All servos (except higher frame rate dedicated tail rotor gyro servos), use standard PPM data. PCM Rx have to convert the signal back to PPM for the servo outputs. There is no such thing as a PCM servo because no PCM Rx outputs PCM data.
...the signal to the servo is a PWM signal not PPM or PCM...
and as far as the JR rx's go 549's and 649's have problems with "noise" the 549 is worse than the 649...
a switching regulator can cause problems with either one of those Rx's and on the 549, digital servos even give it a fit..