All posts by Mike Dimmick
Below are all of Mike Dimmick's postings, with the most recent are at the bottom of the page.Final thought: you have to do a complete wipe of all channels and a full retune from scratch. See TV Re-tune for advice on how to do this for this box. It's possible that the box still has some stored information from the old frequency and is not storing the new information because it clashes. (RG47SH)
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Robert Robinson: I'm afraid this box is on the list of 2K-only equipment at http://www.digitaluk.co.uk/2kequipment . It can't be fixed with a software upgrade. You'll need to get a new Freeview box.
The new multiplexes use an 8K mode which is more robust when reflections are picked up. It's also the mode that most other countries are using. We're fairly unique in ever having used 2K mode. New boxes should all support 2K and 8K modes. (RG47SH)
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Jamie: Digital UK's predictor suggests that it should get better in May, presumably after Ridge Hill, Lark Stoke and The Wrekin complete switchover.
Normally at a distance of 5 km with clear line of sight to the transmitter you wouldn't have a problem, but Pontypool's transmitting aerial is designed to cover the area to the east. You are due south of it, so you get less signal than if you were due east.
The Mynydd Machen transmitter forms a Single-Frequency Network with Pontypool (for the PSB multiplexes), and transmits to the north and north-west. However, there is some terrain blocking your sight of it.
It's possible that rotating the aerial a bit to the west could reduce pickup of unwanted signals and pick up more signal from Mynydd Machen in addition to Pontypool. Aerials do have a fairly wide 'acceptance angle' and this tends to be wider for vertical polarization than for horizontal.
In some weather conditions, signals can travel further than planned, and this can cause interference. This is probably the source of the problem and why it changes over time.
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Saul Warren-Howles: The Salisbury transmitter switches over starting on the 7th of March 2012 and completes on the 21st of March.
There's a chance that additional retunes may be required before and after switchover, depending on whether there are clashes with other transmitters and whether the plans are updated to release C62 at switchover.
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M
Sutton Coldfield (Birmingham, England) Full Freeview transmitteThursday 31 March 2011 5:44PM
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David Luckman: The HD transmissions are on a lower frequency, C34, than most Group B aerials support. It appears that Scalpcliff Hill is blocking line-of-sight, but Digital UK still predicts a useful signal for some reason. It's possible that careful siting of a Group B Yagi-type aerial might give better results than the grid-type.
Grid-type aerials aren't recommended for digital reception although the reason escapes me for the moment. I think it's simply that they don't give much gain. That's fine for watching analogue at 1000 kW, not so good for a 4 kW HD multiplex.
Don't go overboard with a replacement aerial, the high-power DTT multiplexes will be 17 dB louder than the low-power HD multiplex, which is actually larger than the width of the recommended range of signal strength (45 to 60 dBuV).
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Norman Strath: The aerial has to point nearly due south to pick up Durris properly. The tree won't help - is there another place on the building that you could mount the aerial that would avoid the tree?
The aerial could be pointing west, towards the Knock More transmitter, but you're expected to get better results from Durris. In fact there's no prediction for Knock More - there are three mountains between you and it.
The commercial multiplexes are still broadcasting at low power, and you aren't expected to get a usable service from them until the 15th of June.
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Briantist: I mean that that TV aerials will be picking up the signals transmitted by the phones and the base stations, and that will cause problems for TV reception. Sorry that wasn't clear.
As I say, there are already many people having issues with TV reception caused by mobile phones at 900 MHz and emergency services radio at 400 MHz, so I predict more problems at 800 MHz. Let's not compound the problem with phones at 600 MHz as well.
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2/PSB2 and A/COM4 aren't completely identical. 2 and A are in 2K mode, while PSB2 and COM4 are in 8K mode.
This doesn't affect capacity (or by only a very small amount), but does affect the robustness of the signal - 8K mode can cope with more distant/more delayed reflections.
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Bill Martin: Ofcom published some research here:
Ofcom | Cost and Power Consumption Implications of Digital Switchover
That's the increase in domestic power consumption.
It's harder to give an accurate assessment of transmitter power consumption. The quoted figure of max ERP for analogue is the sync peak. There is a period of unmodulated* carrier on every line as the legacy CRT scan beam moves from the end of one line to the start of the next. There is also a longer period at the end of every field as the beam returns from the bottom of the screen to the top. (A frame, one picture, consists of two interlaced fields.) These periods are generally called 'sync' or 'blanking interval'.
* OK - the carrier is now modulated to carry Teletext, Programme Delivery Control and other data in the Vertical Blanking Interval. But the modulation is very small.
The sync pulses are 42% larger *in voltage* than the peak level during the visible part of the picture. That makes them over twice as large in power. The peak level is only seen for a completely black picture - the brighter the picture, the lower the transmitted level. Yes, seems weird, but we used to use 'positive' modulation, where white gave larger values than black, for 405 line VHF TV and there were often reception problems, so we used negative modulation for UHF. A medium-brightness picture might be only be 35% of sync peak in voltage, making sync peak about 8 times the power of the picture information. That's about 9 decibels.
You can see the difference at Wright's Aerials . There are massive narrow spikes at C37, C41, C44, C47 and C52, somewhat towards the left of the centre of the channel. Those are 870kW max ERP. The adjacent digital multiplexes at C40, C43 and C46 have roughly the same level as the actual picture content of the analogue transmissions, but are quoted as 10kW max ERP.
After switchover Emley Moor's digital multiplexes will run at 174kW max ERP. That will actually make the digital signals larger than the average level of the analogue signals.
Bill Wright did actually make some measurements from Emley Moor when it was transmitting test signals a couple of weeks ago, but I don't have the links to hand.
The transmitter equipment at most sites is all-new, and the aerials have largely been replaced with more efficient designs, so there may be some savings from that route. However, there's a chance that the actual electrical power used will actually be greater, not less - the main sites will now transmit six high-power muxes rather than four or five analogue signals, although relays will transmit three muxes rather than four analogue channels, and of course the six low-power muxes at the main sites are switched off.
(ERP = Effective Radiated Power. The actual power emitted by the transmitter, multiplied by the transmitting aerial gain at that frequency, compared to an isotropic radiator.) (RG47SH)
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Thursday 31 March 2011 4:14PM
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Hmm. If it's the shortest distance it could be a problem of too much signal, though the predictor suggests that you weren't expected to get reliable signals before switchover.
Messages on analogue are shown before switchover, but only for about two weeks before the actual switchover date for the transmitter you're watching. I've not heard of any messages longer in advance than that, unless it was an advert from Digital UK (which I would hope was broadcast from the correct transmitter only!)
Of the three main transmitters that you might be watching, Sandy Heath has just started switchover, Sutton Coldfield changes at the start of September and Oxford a week after that. You should get best results from Sandy Heath when switchover is complete, but Oxford may have provided stronger signals before yesterday.
Some boxes store whichever version of the channels they find first, rather than the strongest found. Of these transmitters, Oxford is on the lowest frequencies. It's worth checking to see if there's another copy of the BBC channels at channel 800, and if they work better. If so, see the Digital Region Overlap section for ideas.