The invention relates to encryption apparatus for audio communication through a television system. Audio security is as important as video security. With the advent of satellite links, private two-way audio channels that can be associated with a video channel are desirable, for instance for business teleconferences. Secure audio scramblers are already known for telephone lines. With video channels, however, advantage can be taken of the television system facilities to reduce the cost of scrambling the audio signal in particular by using the randomizing and synchronizing capability of video scramblers already present. Also, speech quality in the scrambling-descrambling process may be improved so as to exceed the telephone-line bandwidth. It is also important to allow for more than one audio system with the same installation.
It is known to scramble a digit stream as part of a video digital transmission system. See for instance "Digital Transmission Techniques" by G. M. Drury pp. 37-49 in IBA Technical Review (Great Britain) 1976 Vol. 9.
It is known from U.S. Pat. No. 3,958,077 of J. Ross et al. to generate pseudo-random numbers with digital shift registers. The patent also shows how to perform pseudo-random scanning in a television system.
In a scrambled television system, U.S. Pat. No. 3,717,206 of V. R. Zopf et al. shows that for subscription purposes coded signals have been associated with the transmitted video signals. This is also shown for unscrambling in U.S. Pat. No. 4,070,693 of H. B. Shutterly.
A pseudo-random generator is disclosed in U.S. Pat. No. 3,681,708 of M. E. Olmstead.
It is known from U.S. Pat. No. 3,105,114 of W. Koenig to divide into segments a signal and to introduce selected time delays between such segments for scrambling effect.
U.S. Pat. No. 3,659,046 of Angleri et al. discloses a message, in binary form, being scrambled in a pseudo-random fashion by logically combining bits.
From U.S. Pat. No. 3,731,197 of J. E. Clark it is known to sample an information-bearing signal and to read the samples in a prearranged abnormal order to obtain unintelligible secured signals.
U.S. Pat. No. 3,824,467 of R. C. French shows an audio signal divided into segments which are rearranged into a new sequence before transmission. Encoding-decoding is provided by binary pseudo-random addressing of storage devices. Each storage device transmits its stored time element while storing a new time element.
U.S. Pat. No. 3,921,151 of G. Guanella teaches dividing an audio signal into segments of equal time intervals which are temporarily stored. Scrambling of the segments by reading-out in a random pattern is achieved so that at each location there is no repetition, nor omission.
U.S. Pat. No. 3,773,977 of G. Guanella, which patent is related to aforementioned U.S. Pat. No. 3,921,151, shows the use of coinciding aperiodic cipher signals from which control signals are derived for determining the storage elements, together with automatic monitoring of the occupancy to avoid omissions, or repetitions.
It is known from U.S. Pat. No. 3,819,852 of Peter Wolf to transmit an audio signal in time-compressed form during the period of a line in the vertical blanking interval of a television system.
The object of the present invention is to add an audio scrambler to a television system, while using to a maximum extent the video installation for scrambling and unscrambling in the transmission and reception of an audio message.