Pay television system

#The present invention relates to pay television systems. Known systems of this type may be divided into two categories. In the first, the television signal itself is encoded and the coding changed for each program. In the second category, no encoding of the television signal takes place, but the frequency of the carrier is changed so that the signal cannot be received on a particular station on the standard television receiver unless a decoder is activated. The former system requires a great deal of equipment since the timing and phase relationships must be very closely controlled. The second category requires considerably less equipment but results in very low security since a simple change of frequency will allow reception.

#The present invention is a pay television system. It comprises means for furnishing a standard video signal, and intermediate frequency carrier signal furnishing means for furnishing an intermediate frequency carrier signal. Modulator means modulate said video signal onto said intermediate frequency carrier signal, thereby creating a modulated intermediate frequency carrier signal. Switch means are also provided. First and second frequency converter means are connected to the modulator means for alternately changing the frequency of said modulated intermediate frequency signal to a first and second output frequency under control of said switch means, thereby creating an encoded television signal having, alternately, a first and second component at said first and second output frequency. Finally, means are provided for sending said encoded television signal to at least one receiving location.

#A standard television receiver is at said receiving location, as are first and second receiver means. The first receiver means is tuned to said first output frequency and responsive only to said first component of said encoded television signal, for receiving said encoded television signal and changing the frequency of said first component to a predetermined decoder frequency, thereby creating a first decoder signal. The second receiver means is tuned to said second output frequency and responsive only to said second component of said encoded television signal. The second receiver means receives said encoded television signal and changes the frequency of said second component to said decoder frequency, thereby creating a second decoder signal, said first and second decoder signals having the same frequency but occurring alternately, Finally, means are provided which are connected to said first and second receiver means, for combining said first and second decoder signals and furnishing a combined signal suitable for reception on said television receiver.

#The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof will thus be understood from the following description of specific embodiments when read in connection with the accompanying drawing.

#FIG. 1 shows the encoding and decoding systems of the present invention in block diagram form;

#FIG. 2 is a schematic diagram of the program code generator and switch control shown in FIG. 1; and

#In FIG. 1, the standard composite television signal including all synchronization signals is applied to one input of a modulator 10 whose other input is connected to an IF oscillator 11. The modulated intermediate frequency carrier signal at the output of modulator 10 is applied to the inputs of two frequency converters 12 and 13. Frequency converter 12 converts the frequency of the signal applied to its input to a first output frequency suitable for cable transmission, while frequency converter 13 changes the frequency of the intermediate frequency carrier signal to a second output frequency different from the first output frequency but also in the cable transmission range. The converters are enabled alternately by means of a switch control 14 which in turn is controlled by a program code generator 15. There is thus an output from frequency converter 12 only in the absence of an output from frequency converter 13, and vice versa. The two signals are combined in a summing unit 16 and applied to the cable for transmission to the subscribers. The outputs of frequency converters 12 and 13 may, for example, be in the UHF range. The intermediate frequency at the inputs of converters 1 and 2 would normally be 45 MHz.

#At the receiving end, the encoded television signal which has, alternately, a first and second component at the first and second output frequency respectively, is split by an RF power splitter 17 into two channels. In the first channel, a tuner-mixer 18 is tuned to the first output frequency, while in the second channel a tuner-mixer 19 is tuned to the second output frequency. In the first channel, the first component of the received television signal is heterodyned to the output of a local oscillator 20 such that an intermediate frequency signal, herein referred to as a first decoder signal, appears at the output of tuner-mixer 18 in response to the first component of the encoded television signal. Similarly, the second component of the encoded television signal is heterodyned in the second channel to the output of a local oscillator 21 whose frequency is so chosen that the intermediate frequency signal at the output of tuner-mixer 19, namely the second decoder signal, is of the same frequency as the first decoder signal. After amplification, respectively, in amplifiers 22 and 23, the first and second decoder signals are summed in a summing circuit 24 whose output is herein referred to as a combined signal. The combined signal is frequency converted to a VHF or UHF signal by a converter 25 cooperating with a local oscillator 26.

#It will be noted that the above-described system would allow a non-subscriber to see and hear the program only for the short interval that it is on one or the other of the RF frequencies. Such a segment is not suitable for viewing.