A real time telemetry system is disclosed for use with a drilling system including a rotary valve 28 controlling the supply of drilling fluid or mud to a downhole tool. The system comprises receiving data to be transmitted, encoding the data as a duration, and controlling the rotation the rotary valve 28 such that the rotary valve 28 is rotated for the said duration at a predetermined rotary speed to cause the formation of pressure fluctuations or waves in the drilling fluid or mud.