Pulse-position modulation
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Pulse-position modulation is a form of signal modulation in which the message information is encoded in the time delay between pulses in a sequence of signal pulses.
This method is not used very frequently, because other methods are usually more suitable for typical data transmission. However, PPM is often used for the radio control of model aircraft, boats and cars. Here the position of each pulse represents the angular position of an analogue control on the transmitter, or binary switch positions can be represented by distinct pulse positions. The number of pulses per frame gives the number of controllable channels available. The advantage of using PPM for this type of application is that the electronics required to decode the signal are extremely simple, which leads to small, light-weight receiver/decoder units. (Model aircraft require parts that are as lightweight as possible). Servos made for model radio control include some of the electronics required to convert the pulse to the motor position - the receiver is merely required to demultiplex the separate channels and feed the pulses to each servo.
More sophisticated R/C systems are now often based on pulse-code modulation, which is more complex but offers greater flexibility and reliability.
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