| 摘要 |
<p>1. A central receiving station for a cable TV network, comprising a receiving antenna (1) which feeds several so-called variable gain channel amplifiers (2 to 17), these amplifiers being each adjusted to a different TV channel and being connected by their respective outputs to the inputs of a channel mixing multiplex (18), the gain of each amplifier being adjusted automatically by so-called error signals (e) which are individually obtained for each channel amplifier (2 to 17) by comparing a characteristic signal sampled from the output of said amplifier with a common reference level, by means of a comparator (31), each error signal, delivered as a voltage, being selectively applied to a voltage storing circuit (49) which is individually associated to a channel amplifier, the gain of which should be controlled by said error signal, characterized in that this station is equipped with a control card (21) which is common to all the channel amplifiers and which mainly comprises a mixer (24) connected on the one hand to the output (S19) of the multiplexer (18) and on the other hand to the output of an inner local oscillator (25) the frequency of which can be adjusted cyclically and step-by-step under the control of an internal clock (32, 33) which ensures a selection of a channel amplifier in view of its adjustment the mixer (24) signal being applied to an extractor circuit (30) which extracts the amplitude level of the TV synchronization pulses, via a detector of the envelope curve, such that the characteristic amplitude signal is obtained which is delivered to the comparator (31) producing each error signal.</p> |
