No no no. MPX got rid of PCM years ago (it is correct that PCM transmission schemes are proprietary). All MPX modern transmitters will transmit primarily in PPM (I know the Profi 4000 still has a MPX PCM mode in case you have old MPX PCM receivers). Why would you want to use a PCM receiver if you own an IPD receiver? IPD is an intelligent version of PPM that will give you the benefits of PPM with the fail safe of PCM. If Ernie says that another brand of crystal will work in a MPX receiver and you get a good range check, then I would be confident that all is well as you will have the best transmission scheme available. I have been flying MPX IPD for well over two years now (IPD 9 for .75 years and IPD 12 for the rest) and never an issue with the INCREDIBLE range that I have NEVER had from Kraft, JR, or Futaba!