1. A PCM-system having a transmitting section (SE) for the parallel scrambling of n binary digital signals (BS1 to BS4), which are respectivement fed to a first input of n modulo-2-adders (MO1 to MO4) whose outputs, in order to form a serial binary signal (SB), are connected to the inputs (DO to D3) of a multiplexer (MUX) and whose second inputs (EM1 to EM4) are connected to the outputs (a, c, d, g) of a scrambler (SC) which are formed by the flip-flops (K1 to K7) of a shift register (SR) provided with feed-back ; and with a receiving section (EM) which corresponds to the transmitting section (SE) and which includes a demultiplexer (DMUX) and a synchronized descrambler (DSC), characterized in that the feed-back path of the shift register contains only one modula-2-adder (MO), that in order to produce a predetermined synchronizing sequence the transmission of the binary digital signals (BS1 to BS4) is blocked, and that the flip-flops (K1 to K7) have setting inputs which are combined via a common resetting input (R) and in order that the synchonizing sequence may be produced are set by a pulse in such manner that the pseudorandom sequences emitted by the scrambler, having been combined, from the synchronizing sequence at the output of the multiplexer (MUX).
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