Digital terrestrial transmissions
Digital Terrestrial TV is transmitted in - the familiar UHF bands
already used for terrestrial analogue TV. The standard channels (Ch.2 I -
Ch.68) are used, shared with the analogue transmissions and DTT signals are
either horizontally or vertically polarised, like analogue transmissions.
The initial transmitter plan includes all 5 existing main transmitter sites
and 30 relays
Multiplexes
Each DTT signal is called a multiplex. Each multiplex carries several
programmes Typically five, together with electronic programme guide (EPG)
information, teletext and eventually a number of other services. Six multiplexes
are being transmitted initially the multiplexes are usually referred to as
Dl to D6. Because of spectrum planning constraints not all transmitters carry
all six multiplexes
The relationship between analogue and digital channel allocations
varies considerably between the different transmitter sites. In some cases
(e.g. Crystal Palace) all six multiplexes are close to the analogue channels.
In others (e.g. Black Hill) some multiplexes are close to the analogue channels
whilst the remainders are far removed. DTT multiplexes are transmitted with
the same polarisation as the analogue services from the same site.
The DTT Signal
The digital signal is very different from the familiar PAL analogue
signal. DTT uses a special form of modulation called COFDM (Coded Orthogonal
Frequency Division Multiplex). The purpose of COFDM is to make the signal
highly immune to multi-path reflections. In other words, up to a point, it
is not affected by ghosting the COFDM signal remains perfectly receivable
under conditions where an analogue signal would suffer intolerable
ghosting
COFDM works by dividing the signal into 1,705 individually modulated
carriers. This means that the power in a signal is spread evenly across its
7.61 MHz bandwidth. In contrast most of the power in an analogue TV signal
is concentrated around the vision carrier
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