A simple synchronization system is proposed for timing recovery in optical PCM repeaters. The basic idea is to exploit the spectral line at the transmission rate, which is already present in the unipolar symbol stream travelling along the fibre. Since a nonlinear circuit is no longer needed, the amplified signal at the photodetector output is directly filtered by a resonant circuit to give the clock recovery by means of a zero-crossing detector. The limits of effectiveness of this synchronization system are then discussed, especially with regard to the accumulation of timing jitter as well as of alignment error.