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Tuesday, 9 April, 2002, 07:05 GMT 08:05 UK
Picking up ITV Digital's pieces
Nick Higham
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Nick Higham
BBC media correspondent
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ITV Digital may not have gone bust, but to judge from the widespread speculation about what may happen if and when it does, most people seem to think its insolvency is inevitable.

So what would follow the demise of ITV Digital? The likeliest outcome, it seems, could be more free-to-view channels on digital terrestrial television, and (perhaps surprisingly) a continuing pay-per-view service, but only the most limited pay-channel service.

The latest edition of the newsletter New Media Markets points out that, in addition to ITV Digital, there is a second pay-TV operator on digital terrestrial television, S4C Digital Networks - or SDN - which operates the multiplex carrying the ITV Select pay-per-view service, a joint venture between S4C and ITV Digital.

Multiplexes
A multiplex is a UHF frequency which can transmit several different digital services in the space taken up by one analogue channel
For example, the BBC's multiplex carries BBC One, Two, Four and Choice as well as CBeebies and CBBC
There are six multiplexes, of which ITV Digital has three
ITV Select could apparently keep operating even if ITV Digital went out of business, alongside the other occupants of the SDN multiplex: Channel 5, the ITN News channel, TV Travel Shop and the adult channel TVX, plus S4C in Wales. It also carried (outside Wales) a Granada-Littlewoods shopping channel, Shop!, which closed on Monday.

New Media Markets also suggests ITV Digital's death could prove a boost rather than a body-blow to digital terrestrial television because the spectrum it now occupies could be re-allocated to the BBC and other terrestrial broadcasters to improve their signals - and presumably launch some additional free channels.

ITV Digital's monkey
Monkey: ITV Digital is still operating, but is in administration
At present, over 50 digital terrestrial channels are squeezed into six digital terrestrial multiplexes.

One is operated by the BBC, another by ITV and Channel 4, ITV Digital has three and SDN has one.

But there are more channels crammed into the multiplexes than ought by rights to be there.

This is thanks to digital compression and a process known as "statistical multiplexing" - "stat muxing" in engineers' jargon, or as Nick Tanton of the BBC's research and development department puts it, "robbing Peter to pay Paul".

Each DTT multiplex has a capacity of 24 Mbits per second. A single television channel requires anything from 1.5 Mbits to 8 Mbits per second, depending on the kind of programming it is carrying.

Entrapment screen grab - ITV Select
ITV Select is the service's pay-per-view offering
Fast-moving sport requires a lot of capacity, studio talking heads or slow-moving arty films a lot less.

"Stat muxing" involves constantly apportioning the available bandwidth between the channels on a millisecond-by-millisecond basis. The result, combined with compression, is a loss of picture detail, especially in things like grass (which often looks "plastic") and faces, and "mosquito noise" around movement.

DTT signals were also transmitted to begin with at relatively low power (a problem now being remedied) with the result that receivers were alarmingly prone to interference from light switches or thermostats going on and off.

Football coverage from Ondigital/ ITV Digital
"Stat muxing" can distort fast-moving images
Combine those two sets of problems and it's not surprising that DTT reception has often provoked complaints about poor quality, and has never lived up to the promise of sharper-than-analogue pictures.

It also explains why the BBC and perhaps other terrestrial broadcasters are eyeing ITV Digital's multiplexes covetously.

They may get lucky. If ITV Digital does go bust, as things stand its licence to broadcast would be readvertised by the Independent Television Commission.

But the ITC says it will only readvertise after talking to the government about "spectrum planning" issues - in other words, carving up the available spectrum in a different way, quite possibly to the terrestrial broadcasters' advantage.

E-mail: nick.higham@bbc.co.uk

A version of this column appears in the BBC magazine Ariel.

See also:

03 Apr 02 | Business
ITV Digital set for showdown
28 Mar 02 | TV and Radio
BBC feels effects of digital saga
27 Mar 02 | TV and Radio
Dyke: Digital TV will survive
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