 |
DTT
Digital Terrestrial Television THE FUTURE OF TELEVISION! |
|
| Home | Contact | Site
Map | Radio
| Amateur
Radio | Cats &
Dogs
| Vinyl
Records | Gallery
| Other
Stuff
| Links | Feedback | About | Blog History of UK Radio | DAB Radio | Useful Links |
||
Digital Terrestrial Television - The
Future Of Television Is Upon Us
All analogue television
in the UK is being switched off between 2007 and 2013
|
TV
viewers
in Wales, the
Borders and
the south west ITV regions were the first to have their analogue
television transmitters switched off as part of the UK-wide timetable
for Digital Switch Over "DSO". After an initial test where the television
transmitter at Ferryside was switched over to digital-only television
in 2005, the Whitehaven transmitter, in the Border region, was the
vanguard for full digital switch over. Whitehaven DSO was in November
2007 with the Selkirk transmitter following in November 2008 and
Caldbeck and Douglas in July 2009.
Media regulator Ofcom set the timetable that will see the UK will be fully digital by 2013. This just misses the UK Government's target date of 2012. The digital switch over will be accomplished region by region. The phasing in of a digital-only service would end with the analogue signal being switched off in the Ulster region in 2013. Initially digital terrestrial signals could reach 73% of UK households, at lower power than after "DSO", with a significant increase in coverage not possible until analogue television is shut down. To continue to receive television programmes after the switch-off of the analogue signal, viewers would have to connect all their TV sets to a digital TV service, via Freeview, cable or satellite. In most cases, that would require them to buy a digital television receiver box (Set Top Box) and possibly to upgrade their aerial. Each region would be converted over a period of several months, giving households time to switch to a digital terrestrial (Freeview), cable, or digital satellite service. [ As a personal aside, I cannot understand why the UK government has been faffing about for years over DSO, and continues with even more faffing about by taking six full years to achieve digital-switch-over on a piecemeal, region by region basis, lengthening the period of viewer confusion! Denmark and the United States of America performed DSO on one clear date. The USA switched off ALL main, high power, analogue television transmitters on 12th June 2009, leaving everyone with just the digital signals. That seems far simpler. I appreciate the huge engineering challenge in equipping all the transmitters for digital, but then maybe the UK should have started preparations earlier, rather than ending the process four years behind the USA and other countries!! In fact Germany* and Switzerland had made the switch to digital by the end of 2008, while Britain still had five more years of faffing to endure.] * except one transmitter |
Some of the best Digital Switch Over information can be found on these web sites:
MB21 - Mike Brown's TV resource - http://tx.mb21.co.uk/dso
Digital UK - clear information for TV viewers - http://www.digitaluk.co.uk
FREEVIEW - official site - http://www.freeview.co.uk
FREEVIEW - postcode checker - http://www.freeview.co.uk/availability
UK Free TV - general free TV information - http://www.ukfree.tv
BBC - digital reception advice - http://www.bbc.co.uk/reception/digitaltv/index.shtml
Digital TV Group - postcode reception checker - http://www.dtg.org.uk/industry/coverage.html
Digital TV Group - with sections for consumers & industry - http://www.dtg.org.uk
Digital TV (government site) - http://www.digitaltelevision.gov.uk
Arqiva - the UK's transmission provider - http://www.arqiva.com
HOW TO RECEIVE FREE DIGITAL TELEVISION
By the end of 2005 about 70 to 75 percent of households could receive around thirty channels of free to view Digital Television from FREEVIEW via an ordinary roof mounted television aerial and an Set Top Box (STB). A set top box is very simple to install and operate. The set-up procedure is generally fully automatic with little need for any technical knowledge.
Installation usually takes less than 5 minutes, with the box auto tuning all the available multiplexes and TV channels. Day to day use is also very simple.
The 75 percent coverage of FREEVIEW will be expanded to near universal population coverage between 2008 and 2013 as the analogue transmitters are switched off thereby allowing additional digital terrestrial television transmitters to be switched on and powers to be increased at all the existing DTT transmitters.
A SUITABLE TV AERIAL
Apart from a set top box like the one shown above, viewers will need a suitable tv aerial. In many cases an existing tv aerial will be sufficient; As long as existing analogue pictures are clear and free from snow, noise and interference (and that DTT - digital television - is transmitted from the transmitter that the aerial is pointing towards) then viewers could reasonably expect to be able to receive the many digital channel available from FREEVIEW. Other commercial and pay TV channels may also be available.
If the pictures were a bit snowy and noisy on analogue (indicating that the aerial is poor anyway) there is a good chance that digital reception will not be possible and that an aerial upgrade will be required. Additionally if the DTT multiplexes are transmitted in different UHF channel groupings than the local analogue channels (more about this below) it may be that the existing aerial may need to be replaced with an aerial of a more suitable group or a 'wideband' aerial (more below). However the transmission providers have attempted, wherever possible, to keep the new digital transmissions within the groupings previously used by the old analogue channels.
A
word of caution about aerial installers: There is no such thing as a "Digital Aerial". As long as the aerial is in good condition and of the correct type - called the "group" - it will receive analogue and digital television equally well - assuming that the local transmitter is providing digital television coverage in the area.
There are many good installers of course, but there are very many cowboys who are trying to leap on the digital switch-over bandwagon and sell inappropriate aerials, badly fitted and with cheap low quality coaxial cable and charge vastly inflated prices! If possible use a CAI approved installer http://www.cai.org.uk/
For unbiased and accurate information about digital television and television aerials do look at the PARAS website - Professional Aerial Riggers Against the Sharks: http://www.paras.org.uk/
Freeview coverage can be indicated by performing the postcode check here: http://www.freeview.co.uk/availability (It does seem to be designed to be a little pessimistic, possibly so that people's hopes are not unduly raised while the Freeview signals are a lower power than they will be after 'digital switch-over'.
MORE ABOUT AERIALS AND UHF CHANNEL GROUPINGS - Technical Stuff
When considering digital television the viewer must also consider the aerial. Although DTT (Freeview etc) - even when being transmitted at full strength after Digital Switch Over (DSO) - will be transmitted at lower powers than traditional analogue TV (about 7dB less) digital television receivers only need signals of about 20 dB less than analogue sets to work properly. HOWEVER with analogue, if your aerial is producing a weak signal the pictures on the viewer's TV will just be a bit weak - i.e. snowy, grainy, noisy. NOT SO with digital television! - If the signal produced by the aerial is a bit too weak then the pictures will simply not appear on the screen at all and the viewer will end up with a black screen. Once the signal threshold has been reached or exceeded then the perfect digital pictures will return - With digital it is an All Or Nothing situation - there is no snowy analogue 'half way house'!
An aerial that is in good condition, not too old and damaged by the crows is essential, as is good quality (NO) VERY HIGH QUALITY screened digital,satellite grade 75 ohm coaxial downlead cable. The aerial must also be of the correct GROUP for the digital signals (see below) and accurately installed and aligned with the appropriate transmitter.
When analogue 625 line colour television was introduced to the UHF band in the 1960's each transmitter site was allocated a group of four channels to transmit the four anticipated tv stations (BBC1, BBC2, ITV and The Fourth Channel). The four UHF channel groupings were close together and repeatedly used at hundreds f sites all over the country to make efficient and organised use of the limited radio spectrum available. Due to the laws of physics aerials can only work effectively over a limited range of frequencies (channels). For this reason The UHF band, which covers a huge range of frequencies from the lowest at 471MHz (UHF Channel 21) to 847MHz (UHF channel 68), was split up into three "GROUPS" or "AERIAL GROUPS":
GROUP A aerials can work efficiently between u.h.f. channel 21 to u.h.f. channel 37
GROUP B aerials can work efficiently between u.h.f. channel 35 to u.h.f. channel 53
GROUP C/D aerials can work efficiently between u.h.f. channel 48 to u.h.f. channel 68
If a grouped aerial is used to try to receive an 'out of group' transmission poor reception will usually result as there will be very little 'gain' and the direction properties of the aerial may also be lost - instead of the main pick up lobe facing directly in front of the aerial, as it should, the lobe will be distorted and face off at some unwanted angle.
More recently new aerial groups have been introduced that cover wider bands, the gain and directional characteristics may be slightly less than the above equivalents, but they are designed to work with larger groups of channels properly:
GROUP K - 21 to 48
GROUP E - 35 to 68
GROUP W - WIDEBAND - All Groups from 21 to 68 (The gain of these aerials may not be quite as high as those for the grouped aerials, A, B, C/D)
GROUP W Aerials have become much more frequently used with the introduction of DTT where wide spacings of u.h.f. channel groups are used at the transmitter. It is not always necessary, however, to use a WIDEBAND aerial simply because one is trying to receive digital television, many transmitters have managed to squeeze in the DTT multiplexes in groupings that fall within the existing analogue grouping.
As an example, in the Sutton Coldfield transmission area five of the six digital multiplexes fall within the original analogue grouping, except the highest multiplex, MUX-D which is on u.h.f. channel 55, channel 55 just falls outside the GROUP B Aerial grouping! Maybe a group B aerial would work okay, but maybe it would be better to install a Wideband aerial just in case? But then the gain of a wideband aerial in the B group part of the spectrum may be a little lower than the gain of an equivalent B group aerial. Difficult
Sutton Coldfield's channel groupings:
Analogue: BBC1 = ch 46 BBC2 = ch40 ITV = ch43 Channel Four = ch50 Five = ch37(Lichfield) : U.H.F channels 37, 40, 43, 46 and 50 fall within AERIAL GROUP B.
The DTT multiplexes are MUX1= ch41 MUX2 = ch44 MUX-A = ch47 MUX-B = ch51 MUX-C = ch52 MUX-D = ch55
Winter Hill's channel groupings:
With Winter Hill it's much easier - all channel groupings fall within the C/D Aerial Group:
WINTER HILL Analogue TV : Channels BBC1= ch55 BBC2=ch62 ITV1=ch59 CH4=65 FIVE=48 (very low power)
Digital TV: MUX1=ch56 MUX2=ch66 MUXA=ch68(slightly lower power) MUXB=ch67 MUXC=ch60 MUXD=ch63
THE MOVE TO DIGITAL TERRESTRIAL
The FREEVIEW DTT service was established jointly by the BBC, Crown Castle International (the then transmission service provider) and SKY to bring Multi-Channel television reception to a wider audience and via the existing infrastructure of terrestrial television masts that could be received through standard rooftop aerials.
FREEVIEW digitally transmits multiple television services in blocks called "Multiplexes"* The BBC channels BBC1, 2, 3, 4, BBC News 24, CBBC, CBeebies and BBC Radio etc. are all transmitted together in the BBC's dedicated multiplexes; ITV1, ITV2, ITV3, ITV News etc are lumped together into another multiplex and other TV services such as Channel Four, FIVE, Sky News etc arrive in further multiplexes. Many more TV and radio channels are available by this method than would otherwise be possible by traditional analogue methods. Further services, in addition to FREEVIEW, are also available via the normal rooftop television aerial and set top box as pay to view services providing a number of so called 'premium' channels. These channels are packaged into further multiplex space and require a viewing card, bought at additional cost, and a dedicated set top box that is equipped with the necessary viewing card slot - conditional access. These pay tv boxes will also receive the subscription free FREEVIEW services.
Many of the main terrestrial television transmitter sites, along with some of the larger relay transmitter sites, started carrying the digital Freeview services years in advance of analogue switch off and full DSO. However to squeeze in digital television alongside existing analogue television services meant that not all transmitter sites could be used to transmit digital TV due to the lack of spare channels / frequencies available in the UHF television band between channel 21 and 69. Another consequence of the limited number of UHF channels was the fact that Freeview (and the other Pay TV channels) had to be transmitted at significantly lower power than would be the case after Digital Switch Over was complete.
The vast majority of small relay transmitters (of which there are hundreds) could not initially carry Freeview and other stations (Pay TV etc) at all due to the severe lack of frequencies / channel space. This lack of spectrum space would hold up the expansion of DTT until all the analogue television transmitters were switched off - freeing up spectrum for the smaller relay transmitters to radiate digital TV signals.
Initially around 70 - 75 percent of the population could receive DTT but very much less of the geographical area of the UK would be covered by DTT signals. This is because for both economic reasons and the need for radio spectrum efficiency, television signals must be concentrated on areas of significant population. Even with the 99.6% population coverage of the analogue television network there would be vast geographical areas that could not receive any signals - but this is generally not a consideration if there are only one or two households in a sparsely populated area. The later analogue TV relays to be commissioned at great expense only provided TV reception to populations of only one or two hundred households. With the initial DTT network it was relatively easy to get coverage to densely populated areas with a just few dozen transmitters, but to bring coverage to the last 25 to 30 percent of the population requires many hundreds more transmitters to be built - incredibly expensive of course, and really demonstrating the Law Of Diminishing Returns!
When all the analogue transmitters (UHF PAL Colour) of BBC1, BBC2, ITV1, Channel Four and FIVE are switched off by 2013, digital terrestrial television will be able to be transmitted from all necessary low power relay transmitters (though not all of those previously needed by analogue television), and at full power from the main transmitter masts, to provide the 99+% coverage that was previously available via the old analogue TV networks. The switching off of analogue television was, however, a politically a hot potato because millions of people could have been left without any television, perhaps unwilling to pay for a new set top box and a possible aerial upgrade or adjustment.
*NOTE: A MULTIPLEX is a special method of digital broadcasting in which the a single transmission can be used to combine and transmit several television services. The use of sophisticated digital compression techniques, such as MPEG4, enables many television and radio services to be accommodated (squashed into) a single multiplex. Several multiplexes will be used from each television transmitter to bring a multitude of channels to the viewer/listener. A multiplex is broadcast on a single UHF channel just like an analogue TV station, but using digital technology. This digital technology uses very clever compression techniques to shoe-horn in five or more television services into the space normally occupied by a single analogue station.
"FREESAT" - Subscription Free
Satellite Television
As an alternative to FREEVIEW via an aerial, free digital television channels are available via the FREESAT service which has been launched, independently of Sky TV, by the BBC and ITV. FREESAT is free to air unlike Sky Television - this means that there is no monthly subscription millstone. All that is required is a small set-top box that costs about £50.00 and a mini-dish (satellite aerial). There may already be a minidish fitted to the residence from former residents or a lapsed Sky Television installation. As long as the minidish and LNB are in working order this can be connected directly to a new "Freesat" set top box for instant subscription free satellite television.
[Other methods of receiving digital television are via SKY and paid subscription to the SKY satellite service, or paid for monthly subscription to a local cable company, if there is one. Additionally Sky also offer their version of a free satellite service.]
LINKS TO OTHER HELPFUL WEBSITES:
(All links will open in a new window)
PARAS - Professional Aerial Riggers Against the Sharks
www.aerialsandtv.com - Aerial installations, theory and practice
www.atvcircuitsandservicemanuals.co.uk - TV service information
http://www.wrightsaerials.tv/ - Bill Wright's aerial installers website
FREESAT official website - http://www.freesat.co.uk
Goodmans Digital UK - Manufacturer of FREESAT Set Top Boxes
It was a Reader's question that prompted me to include this page back in 2005:
Peter W Robinson asked: "I live in a valley. Please can you tell me when we will have a transmitter able to transmit Digital TV to our homes - other than BBC which is very good. Of course we could get a dish for sky etc. but I believe that sooner or later we should be able to receive the Freeview signals from ITV and other channels. Please let me know what is holding up this work?"
In essence the answer to the question is this:
How Analogue Television Was Arranged
The five 'ordinary' analogue television channels, BBC1, BBC2, ITV1, Channel Four (or S4C) and FIVE are broadcast on the UHF band (Ultra High Frequency radio band) using the analogue PAL (Phase Alternate Line) colour standard in the UK on frequencies between 471 MHz and 847 MHz which are divided into 'channels' numbered from 21 to 69. There have been over 900 television transmitter masts constructed jointly by the BBC and IBA between the 1950's and 1990's (and latterly some further small relay masts installed by Crown Castle and NTL). Almost all of these transmitting stations transmit the four terrestrial TV stations; BBC1, BBC2, ITV1 and Channel Four/S4C. Each TV service having being carefully allocated a specific 'UHF Channel' number to be transmitted on from the mast. Eg the Winter Hill mast in Lancashire transmits on these channels:
BBC1 on UHF channel 55
BBC2 on UHF channel 62
ITV1 on UHF channel 59
Channel Four on UHF channel 65
Some 50 of these masts are designated as 'Main Stations' and transmit at high or very high powers in the order of 20,000 to 1,000,000 Watts effective radiated power. These 'main stations' reach the majority of the UK population. The remaining population has to be served from hundreds of medium or low power relay transmitters using powers typically in the order of between 2 Watts to 10,000 Watts. In this way 99.6% of the population of the UK is served with four channels of analogue TV.
Due to the fact that the UHF television band was originally planned in the 1960's to accommodate four television services it was a challenge for the DTI, ITC , NTL and BBC to factor in a fifth television service in the early - mid 1990's in the form of Channel Five Television. Because of the limits of radio spectrum space available FIVE TV, as it is now known, could be allocated a total of (only) 47 transmitters and so reaches around 80% of the UK population. Some of Five's transmitters are at the same or similar power as the other four analogue channels, but there are many that are at lower power; for example FIVE TV is transmitted from Winter Hill on UHF Channel 48, but at much lower power than the other four TV services so as not to cause interference to existing transmitters elsewhere. This is also the case at other FIVE TV transmitters where lower power must be used to avoid interference to viewers of other stations. Powers for FIVE's analogue transmitters ranged from 33 Watts to 1,000,000 Watts of effective radiated power.
What UHF Channels Will Be Used For D.T.T. Post Digital Switch Over?
It will be all change post digital switch over, not only will there be no more analogue television, but there will be a new arrangement for the allocation of UHF channels used in the UK for broadcasting. Not all of the channels previously used for analogue (PAL) television will be used for digital broadcasting. The unused channels will probably be auctioned off by the government / Ofcom to raise money for the treasury.
These auctioned off channels may go to the highest bidder and may or may not be used for transmitting television. There is a possibility that the auctioned off channels could be sold to other commercial TV operators, possibly used for high definition (HD) television services, or may be used for something entirely different such as data communication.
Our
friend and correspondent, Martin Watkins, has complied one of his
typically
marvellous transmitter lists which you can download here - the list (v9) was compiled
in November 2006 and shows the ITU UK DTT allocations that were
anticipated at that time, however things have been continually changing
since then so this list is now out of date (November 2009) but we have
left it in place here merely out of historical reference.
Home | Contact | Site Map | Radio | Amateur Radio | Cats & Dogs | Vinyl Records | Gallery | Other Stuff | Links | Feedback | About | Blog
History of UK Radio | DAB Radio | Useful Links
.