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

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.

Reply to
usenet
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The easy way to check is by slowly removing/connecting one of the SCART cables at an angle - you can usually make one of the RGB feeds break up first.

It's often difficult to tell - there isn't always a vast difference depending on programme material.

But if the VCR is looping through RGB, I'd expect it to also have to convert RGB internally as well for recording purposes. Now some Philips ones do or did, but it's not common.

An RGB SCART also carries composite in and out. It's a devilish system to get your head round at times.

I built an RGB DA to feed the ONDodgy box to the set in the kitchen as well as the main set. The STB obviously is powered all the time, but the DA is powered off my Audio/TV supply that all switches from the AV amp on/off. If I power down the DA, and leave everything else on, the kitchen set reverts to composite off the SCART cable since this is looped straight through - it's only supplying the syncs so I didn't bother with 'DAing' it. And often you can't tell the difference. Seemed like a good idea at the time...

Reply to
Dave Plowman

How would that work for wrapping? Surely the next customer (if it's rejected) won't be happy about getting a box that's clearly been opened?

I always assumed that was _why_ they had to follow distance-selling rules. Non-catalogue shops have the article on display, or at least will have a store model. Argos don't have a lot of floor space, which only really gets used for promotions.

F.

Reply to
Fraser

Tiny niggle, but I thought 28 days was the magic number?

F.

Reply to
Fraser

There is no halfway stage. If you get lockup and blocking at all, then it's not working!! ;-)

The data has degenerated to the point where it is useless, and the TV loses it.

F.

Reply to
Fraser

Argos don't have to show you the goods.

Reply to
IMM

In article , Fraser writes

Humm...a lot of people in the broadcast industry would disagree with that statement. Still, its an opinion and its mine:--)))

Reply to
tony sayer

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.

Reply to
Niall

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.

Reply to
Bob Mannix

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 :-)

Reply to
The Natural Philosopher

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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Reply to
Andy Hall

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

.andy

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Reply to
Andy Hall

In article , Niall writes

Well is this how you look at the rest of your DIY projects?...

How much do you spend on your TV?, isn't it worthwhile spending a bit more to ensure it works properly?...

Reply to
tony sayer

Thats why I mentioned unwanted signals. Those are intereference.

Reply to
The Natural Philosopher

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.

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.

I had problems at first finding the former ITC DTT TX pages on the new Ofcom site. The new URL is .

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.

Reply to
Andy Wade

Andy, you need to read up on the basics of COFDM modulation (as used for DVB-T and DAB). Its whole raison d'être 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.

Reply to
Andy Wade

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

Reply to
stefek.zaba

I am familiar with the basics of COFDM as well as the rival U.S. system and have read the papers by Stott and others.

While COFDM should give good multipath performance, the code rate and the guard intervals mean a compromise between the data rate and signal robustness. I understand that there are various issues around the UK system because of the limitations imposed by the current transmission arrangements, but I haven't looked at that in depth.

In practice I have seen cases where the bit error rate is noticably worse when cabling is poor or the antenna is not aligned correctly and there is visual evidence of multipath reception on analogue.

I suspect, therefore, that as Stott admits in at least one of his papers, while one can model some aspects of COFDM mathematically, some issues can only be determined experimentally. It would appear that the practicality may not be quite what the theory predicts.

DAB suffers in practice from limitations arising from the broadcasters trying to squeeze more into the spectrum than will give the best results. I haven't looked at the detail, but it would appear that DVB-T has some issues as well.

.andy

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Reply to
Andy Hall

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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Reply to
Andy Hall

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?
Reply to
usenet

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