Specifications

[
StarServe Installers Guide
]
page 39
Same Coverage
A planning requirement for all service areas is the concept of
‘same coverage’, where the introduced DTTB service is
received by viewers currently receiving an analogue service.
It is important to ensure the signal level variation is adequate
for the long-term robustness of the DTTB reception, rather
than installing a ‘just adequate’ antenna system. This
requirement for successful long-term DTTB reception may
usually be met by ensuring the receiving antenna provides
successful and robust analogue reception.
The greater efficiency of digital modulation results in less
power required to provide the ‘same coverage’. Depending on
the planning requirements of the particular area’s digital
coverage, the power for digital may be as low as one-tenth of
the analogue power.
The digital services will normally be transmitted via the same
antennas as the current analogue services. Hence the ratios
of digital to analogue powers will be similar in all directions.
The received signal level required in a particular location to
achieve long-term successful reception of all digital services
will depend on the following.
The Digital Cliff Edge
Unlike an analogue TV signal, which can be viewed at weak
signal strengths or in corrupted conditions, DTTB TV picture
and sound will either be perfect, in the process of breaking
up or non-existent.
The penalty for near perfect pictures and sound is that DTTB
reception exhibits a very rapid change from being excellent
to disappearing. This phenomenon in general is referred to as
the ‘digital cliff edge’ or ‘threshold’.
For reliable long-term reception of a digital service, the level
and quality of the signal needs to have sufficient margin to
stay away from the cliff edge!
Digital TV Signal Levels
The measurement of the signal level delivered to the input of
the receiver is the initial easy check on the suitability of the
installation. The levels recommended below have allowances
to cater for reception of Band III, IV and V in most environments
experienced in city and rural locations.
The rule of thumb developed in analogue reception is 1mV or
60dB µV for a successful installation. The potential rule of thumb
for digital TV reception is 0.5mV or 54dB µV. This guide for
digital signal levels is correct if the analogue service in the
presence of the digital service is of fair quality, i.e., if the
analogue service has moderate ghosting and little interference.
Other important information on Digital TV Signal Levels:-
a) When the antenna system has an amplifier, the maximum
limit is still applicable to the receiver input, but the minimum
and preferred levels are now applicable to the amplifier’s input.
In amplified systems, the Carrier to Noise ratio should be
measured and should be a minimum of 29dB (preferably 43dB).
b) The lowest level that can be received and still provide
picture and sound (the threshold or cliff edge), is approximately
31dBµV, when there is little multipath and no interference.
c) The nominal modulation parameters are 64QAM with a
2/3 FEC. Other alternate modulations, for example of 64QAM
with a 3/4 FEC, will have a different minimum reception level
of approximately 33dBµV.
The cliff edge characteristic invites the definition of a margin
the signal has to achieve before the cliff edge is reached. By
the insertion of an attenuator in the input of the receiver or
amplifier, the level margin may be found.
d) The recommended minimum level margin is 9dB for the
first level of caution. This will cater for short and long-term
variations. Obtaining the preferred level, in ideal reception
conditions, would produce a preferred level margin of greater
than 20dB. With more complex multi-path reception conditions
and modulation using 3/4 FEC, this level margin may be
reduced to perhaps 16dB.
All the channels expected to be received must be received
reliably. Hence the level margin must be checked on all
channels. Further quality checking must also be assessed
on all channels.
Analogue
Digital