A subscription television system includes means broadcasting a scrambled television signal encoded with p-bit program and subscriber authorization codes and m-bit memory write and read codes. Each subscriber is provided with a decoder having (2.sup.m) p-bit memory locations, the decoder being operable for storing a broadcast p-bit subscriber authorization code in a memory location defined by the broadcast m-bit memory write code for updating the decode authorization status of the subscriber and being operable for comparing a broadcast program code with a subscriber authorization code stored in a memory location defined by the broadcast m-bit memory read code for developing a decode authorization signal to effect unscrambling of the broadcast television signal.

A poach-resistant system enabling cable television subscribers to receive upon request certain programs otherwise unavailable (i.e., "secure") while precluding such reception by non-requesting subscribers as well as non-subscribers. Before transmission of such a secure program the roster of subscribers is scanned at a central control station having complied subscribers' requests for that program, and a resulting string of pulses corresponding in sequence to the respective subscribers is punctuated with program command pulses timed to designate those subscribers who have requested the program. Transmission of the resultant program command signal string over the cable to junctions with leads to the respective subscribers' television sets results in unblocking of such junction for each of the requesting subscribers only. Allocation of plural clocking pulses to each subscriber enables program availability to be tiered as desired. Appropriate circuitry in the junction of each subscriber's lead with the transmitting cable retains control there so that unauthorized program signals never enter a subscriber's premises, where security could be defeated.

A technique for toggling the coding mode in a television signal encoder comprises establishing a rectangular window defining a relatively small and substantially centrally located portion of the field image intervals of the television signal. The coding mode of the encoder is toggled whenever the APL of the television signal occurring during the window exhibits a change for successive video fields exceeding a predetermined threshold level.

An LCD driver compares an image signal with a count of a counter repeatedly counting a clock. Based on the result of the comparison, the LCD driver outputs pulses each having a duty factor in accord with the image signal. When the count is not between first and second predetermined numbers, the frequency of the clock supplied to the counter is switched from a fundamental frequency to a low-frequency, thereby controlling the width of a drive voltage of a liquid crystal display cell and correcting the image signal in association with the S-shape characteristic of optical transmittance of the liquid crystal display cell. This LCD driver is advantageously simple in structure.

A system for controlling the acquisition of encrypted program signals such as television or other signals utilizing existing equipment in use in telecommunications networks. The transmitted program signals are encrypted subject to decryption for presentation by a receiver such as a television receiver. A subscriber unit associated with the receiver receives the signals and is connected to the telecommunications network. The subscriber unit also receives program order commands from a subscriber and responsive to such a command transmits to the telecommunications network an order which enables the telecommunications network to identify the directory number to which the subscriber unit is connected and the number of the subscriber unit. Upon validation the telecommunications network returns to the subscriber unit a first decryption key which is stored in the subscriber unit. At substantially the same time the telecommunications network delivers to the program provider the order and identification of the ordering entity. The program provider returns via the telecommunications network to the subscriber unit a second decryption key and signal activating the first decryption key. Upon commencement of transmission of the ordered program an auxiliary signal is transmitted to the subscriber unit activating the second decryption key. The activation of both decryption keys and the use thereof provides decryption of the encrypted program signal and delivery of the decrypted signal to the television receiver. The signaling through the telecommunications network does not require call connection.