The comments about PCM are due to the failsafe and not to PCM. PCM code can be sent without checksums and the Rx simply convert the binary code back into PPM for the servos, including all the errors in the binary code. Even by adding a checksum and detecting an error does nothing, since it could be programmed to detect the existence of an error but still pass it on to the servos anyway. PCM can be made without error detection and without failsafe and it will glitch and crash the same as PPM. A few years ago I was at a demo by Philips about CD technology and the difference between digital and analogue. They demonstrated a CD player in which they could turn off the error detection and error correction circuits and let you hear raw digital music. It was worse than a scratched vinyl record, the digital data on a CD is very poor and if passed on to the speakers without the "failsafe" it shows all the "glitching" of analogue. It is only useable when the CD's version of failsafe is switched on.

It is the failsafe mechanism, not PCM, that does the brief hold that avoids glitches, and then goes into programmed positions. For this both PCM and PPM failsafe are exactly the same, the only difference is their methods of detecting errors. Failsafe uses two sets of memory. In one set it stores the pre-programmed settings for full failsafe, and in the other it stores the last good data packet which is constantly being erased and replaced with the next good data packet. When the error is detected the faulty data is discarded and the last good packet is taken from memory and sent to the servos instead. This avoids momentary glitches. After some time if the incoming data is still corrupted the data from the pre-programmed memory is sent to the servos instead. This sytem is absolutely common to both PCM and PPM failsafes. It is totally independent of the data modulation type, only the error detection mechanism is different between PPM and PCM.

I use exclusively PPM failsafe system. It has all the same features as PCM with failsafe. It does not glitch and if the corruption lasts 1/2 second it goes into the programmed failsafe positions. The PPM failsafe Rx is lighter, smaller and cheaper than PCM, it gives full status to 12 channels at a frame rate that PCM systems do not yet match, and if I should ever change my brand of Tx it will work just as well with another brand, I am not locked into one system. It also gives me the higher resolution of PPM, I have demonstrated my PPM system resolving to greater than 1400 places, though in practice given the fuzziness of servo pots and gear slop this is somewhat academic.

FMA has gone even further. The system I use looks for errors in the servo data. FMA has developed a system that also looks for errors in all the data that is not servo position data, such as numbers of channels in the frame, length of synchro pulse and so on, all the little bits that create a fingerprint of your individual Tx that distinguishes it from other Tx even on your frequency. They claim a very secure link between Tx and Rx because of this.

H