T.E.M. A07B500S, Руководство пользователя

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A07B500S manual
Pag. 58
transmitter, but this is often noticed immediately: if listening is improved at one point by regulating the delay
of the two signals with a precision of microseconds, listening is inevitably worse at another point.
Ultimately, the area of overlap in which high quality listening is possible covers fewer square kilometers. It
is immediately obvious that the only condition in which synchronized devices can operate is that created on
the Milan-Rome highway in Italy: transmission is entirely in mono; the transmitters are weak in power and
are positioned in a straight line a few kilometers apart. The antennas are Yagi-type, and are all positioned in
the same direction; all have an increased front-to-back ratio, each radiation pattern has a delay equalizer, and
modulation is completely synchronous. In this model, the zone of reciprocal interference is reduced to areas
of a maximum of a few hundred meters in which two, and no more, transmitters are found. The position of
such a zone is just behind each antenna.
A European Resolution exists which helps to clarify limits that can be reached by these types
of systems. (Rec.ITU-R BS.412-9 planning standards for terrestrial FM sound broadcasting
at VHF)
ANNEX 3 reads: Protection ratio for FM sound broadcasting in the case of the same programme and
synchronized signals.
This regulation is easily downloaded from the Internet and relates to the topic in scientific terms, publishing
results and limits which
should be
referenced in order to assure high listening quality. Here below are the
conclusions reached from the recommended regulations for those about to install a synchronous system.
Conclusion
On the basis of the collected data and of the results obtained, it is possible to plan a synchronized
monophonic network for special applications with protection ratios of only 2 dB, provided that the
relative time delay between the modulating signals is maintained within 5 µs over the whole area to
be served and that the maximum deviation does not exceed
?
75 kHz.
Thus in the case of co-channel interference, the protection ratio evaluations made for synchronized
wanted and interfering transmitters broadcast the same programme, give values very much below
those indicated in Fig.1.
In the case of stereophonic mode there is a much more intense influence of the stereophonic content
and the delay value. On the basis of the complementary investigations made in Italy it can be
assumed that:
– the minimum reference value of protection ratios for stereo audio signal in isofrequency and isomodulation should
not be lower than 16 dB for impairment 4 in the assumption of the delay equalization within 10 µs;
– in the reception areas affected by noticeable propagation delay, or for musical excerpts with a high stereo content,
the protection ratio required for quality 4 impairment grows up to about 30 to 38 dB respectively for continuous
interference.
Further evaluations should be pursued for a larger number of configurations including
transmissions with multiplexed data signals.