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All posts by Mike Dimmick

Below are all of Mike Dimmick's postings, with the most recent are at the bottom of the page.


Tony: It depends whether the device is reporting error counts (or 100% less error count) before or after the two stages of error correction. Most Freeview boxes, and PC cards, report quality statistics after both stages of error correction and are therefore completely useless for setting up for best reception.

The HD-TM does show pre-Viterbi error rates and an indication of carrier-to-noise ratio. You should adjust your system to minimize the pre-Viterbi BER. Before switchover it was generally recommended that you should keep digital signals in the 45-60 dBuV range, after switchover you can probably add a bit more headroom (this range was recommended to keep the *analogue* signals, typically 10-20 dB louder than the digital ones, in the 60-80 dBuV recommended range.)

TVs, boxes, tuner cards and dongles usually don't display pre-Viterbi statistics as the decoded signal has to be re-encoded to compute them, which requires extra logic.

Raw bit error counts for HD signals will be greater than for SD signals, because the HD mux carries approximately 66% more data. Still, they should be about the same order of magnitude. They're more normally quoted as rates, number of bits in error compared to the total number of bits, which should be about the same.

Be aware that BER is quoted in exponential notation and it's a very small value. A larger exponent (the value after the E) is *better*, not worse. 1E-4 is 1 in 10,000, 1E-6 is 1 in 1,000,000. "Quasi-Error-Free" is given in the DVB-T spec as a figure of 2E-4 *after* Viterbi decoding (i.e. post-Viterbi).

I'm not sure which predictor you're using. Digital UK is the only one I know of that uses correct data (mainly because the broadcasters are the only ones that have all the necessary data) and correct algorithms. Digital UK's figures are percentage of locations within your grid square that are expected to have sufficient margin for 99% of the time (50% for the second column), they are not field strengths or terminated voltages. Wolfbane is widely considered to give results that are too low; even so, the predicted values given are electric field strength in dBuV/m, not terminated voltages in dBuV. The conversion is related to the aerial's dimensions and gain.

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John Gleeson: I usually use the http://lyngsat.com/28east.html or Astra 1N / Astra 2A / Astra 2B / Astra 2D / Eutelsat 28A (28.2°E) - All transmissions - frequencies - KingOfSat websites. However, both of those are updated by user contributions only.

The satellite broadcasters concentrate on getting the information to the EPG providers (Sky and Freesat, in the UK) and generally don't publish manual tuning information anywhere. The BBC are rare in reporting what they use: see BBC - Help receiving TV and radio - Satellite frequencies of BBC channels . They usually also write a post on the BBC Internet blog saying what they're doing, for example at BBC - BBC Internet Blog: Changes to BBC services on satellite and cable this autumn .

The 24 Olympic streams are being provided by Sky on the BBC's behalf, though they will be free-to-air and available through Freesat. I don't know whether the BBC will make that information available.

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Logomage: If you've lost the channels on the SDN multiplex (e.g. ITV3) or on ArqA (e.g. Pick TV, Dave), it's probably down to the change of FEC mode. This change has increased the capacity but made it more difficult to receive these multiplexes.

The Forward Error Correction mode controls how much redundant data is transmitted to allow the receiver to detect and automatically correct errors. With less FEC data, fewer errors can be corrected.

You will always get better results with the aerial outside. Consider that as an option. If you really can't do that, see http://www.wrightsaerials….pdf .

Also, note that if you are using Bromsgrove (the aerial should be oriented with the elements running up-and-down, rather than side-to-side, and it should be pointing roughly south-west), the aerial group changed at switchover. The PSB multiplexes are still within Group A, which was all you needed for analogue and for digital before switchover, but the commercial multiplexes SDN, ArqA and ArqB are now in Group B. You should consider getting a Group K or wideband aerial if the old aerial was Group A.

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Logomage: Bromsgrove SDN and ArqA switched FEC mode on 1 May. Sutton Coldfield changed on 24 April. So far the new capacity isn't being used, but now that nearly 90% of the country has switched over, services might launch soon, though after the Olympics is now more likely.

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C22 (482.0MHz) after switchover
Tuesday 22 May 2012 2:05PM

Nichola: You're probably getting the HD signal from the same transmitter as the SD version of ITV1. However, ITV don't yet have HD versions of all the SD regions; for places where they don't have an equivalent, they transmit a different version from the same region, or from a nearby region.

At the moment, there is no Southampton version of ITV1 HD, so ITV transmit the Meridian South-East version (produced in Maidstone) instead.

The same is true for ITV1+1 (33). I'm not actually sure whether the +1 version is Southampton or Maidstone, some sources have one and some the other.

It could be worse: until recently, viewers in the Anglia region *also* got Meridian South-East.

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C22 (482.0MHz) after switchover
Tuesday 22 May 2012 2:17PM

barry: Reception can vary noticeably even over quite short distances due to the effects of different terrain heights and reflections. Your equipment may be less sensitive than theirs, and your aerial system may have less gain or be losing more signal than theirs.

Digital UK's predictor does show the lowest probability of receiving the channels on C22. You may benefit from having the aerial rotated so that the elements go up-and-down rather than side-to-side: this raises the predicted coverage from 'variable' to 'good'. (This only applies to the Rowridge transmitter, which now transmits on vertical polarization as well as horizontal.) 

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John: No fault is reported at the transmitter and no work is scheduled. This usually means it's a problem at your end. Weather conditions affect signal propogation and can cause interfering signals to travel further in some conditions than others, and the wanted signal not to travel as well as average. There doesn't appear to be particularly abnormal weather at the moment, though.

Switchover starts in slightly over three weeks, so it really isn't worth calling anyone out to look at it. You might do a visual inspection just to check that all the aerial elements are present and pointing the right way, and that the cabling is properly connected and not worn through. Check that the analogue pictures are as good as the last time you checked. If they're noticeably worse, the aerial system should be checked over.

The high-power BBC multiplex will take over BBC Two's old frequency in the early hours of 13 June, after BBC Two shuts down. If you can get good results on BBC Two now, you should be good after switchover. Check the teletext test on p284 (
Digital UK - The condition of your aerial
) on BBC One, ITV1 and BBC Two to see whether you might need the aerial system serviced.

If the signal strength on Muxes B, C and D are lower than on 2 and A, that may indicate that your aerial was actually designed for Crystal Palace and was rotated to point at Bluebell Hill. In this case you should have it replaced.

Do also be aware that a lot of equipment just stores the first version of channels that it finds. If you've retuned since 4 April, the box may be storing Crystal Palace services (received too weakly to use reliably). Check for another version of the channels somewhere else in the channel list, usually at 800 onwards. Though you did say you'd retuned manually, some equipment needs the old list to be cleared out as a separate step first.

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Mr P Watmough: They are not. Check that you are looking at signal *strength* and not signal *quality*. *Strength* should change gradually, it should only change sharply if an amplifier in the aerial system is switched on or off. If it really is, and you're using a communal aerial system, it's probably a fault in that.

Quality is a function of both the wanted signal strength and of the level of interference. The reported quality is usually some percentage of datablocks received without errors. The more interference, the more errors, but the error correction scheme automatically corrects as many as it can, up to the threshold of what's possible. Many boxes report only the rate of what couldn't be corrected. On these boxes, the aerial system should be set up so that the quality meter always reports 100%, even in the worst conditions.

Every frequency used for TV transmission in the UK is used at numerous different sites. They have been spaced far enough apart that they shouldn't interfere with each other, within the expected coverage area, for more than 1% of the time. However, for the COM muxes (SDN, ArqA and ArqB), there are usually more or closer interferers than for the PSBs (BBC A, D3&4, BBC B).

Changes in weather conditions cause changes in signal propogation. If signals usually change overnight, it can just be down to the regular daily warming or cooling of the atmosphere, which is termed 'tropospheric enhancement' as it enhances the signals from distant transmitters (i.e. interference).

If you really think you have discovered a fault, you should contact Sky or one of the other broadcasters on the affected multiplex. Arqiva do not take fault calls from the public, except for a small selected group of people who monitor their local transmitter. If the fault is occurring on one of the BBC multiplexes, you can report that through their engineering page at Radio & Television Investigation Service - Diagnostic form . Keep saying 'no' when it asks if this has answered your question, and eventually you'll get to a form that enables you to send an email.

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Nicky: The problem is that the "HD Ready" and "HD TV" logos were created (by EICTA) several years before the decision on how to transmit HD signals in the UK was made.

"HD Ready" just means getting an HD signal from another source. It didn't even have to include any kind of digital tuner, although other guidance at the time meant that they usually did. The tuners usually only have MPEG-2 decoding and usually only at 'main level' (i.e. enough memory and speed to decode SD pictures only).

EICTA's "HD TV" logo does require a digital tuner, and does require MPEG-4 AVC/H.264 video decoding, but only requires the DVB-T standard. At the time the logo was created, DVB-T2 was still a couple of years away.

DVB-T2 allows about 66% more capacity per channel for the same coverage area: basically, able to carry five HD channels in the space that would only be able to carry three with DVB-T.

Other countries *have* launched HD services with DVB-T, and some using MPEG-2 compression (at High Level) rather than AVC. In those countries, e.g. France, Ireland, the "HD TV" logo is enough. Over here, you need Freeview HD-branded equipment.

I'd now like to see the 'HD Ready' and 'HD TV' logos removed from anything on sale that isn't Freeview HD compatible, though the manufacturers would no doubt argue that the packaging is printed for more than one country.

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Dave Lindsay, Gareth: Don't assume that the grid reference is accurate! All Ofcom's document says for the location is 'south-east London' and that it targets 100 households. Some of the grid references on MB21 are accurate, but only because someone has been out and found the site already (in which case it's probably been photographed).

The final Installer Newsletter for Crystal Palace ( http://www.digitaluk.co.u….pdf ) stated that Deptford was still active at the time of writing, but that they had not yet decided whether to apply for a digital licence.

It was listed as a transposer in Ofcom's guide, which means it uses different channels from those used by Crystal Palace. The other sort of relay is an Active Deflector, which uses the same channels.

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