Abstract: A scrambler for a television system alternately inverts the phase of the carrier which is modulated by the television signal. A binary coding signal controls the manner of the carrier-phase inversion. Phase inversion is obtained by combining the modulated normal carrier with an out of phase carrier having a maximum peak amplitude greater than the peak amplitude of the modulated carrier. The coding signal also frequency modulates the carrier for the scrambled television signal thereby providing a key for decoding at the television receiver. The decoder at the receiver reproduces the coding signal from the scrambled signal, removes the frequency modulation and combines the scrambled signal with a reproduced normal carrier at the time intervals corresponding to the presence of the phase-inverted carrier.

Abstract: A jamming circuit for making unusable the television signal of a preselected channel includes a bridged T-trap filter having a pair of high Q circuits adapted to be tuned to the carrier frequency of the preselected channel, and a device for cyclically sweeping each tunable circuit through the video carrier frequency for the preselected channel. Each circuit is varied approximately .+-. 500 KHz from the carrier frequency, the cyclic sweeping rate being 14 KHz for one tunable circuit and 16 KHz for the other. Cyclic tuning is achieved by using varactors as part of the tuning capacity for each circuit, the varactors receiving a sawtooth waveform produced by two free-running oscillators. As a result of the jamming circuit, the television signal is modulated at a 14 KHz, 16 KHz and 2 KHz rate, thereby producing severe amplitude and phase distortion.

Abstract: An improved scrambler and decoder for a television signal is described wherein a television carrier is inverted in phase and subjected to low frequency amplitude modulation to obtain enhanced channel security. The carrier amplitude modulation is obtained with a coding signal which is transmitted as a pilot signal in the form of frequency modulation of the carrier. The coding signal is recovered at the decoder with a phase lock network and used to regenerate an unscrambled carrier. The coding signal provides information for special functions such as subscriber access control, program selection and the like.