By introducing certain inaudible distortion into the audio wave-forms used to modulate an s.s.b. transmitter it becomes possible to detect any frequency translations in the system. Audio frequency circuits in the receiver generate an a.f.c. voltage proportional to any such translation, and and this is used automatically to correct the frequency of the local carrier generator without the need for pilot tones or carrier. At present the system has a lock-in range in excess of ± 200 Hz, and a locked-in error of a few hertz. It may make practicable s.s.b. operation at v.h.f, as well as simplifying the operation of h.f. equipment.