Local exchange equipment for a cable network

Claim 1

Switching system comprising an exchange and a number of subscriber posts connected to said exchange through a cable network, whereby each subscriber post comprises selector means for generating selection signals for selecting one out of a number of program or data signals accomodated into channels received in the exchange over a multiplex line, and said exchange furthermore comprises for each subscriber post a mixing stage and a corresponding voltage controlled oscillator with a voltage memory, and furthermore comprises means for scanning the selection signals of the subscriber posts by means of a multiplexer, means for deriving a frequency signal, related to the nominal oscillator frequency, out of each selection signal, means for deriving a signal from the oscillator assigned to the corresponding subscribing post through a multiplexer and means for comparing said derived oscillator signal with said frequency signal related to the nominal oscillator frequency to generate a control signal for the voltage memory assigned to the measured oscillator which control signal is delivered thereto through a multiplexer for adjusting the oscillator such, that the program- or data signals chosen by means of the selection signals are transmitted from the exchange through the mixing stage and the cable network to the corresponding subscriber, characterized in that each of the subscriber posts comprises a buffer memory for storing selection signals and the exchange comprises means for distinguishing a new selection signal from an unaltered selection signal, a selection signal memory in which the new selection signals received from the buffer memories in the subscriber posts are stored, a conversion memory for deriving a digital channel indication signal out of a selection signal from the selection signal memory, a digital to analogue converter for converting said digital channel indication signal into a control signal for the related voltage memory, a frequency measuring unit for measuring each time the frequency of one of the voltage controlled oscillators, and a comparator for comparing the output signal of the frequency measuring unit with the corresponding digital channel indication signal and deriving a correction signal for the related voltage memory, whereby in a first circuit the buffer memories in the subscriber posts are scanned in a predetermined sequence and the read out new selection signals are stored into the selection signal memory, that in a second circuit which is in principle independent from said first mentioned circuit each time one of the selection signals is read out from the selection signal memory and is converted by means of a conversion memory into the digital channel indication signal which is used to adjust the related voltage memory through the digital to analogue converter, that in a third circuit in principle independent from said first and said second circuit the voltage controlled oscillators are scanned in a predetermined sequence and the frequencies thereof are measured by means of the frequency measuring unit, after which the value delivered by the frequency measuring unit is each time compared with the channel indication signal, corresponding to the related oscillator and supplied by the conversion memory, whereby a deviation between both results into a correction signal delivered to the voltage memory.