Abstract: A synchronous time division multiplex (STDM) system for sampling 1 through N digital input data signals from their respective 1 through N input channels at their respective frequencies F.sub.1 through F.sub.N for concentration and retransmission over a single output channel at a frequency F.sub.0 >.SIGMA. F.sub.n is disclosed. The data bit stream of each input channel is alternately coupled to first and second similar sections, A and B, of each input-channel-associated concentrator at fixed successive sample times T.sub.S. While the input data bits are coupled to and stored in the first section A during the sample time 1T.sub.S, the input data bits previously coupled to and stored in the second section B during the immediately previous sample time OT.sub.S are being retransmitted in their concentrated form on the output channel. On the next successive sample time 2T.sub.