Abstract: An ignition timing control system for an internal combustion engine (18) varies the timing of a magneto (14) in accordance with a pre-programmed ignition advance schedule. An optical pick-up (22) senses the passage of slots (26) in a disc (24) which is rotated at the speed of a distributor (16) and produces constant angular width reference pulses corresponding to fixed ignition timing in the engine's cylinders (13). A magnetic pick-up (36) detects incremental rotation of the engine's crankshaft (33) and produces timing control signals proportional to engine speed. The timing control signals and reference pulses are converted to a multi-bit data word which is used to synthesize an analog control function representing variable ignition timing using a pre-programmed digital to analog converter (58).