1 April 2003

Much has been said about the Government’s plan to switch off analogue television broadcasts, but what exactly will happen when a so-called analogue ‘switch off’ takes place?

Basically speaking it means that the UK will abandon using the ‘conventional’ analogue terrestrial broadcasting method in preference to the use of digital broadcasting via cable, satellite (Sky Digital) or terrestrial (DTT – Digital Terrestrial Television, as used by Freeview) methods.

Sky Digital

DTT can (and does) use the same transmitters and similar frequencies as analogue terrestrial broadcasts but can use the same space that one channel occupies to transmit five or more channels.

However DTT broadcasts can only make use of frequencies that are unused by the existing analogue broadcasts, so at the moment DTT is forced to use the gaps in the frequencies.

Analogue terrestrial broadcasts occupy what are known as channel numbers, with each number representing a particular frequency (like those shown on the dial of a radio except they are different values) and put simply, an individual channel number is used to broadcast a single television channel.

In the UK, channel numbers between 21 and 68 are used for terrestrial television and the frequencies used are UHF (ultra high frequency) ones as opposed to the VHF (very high frequency) ones which were used in the UK for 405 line broadcasts up to 1985 and are still used for television broadcasting in various other countries.

Although there are 47 channels for broadcasting television pictures (between 21 and 68), each transmitter has a relatively limited range which is also restricted by hills and mountains as well as tall buildings.

With analogue transmissions you cannot have two transmitters using the same channel number if there is any chance of both transmissions being receivable from one location otherwise there would be interference between the two signals, so adjacent transmitters have to use different channel numbers for the same transmissions to avoid interference.

Certain unusual weather conditions can make television transmissions travel unusually long distances, which is the reason why there are occasional warnings that television pictures can suffer what is known as “co-channel interference”, so to minimise the risk of this happening the channel numbers have to be as different as possible within a very wide region.

Also to maintain good quality pictures, the analogue channels in a particular area are spaced apart to further reduce the risk of interference between two adjacent channels, so for example in a particular area the main transmitter has BBC1, BBC2, ITV and Channel 4 on channel numbers 31, 24, 27, and 21 respectively.

Channel Five is an interesting case since the transmitter network frequency plan was originally devised for four channels, so analogue transmissions for Channel Five often have to exploit coverage ‘gaps’ in the existing network.

They sometimes they use a different transmitter site as well as often having lower power transmissions compared with the other four channels.

The same rules apply to the new breed of RSL (restricted station licence) local television stations, though since they only cover a small area anyway they don’t cause too much of a problem.

Digital terrestrial television (DTT) is also a recent introduction that makes use of spare channels within the terrestrial transmitter network, and although a digital transmission cannot occupy the same channel as a nearby analogue transmission, it is less likely to cause interference to adjacent analogue transmissions as well as covering a larger area using the same transmitter power.

The key advantage of DTT is its ability to transmit more than one television service using a single channel number as a ‘multiplex’ (or MUX for short). A single channel number can now be used to transmit (for example) BBC1, BBC2, BBC3, BBC4, BBC News 24 and other services such as BBC Parliament simultaneously using DTT.

It can be seen that if all the existing analogue channels were disposed of there would be a huge additional capacity for many more digital services that use DTT and other digital systems.

Since the main four terrestrial channels are available in most parts of the country, that effectively means that four multiplexes in total have a very wide potential coverage area and that is just by using the standard transmitters and frequencies relatively unaltered.

Minor adjustments to the resulting digital network that take account of the digital signal’s greater range and robustness will make an even greater potential transmission capacity available, though it is very likely that the analogue shutdown will be used as an opportunity for a major reorganisation of the frequency allocations in order to exploit the coverage improvements.

However, radical frequency reallocations may require many people to have their aerial upgraded to a ‘wide band’ type as well as buying a digital set top box. Many existing aerials are only sensitive to signals within a specified channel range (or a narrow band), so if broadcast frequencies in a particular area are relocated a change of aerial may be required as what was often needed for many existing digital terrestrial viewers.

ONdigital – for example – offered to swap aerials free of charge for subscribers, but since there is no longer a subscription service available the customer would have to pay for an upgrade unless free or subsidised aerial upgrades are made available.

The first scenario is the one that has been mentioned the most and currently seems the most likely to happen, which is at a set date sometime in the future all analogue television broadcasting will cease.

This means all existing televisions will need an additional set top box in order to carry on watching any broadcasts if they haven’t got a digital decoder built inside them (which will happen eventually for all scenarios), but using such a box with a pocket television is either inconvenient (unless someone produces a tiny pocket-sized digital decoder box) or even impossible.

This ‘day zero’ method of switchover means that a large chunk of spectrum (i.e. broadcast frequencies) will become available at the same time, but this method has a disadvantage relating to the fact that there may not be a use for all of the freed space immediately on the day it happens (plus it may remain free for a significant amount of time).

Therefore it wouldn’t be completely ‘revenue earning’ so to speak, as well as being potentially embarrassing from a political point of view for the current government.

As well as the ‘sudden death’ scenario, there have been other proposals that have been discussed to a certain extent. All of them have the eventual aim of switching off all the conventional analogue broadcasts by a certain date.

One approach which has been openly mentioned in the media is to selectively reduce the power of various main transmitters, therefore allowing some of the existing DTT broadcasts to increase their power, but this approach is only feasible if there are none or very few relay stations associated with that particular transmitter (since they will all be affected by a drop in power) as well as there being nearby DTT broadcasts that happen to use the same frequency which stand to benefit from such a move.

So unless it is planned to deprive whole areas of a particular channel (which is akin to a partial shutdown) this approach would have to be planned very carefully and may not benefit a huge number of people as a result.

Another approach to a gradual shutdown of the analogue transmitter network is to close down one or more of the existing channels to begin with, therefore viewers of analogue broadcasts will be forced to buy digital equipment if they wish to continue watching all the channels they have got used to watching, but it will keep an analogue television service (of sorts) going until all channels have been shut down.

The advantage of this approach is that it would enable DTT coverage to match (and probably exceed) 100% of the existing analogue signal coverage before a final analogue shutdown takes place, therefore this form of transition may be more ‘user-friendly’ compared to some of the alternatives, though another issue is raised as a result; which channels should go and which should stay until the very end?

Naturally, with their interests at heart, ITV and Channel 4 have put together a joint proposal concerning what should happen during such a transition. Both channels obviously think that any channels (i.e. ITV1 and Channel 4, though Channel Five also fits into this category) that gain their principal revenue from advertising shouldn’t have their analogue transmissions switched off until the digital audience for the same channels has been properly established.

So their idea is for analogue BBC Two to be switched off and replaced with the BBC Digital MUX (using all existing transmitters and relays).

The next step, according to this proposal, would be to switch off analogue BBC One and use the BBC One frequencies for digital MUX 3 and 4, which would then later be followed by the switch off of the remainder of the analogue services and the reorganisation of all the remaining frequencies.

However this ITV/C4 proposal relies on the controversial step of the BBC broadcasts being sacrificed first, which means that this is obviously weighted strongly in the favour of the commercial broadcasters at least for the relatively short transition period between a full analogue service and a complete analogue shutdown.

Although it might seem unusual to go about a partial shutdown of the analogue service in this manner, it would enable such a task to be achieved without the prospect of having to pay any form of compensation to a commercial broadcaster for any resultant short term loss of viewers (and possible advertising revenue) if a commercial channel was to be switched off at any time before BBC One and BBC Two.

The government also likes to maintain a cordial relationship with the commercial television broadcasters, so it could be argued that it would undergo a partial analogue shutdown in this manner in order to placate the commercial sector, but a crucial point has been (some might say conveniently) overlooked in the previous scenario.

Channel 4 – although funded by advertising – is actually a state-owned broadcaster, therefore it may be possible to perform a partial shutdown of the analogue network in a slightly different way by sacrificing analogue Channel 4 at either the same time as BBC Two or soon afterwards.

Indeed, the analogue channels left after such a move would be BBC One and ITV1 (plus Channel Five in some areas), and for anyone who is familiar with the history of British broadcasting would realise that this is almost as if it is the channels were being switched off in the reverse order that they had been introduced.

When all analogue broadcasts have ceased (which will occur sometime after 2006 at the earliest, only three to four years from now), what will happen next?

Apart from DTT broadcasts reaching at least 100% of the coverage provided by the previous analogue broadcasts, most of the proposals seem to suggest that certain blocks of channels (e.g. channel 63 upwards) could be released from TV broadcasting (as they are in certain other countries) and used for other purposes, though currently these frequencies can only be used for one way broadcasting in Europe unless the rules are changed.

Channel 37 is relatively unusual since the frequency is relatively ‘isolated’ from its neighbours, therefore it is ideal for use for local digital television broadcasts, but whatever is planned to happen after the complete analogue broadcast shutdown may effect the planning of the actual shutdown itself.

If only part of what was used for analogue broadcasts is going to be reused in the short term then it may become theoretically possible to implement a partial shutdown plan so that the Government can make use of the freed capacity for other purposes (and make money out of it) as well as speeding up the shutdown process.

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