Any large-scale digital equipment, such as a computer, requires a number of timing waveforms. In general, these are produced by central pulse-generating equipment and are then distributed throughout the system. In the design of transistorized systems, certain problems arise, principally owing to the limited power output of transistors and their relatively slower switching speeds compared with hard valves. As a result, considerable difficulty is experienced in ensuring adequate synchronism between the different waveforms. An automatic method of maintaining synchronism by means of servo control system has therefore been developed. The output waveforms are examined and compared, and correction signals are generated which control variable-delay devices inserted in the appropriate amplifier chains.