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Technical regulations governing analog terrestrial
television service in Brazil do not permit the use of co-located
adjacent channels.
In 2003, a manufacturer of television transmission
equipment was authorized by Anatel to conduct tests to demonstrate
that reducing the levels of spurious signals in UHF terrestrial analog
television transmission would permit the operation of co-located
adjacent channels.
To gather data on the possibilities afforded by the
use of a spurious signal rejection filter, it was suggested that field
tests be conducted for the subjective evaluation of video and audio
quality and to determine technical feasibility criteria for an analog
channel operating adjacently to other analog channels, whether
co-located or not, in the UHF band. The intent is to use the results
of tests with co-located channels to establish maximum ERP operating
values for analog stations transmitting over adjacent channels,
according to the sensitivity of television receivers to interference
caused by intermodulation at those devices’ RF inputs. In the second
case, it is to verify appropriate protection ratios for upper and
lower adjacent channels in the operation of transmitters equipped with
filters that can reduce spurious signals to at least 70 dB below the
peak power of the video carrier. The co-location of analog
transmitters operating on adjacent UHF channels, or simply improving
the protection ratio for non-co-located adjacent channels, would pave
the way for the formation of digital channel frequency blocks, which
would minimize the aforementioned possibilities of interference.
The level of saturation of the radioelectric
spectrum in various metropolitan areas, together with the diversity of
transmission sites, makes it difficult to co-locate a channel for
terrestrial transmission of digital television with its adjacent
analog channel. It encourages interference that could degrade the
quality of the analog service now in place.
Reducing the transmitted power of digital channels,
which is necessary to prevent interference with their adjacent analog
channels, but which results in less coverage than that provided by
analog channels simultaneously carrying the same programs, affects the
length of the transition period--and also its cost, should it be
necessary to install additional stations to boost the signals of the
digital channels.
Therefore, in order to include the evaluation of
protection ratios between non-co-located adjacent analog channels in
terms of the reduction of spurious signals in terrestrial analog
television transmission, and to include digital television
transmissions by using a digital television transmitter as a source of
interference, Anatel published Public Survey no. 263/2005, which
proposes experiments using the following criteria and infrastructure:
a) adoption of Recommendations ITU-R BT.654 and
ITU-R BS.1116 as primary points of reference;
b) installation of three co-located television
transmitters (no more than 400m apart), operating on UHF channels
“n-1”, “n,” and “n+1,” in a location with a high density of
radioelectric signals;
c) installation, over 2000 meters away, of a
transmitter capable of operating on any of the three channels, solely
for adjacency tests of non-co-located channels;
d) adaptation of each transmitter used in the tests
so that:
i. adjacent channels have spurious signal
rejection filters;
ii. emission of spurious signals on adjacent
channels is lower than 70 dB with respect to the video carrier’s
peak power level and all other technical characteristics specified
in the regulations remain unchanged;
iii. the audio carrier level can be adjusted to
between 10 and 17 dB below the video carrier level in maximum
increments of 1dB; and
iv. a transmitter power adjustment allows the
peak power to be reduced by up to 20 dB; and
e) with channels “n-1,” “n,” and “n+1” in
operation, proceed with evaluations and measurements in all possible
configurations (only channel “n” activated; channels “n-1” and “n”
activated; channels “n” and “n+1” activated; and the three channels
activated simultaneously).
Pedro Humberto de Andrade Lobo
Gerente de Regulamentação Técnica e Administração de Planos
Superintendência de Comunicação de Massa
Anatel
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