Pulse position modulation, or PPM, is a signal modulation used for digital and analog transmissions. PPM is used mostly for optical communication systems such as fiber optic and infrared remotes, when efficiency is needed and there is little chance of interference.
In PPM, data transmitted in short pulses has the same width and amplitude. The PPM changes the delay between pulses.
Modulating a digital signal's pulse position is straightforward. Each delay between pulses is represented by a zero or a one. A small delay is represented by zero, a long delay is represented by one. The duration of delay varies according to the system's requirements. For example, in a Sony infrared remote protocol that uses PPM, a short delay of 1.2 meters per second is represented by zero, and a longer delay of 1.8 meters per second is represented by one.
PPM is very sensitive to external interferences. Although interference is usually impossible to detect, it can cause complete data corruption. Therefore, PPM is not used in cable communications, which are subject to electromagnetic interference. PPM is, however, used in fiber optic cables, which are not subject to interference.
Another disadvantage is that signal decoding requires a transmitter with a perfectly synchronized clock, but the signal does not provide the decoder with a way to reconstruct the clock. That is why differential pulse position modulation is used. D-PPM is a form of PPM that transmits data without a clock. D-PPM provides efficient data transmission due to its compressed format.