Wednesday, 12 March 2008


High definition television transmits more picture information and better quality sound than traditional standard definition television. This enables picture quality to be sharper and more immersive and the sound can be surround sound.
Standard definition refers to the conventional television pictures which do not deploy the HD format.
720p and 1080i
Standard definition television pictures are made up of 576 horizontal lines refreshed at a rate of 25 times per second. The increased amount of picture information transmitted for HDTV means pictures are made up of more lines – i.e. there is a greater level of picture resolution. The accepted standards for HD picture resolution are currently 720p (720 progressive – 720 lines refreshed at 50 times per second) or 1080i (1080 interlaced – 1080 lines refreshed at 25 times per second). The BBC Executive’s application proposes to use both these standards. 1080p is also becoming available (1080 lines refreshed at 50 times per second).
Digital terrestrial television is the form of digital television which viewers can receive through their normal television aerials. The platform comprises six multiplexes broadcasting both free-to-view and pay-television channels.
Freeview refers to the bundle of free-to-view channels on DTT collectively marketed under the Freeview brand.
A multiplex is a fixed 8MHz block of UHF spectrum which carries a bundle of digital television channels. One digital multiplex occupies the same amount of spectrum as a single analogue television channel.
Internet protocol television is a form of digital television delivered over a broadband network using internet protocol.
MPEG-2 and MPEG-4
Compression is used to encode a television channel for digital transmission. To reduce the volume of data that needs to be transmitted, and thereby fit more channels onto a multiplex, compression reduces the amount of information that needs to be sent from one second to the next. Information that is constant – e.g. a plain coloured background – can be reduced so that the capacity can be concentrated on changing information. The process by which this has been done hitherto is called MPEG-2. A new, more efficient process called MPEG-4 will be applied to HDTV services on satellite and DTT. Channels encoded in MPEG-2 and MPEG-4 can be transmitted on the same multiplex. But current Freeview set-top boxes are capable of decoding only MPEG-2 services, so consumers would need to upgrade their receiving equipment in order to receive HD channels in MPEG-4 alongside standard MPEG-2 services
DVB-T and DVB-T2
The technology by which a digital television signal is transmitted is referred to as modulation. The current standard is called DVB-T. A replacement, DVB-T2, is in development. This will allow more efficient use of spectrum capacity. DVB-T and DVB-T2 cannot be mixed on the same multiplex. Current Freeview set-top boxes are compatible only with DVB-T, so consumers would need to upgrade their receiving equipment in order to view channels transmitted using DVB-T2. DVB-T2 equipment is expected to be available to consumers by late 2009.
16 QAM and 64 QAM
Digital multiplex transmission is subject to trade-offs between coverage, capacity and resilience to interference. This is referred to as the mode. At present, digital television is transmitted in the spectrum gaps between analogue television channels. So the BBC makes a cautious trade-off – at 16 QAM mode – between coverage, capacity and interference in order to ensure consumers receive a robust signal. As digital switchover proceeds, and interference from analogue channels ceases to be an issue, the BBC will convert its multiplexes to 64 QAM mode – enabling more channels to be carried in its capacity.

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