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Old 06-03-2003, 10:04 PM
  #34  
HarryC
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Default Should PCM receivers really cost more?

Originally posted by OhD
The update rate is determined in the transmitter
The Multiplex transmitters might be faster but not by much. If you are happy enjoy IPD but don't tell anyone it is better than PCM.
Mpx is not any faster as it uses standard PPM. Mpx claim that PPM is faster than PCM, I have always had doubts about that. Since each brand has its own PCM system which PCM are they comparing to? I guess it is to their own PCM which they no longer make. I believe early PCMs had a slower update but more modern versions give differing priorities to which channels get updated in each cycle and therefore can keep up the refresh rate on those channels that need it. I don't know for sure but I have a sneaking suspicion that the Mpx PCM was a bit outdated and therefore slower than the more modern systems, hence Mpx claim for PPM.

I, for one, have not said that IPD is better than PCM, though some others have said it. We went through this a while back (on the jet forum I think it was) and I pointed out that PCM does have an advantage over IPD. Since IPD is PPM it relies on a synchro pulse and uncorrupted data all the way through each cycle. Any corruption loses all subsequent data in that cycle even if it was good data since the Rx loses where it is in the cycle and needs to wait for the next synchro pulse. If the synchro pulse alone is hit and the Rx can't find the start of the cycle, all the good data for all the servos is lost for that entire cycle. Some PCM systems break the cycle into two halves each with its own PCM version of synchro so you don't lose as much data if it cam't synchronise, and if each servo's data is individually identified rather than just relying upon the sequence, any little hits on one servo don't necessarily scupper the data for all the subsequent servos in that cycle. So when interference is short in time and repeating rapidly, such as spark ignition, PCM still has an advantage as although it experiences the same hits on an individual servo, it does not tend to lose all the following servos' data in that cycle like PPM does.

H