Specifications

page 38
[
StarServe Installers Guide
]
The ‘digital’ signal used for Digital Terrestrial Television Broad-
casting (DTTB) is different to the PAL analogue signal. DTTB
utilises a form of modulation called COFDM (Coded
Orthogonal Frequency Division Multiplex) and is defined by
DVB-T standards. The purpose of COFDM is to provide a
vehicle for the carriage of digital signals that is highly robust
to the effects of echoes or ghosts.
The Digital Television
Signal
COFDM spreads data between
many carriers. Either of two modes
of COFDM may be employed, with
the ‘2k mod’ having 1705 carriers
and the ‘8k mode’ having 6817
carriers. The power is spread evenly
over a 6.7MHz portion of the 7MHz
channel. This is in contrast to an
analogue service, with its
concentration of power in the
vision carrier at the lower end of
the spectrum. The difference in
power distribution has important
consequences for the measure-
ment of signal level.
How DTTB is
transmitted
Digital Terrestrial Television Broadcasting is transmitted
using digital modulation in the channels available in Bands
III, IV and V, sharing the spectrum with the current analogue
transmissions. Unlike analogue transmissions, there is no
Band I or Band II transmissions for DTTB.
Existing TV transmitters will generally be used for DTTB
transmissions. Although successful reception via existing
antennas will normally be experienced, optimum reception of
DTTB may require some modifications.
Digital Services
Each Digital Service will occupy a 7 MHz channel and may
contain a HDTV programme, a SDTV programme or multiple
programmes, all available in 16:9 display format. Associated
with the picture will be surround sound up to ‘5.1’ channels,
together with text and data information, closed captions for
the hearing impaired and the potential for interactive
enhancements.
Spectrum Utilisation
For those VHF allocations in state capital cities, a spectrum
arrangement has been adopted where the adjacent
channels will be used to transmit DTTB at a lower
power than the existing VHF analogue PAL
transmissions.
The actual ratios between PAL and DTTB
will depend upon the detailed planning
and interference restrictions in and
around the particular city.