine, who posts here occasionally.
IFFT for the COFDM demod, soft-decision Viterbi and Reed-solomon FEC decoding & then MPEG decoding. Try doing all that with valves (he said, being half-way through reading Gerogina Ferry's book about LEO).

Yes.
Interesting - I think I first heard "one foot per ns" from somebody at Mullard; I didn't know there was connection with Ms Hopper. Didn't she invent the concept of the compiler?

That's how its currently used for UK DTT. Another application is the 'single frequency network' concept used for the national DAB multiplexes. This depends on having precisely synchronised transmitters (done using GPS for the timing, I think) all radiating the same stream, and, obviously, a longer guard interval -- 246us for Mode 1 DAB (the mode used on Band III). From the receiver's point of view there's no difference between a reflected signal and one from another synchronous TX. Clever innit?
--
Andy



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I don't think she's the one we thank for compilers: early FORTRAN beat her to that punch. But we do have her to thank for COBOL (I presume you know the one about its Stroustrup-analogue object-oriented follow-on, ADD ONE TO COBOL. ?) And the story goes she used to enliven her public talks by handing out 'nanoseconds' - one-foot lengths of copper wire.

You've answered the follow-up question I stopped myself from asking! Couldn't immediately see how the DAB network would cope given single-frequency transmission with the maths working out as for Freeview, but wasn't curious enough to either think through the obvious answer of "much longer guard interval", or to ask. And here it comes, unasked for but no less gratefully received ;-)
Cheers, Stefek
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snipped-for-privacy@hp.com wrote:

You sure about that?
I assumed COBOL predated Fortan.

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wrote:

No. Fortran (FORmula TRAnslation) was the first high level language, well 3 GL. COBOL was in 1959. It was a temporary measure that they expected to last no more than 5 years.
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