Optical TDM transmission system

In an optical TDM transmission system in which a set of N tributary channels at one bit rate are multiplexed on to a single multiplexed channel at N times the bit rate of the tributary channels, an amplitude modulation with a fundamental frequency matched to a subharmonic of the tributary bit rate is impressed on the multiplexed signal. At the demultiplexer this modulation is detected to provide a clock for separating the tributary channels in a manner which directs specific tributary channels to specific outputs without having to have recourse to reading any of the transmitted data itself.