TDM SYSTEM WITH MEANS FOR CROSSTALK REDUCTION BY CHANGING THE SLOT POSITIONS OF THE CHANNELS AFTER EACH FRAME PERIOD

1,024,338. Automatic exchange systems. SIEMENS & HALSKE A.G. Sept. 22, 1964 [Sept. 23, 1963], No. 38522/64. Heading H4K. The time slots of pulses gating a channel over a time division multiplex system are shuffled from one cycle to the next so as to render unintelligible the cross-talk which would exist between channels using invariable time slots. In a system having a synchronizing channel O and twelve time slots for channels 1 to 12, Fig. 1a shows the fixed adjacency of channels accorded invariable time slots. As shown in Fig. 1b time slots are shuffled according to a scheme in which the " odd and even" time slot pairs of a cycle are interchanged to produce the pattern of the next cycle, the end time slots of that next cycle being left undisturbed while the " even and odd" pairs are interchanged to produce the pattern for the following cycle, and so on. Where a line identity is stored in parallel in a circulating delay line system VS, Fig. 4, the delay lines having a delay time of T - T, T being a time slot cycle and # being the width of a time slot, the line identity can be recirculated via gates Sf to be advanced one time slot in the cycle, or it can be recirculated via a delay of # and gates Sn to maintain its position, or it can be recirculated via a delay of 2# and gates Ss to be delayed by one time slot in the cycle. The pulses s, n, f, necessary to effect the shuffling referred to above, are produced by a circuit BG described in detail with reference to Figs. 5 and 6, not shown. By use of tappings on a magnetostrictive delay time at points T - #, T, and T + #, the channels may be shuffled in the same way. With this arrangement, and in combination with a series to parallel converter, the line identity may be circulated serially. Since the shuffling process interferes with the frequency of operation of the line gates these gates may all be operated at the time slot P 0 to control in accordance with speech level a storage device which is gated to the highway by pulses in the shuffled time slots. Where line identity is stored in a magnetic cored matrix read out to control the line gate under control of an access register, the accessing may be done at random to produce the shuffling.