Pulse-pause modulation
The procedure known colloquially as pulse-pause modulation is a coding procedure for analog values in model making radio remote controls . The abbreviation PPM and the English term pulse position modulation is also used for pulse phase modulation . This leads to confusion, since the differences between the two methods are not always clearly pointed out.
Pulse-pause modulation is a baseband modulation method for data transmission . The analog variable to be transmitted is coded as a pause between successive pulses . The impulses are of the same height and duration. The main difference to pulse phase modulation is that a reference clock does not have to be estimated by the receiver or transmitted on a separate channel, because the previous pulse provides the time reference. The pulse-pause modulation can be understood as differential pulse-phase modulation.
The PPM was primarily used in model making radio remote controls to transfer the target position of servos from the radio remote control to the model. N +1 pulses were sent for N channels . A longer pause separates successive samples. In the illustration opposite, the deflection of the servomotors of channels 2, 4, 5 and 6 is in the middle position. The servo on channel 1 has a shorter position, the servo on channel 3 has a longer position.
In RC model making , eight values / positions / sizes are usually transmitted by radio per sequence . The eight pauses are between 9 pulses of 0.5 ms duration and usually a ( TTL ) level of 5 V. Each pause is 0.5 to 1.5 ms long. The baseband signal is modulated onto a carrier using binary shift keying .
In the model there is a receiver set to its frequency , which waits for the first pulse after a> 1.5 ms pause in order to change the servo channel with each rising pulse edge of the sequence and to give the servo a pulse the length of the time between the rising pulse edges to send.