Patent GB-1528273-A (GB)
Title: Methods of and apparatus for the encoded transmission of information
IPC: H04B14/04
Filed: 1976-02-24
Abstract: 1528273 Time division scrambling system PATELHOLD PATENTVERWERTUNGS- & ELEKTRO HOLDING AG 24 Feb 1976 [26 Feb 1975] 07290/76 Heading H4R In a secret transmission in which signal sections of equal length are interchanged in timing at the transmitter and rearranged to their original timing at the receiver, the interchange and rearrangement is effected by introducing each signal section into a store, allotting each section an age number, which is incremented by one after each section length of storage time, and generating corresponding age signals at each station, the age signals changing in a pseudo random fashion within certain limits. At one station the age number allotted to each section initially has one extreme value and alters stepwise until it corresponds with a simultaneously produced age signal, whereupon the section is withdrawn from the store, transmitted, and replaced by a new section of the signal, while at the other station the age number allotted to a signal section newly applied to the store is the same as the age signal appearing at that instant and it is thereafter altered stepwise until it corresponds to the second extreme value, whereupon it is withdrawn from the store and replaced in the store by the new element, the withdrawn elements being reassembled in their original order with a total delay corresponding to the difference between the two extreme age number values. As described, at a transmitter Fig. 4, age signals dB are compared by comparators K with the outputs of counters Z and when coincidence is established the appropriate switch U is operated to feed the existing signal section out of the corresponding circulating store R and feed a new signal section in to replace it. Simultaneously the respective counter Z is reset, so that the count always corresponds to the age of the signal section in the corresponding store. The age signals must be such that the maximum age limit is not exceeded and that no omission or repetition of individual elements occurs, this is ensured by an age corrector AK Fig. 6 (not shown) which operates on age signals derived from pseudo random encoding signals by logic circuits. At the receiver, Fig. 5, similar age signals, after correction in corrector AK* Fig. 7 (not shown) are fed into a counter Z, to reset the counter to the corresponding age, when that counter reaches its maximum count, at the same time as the respective store R has its contents fed out and a fresh signal section is fed in from the line. The signal corrector AK at the transmitter has an arrangement which rejects age signals which do not correspond to an age in any of the counters Z and substitutes the succeeding age, and also detects the reaching of the maximum permitted count in any of the counters, overriding the normal age control and instead causing immediate transmission of the signal section corresponding to that counter. At the receiver the age corrector AK* Fig. 7 (not shown) rejects age signals which correspond to the instantaneous age of a signal segment already in the storage device, by comparison of the age signal with the state of the counters K at that instant, and bring forward the succeeding age signal, and also provide a direct connection from input to output if no segment of maximum age, as indicated by counters K, is present in the store. In an alternative signal segment storage arrangement Fig. 9, the stores Q are arranged in series, with switches U by which the circulation of signal segments can be interrupted and one segment fed out and replaced by a new segment from the input line X. The switching signals j for such an arrangement can be derived from the switching signals i for the parallel store arrangements of Figs. 4 and 5 by a simple transposition in unit RQ, Figs. 9a, 9b, 9c (not shown). Similarly switching signals developed for a series storage arrangement can be transposed (in a unit QR, Fig. 10, 10a, 10b, 10c, not shown). In an alternative method of deriving the switching signals, Fig. 17, the correction circuit AK takes the form of a model of a coder utilizing a shift register SR 1 , having a total delay equal the total delay of the system overall. Such a coder is wasteful of delay capacity since only half the stages are occupied at any time. The model is used to determine the age signals which are then converted in the arrangement AP to the form suitable to operate a system of stores VE having optimum storage utilization. In the age signal corrector the register SR 1  is initially occupied by address impulses or empty impulses, corresponding to a signal element awaiting transmission or a signal element already transmitted in a block of successive signal elements. Between each transmission of a signal element an age signal "a" is checked with the contents of the register, first to determine whether the last stage P 16  is full, in which case that address, corresponding to maximum storage age, must be transmitted, then the other stages P 11  to P 15  are checked in turn to determine that the age signal a<SP>1</SP> corresponds in time to a storage location which contains an address impulse rather than an "empty" impulse. If the appropriate address impulse is present in the selected location that address impulse is transmitted to the code converter to produce, according to its time of arrival, an appropriate digital signal dB, if they are not present then the next a<SP>1</SP> signal is called up and checked after feed of a valid age signal to the output line as signal dB, the impulses in the store are moved one place up, and a new address pulse inserted in the first stage P 11 . The age-location converter AP stores the ages of the signal sections contained in stores R 1 , R 2  and R 3 , in a store formed by the register SR 2 , stage SU and the register SR 21 , the ages being stored as three digit binary numbers, together with the zero age value, obtained by transfer of the input signal direct to output through switch U o . Stage SU is used to increment the stored ages at each step of the basic clock, i.e. at each signal section transmission, and the register SR 21 , holds each age value in turn, once each step of the basic clock, to compare the stored ages with the age signal input dB. When a match is found a coincidence signal K is produced which is inserted into register SR 3  which is stepped at the same time as the stored age values are stepped through store SR 21 , so that the final position of the pulse k in register SR 3  depends on the position of the match found in the age storage registers, and so determines the switching of the appropriate one of coding switches U o  to U 3 . The occurrence of coincidence signal k also causes operation of gate T 3  which transfers the contents of store FS 1 , representing a null signal, into the register SR 21 , which corresponds to setting the age number to zero, representing the new age of the store contents. For operation at the receiver switches RW, and RW 2  are set to the position B so that the comparator KO is fed with a signal from fixed store FS 2  corresponding to the maximum age so the coincidence is detected whenever the age signal S m  in register SR 21  reaches the maximum age, causing the signal to be applied via shift register SR 3  to the appropriate switch U; and the gate T 3  to transfer into register SR 21  the fresh age signal dB to replace the maximum age signal therein. The age corrector of Fig. 17 may be replaced by an arrangement more like the arrangement used with Figs. 4 and 5, Fig. 19 (not shown). Also described are arrangements for generating age signals with a required probability distribution Fig. 14 (not shown) and arrangements in which the age signals are stored with the signal sections, and in which the signal section signals, in sampled digital form are stored interleaved with each other, Fig. 25 (not shown). It is also shown that the arrangements described above use simultaneously appearing age signals at the transmitter and receiver. If however the roles of transmitter and receiver are interchanged the equipment will function satisfactorily provided the equipment used at the receiver delays the age signal by the maximum delay of the system.