(57) The frequency of the "isofrequency" radiophonic signal (fI) is converted by beating together with a fixed frequency generated by a local oscillator
(12) controlled by a reference oscillator (14) according to a constant factor. By
using band-pass filtering, an intermediate carrier frequency signal (fP), equal to the difference between the frequency of the radiophonic signal and the
fixed frequency, is extracted and transmitted over the radio relay system together
with the reference frequency. At the receiving end, the signal received from the radio
relay system is filtered in two band-pass filters (20, 22) so as to isolate the intermediate
signal and the reference frequency. The fixed frequency is reconstructed from the
reference frequency by multiplying by the above mentioned factor, and the intermediate
signal is reconverted by beating together with the fixed frequency; filtering so as
to reobtain the original radiophonic signal. The method can be extended to the transmission
of a plurality of radiophonic signals with a single reference frequency on a same
radio relay system.
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