The DSR-4520X is 2U and is mainly used for IP data over it's secondary ethernet port. For example, it's used with HBO networks for broadcasting video on demand to video servers, They are mpeg2 files delivered via multicast through the secondary ethernet port. It has regular video, audio, and ASI and DHEI output as well.
The DSR-4500X is an 2U receiver that is basically the same as the 4520X except it has no secondary ethernet output for multicast IP data. It has an ethernet port for SNMP monitoring. It also has the same outputs as the 4520X as well as the 8 RF inputs (can only use one at a time).
The DSR-4402X is a 1U receiver with two RF inputs , audio, video, and ASI outputs. It has no ethernet port for monitoring or multicast IP data.
I'd get the DSR-4402X because it's 1U, and would be cheaper that the two others because it has no ethernet port, and you don't need it for your application. But, if you are getting a receiver used, get whichever one is cheaper, and the digicipher II number is intact, that the number on the sticker on the bottom of the receiver matches what's in the menu. If it's not the same, it will have to be repaired to get any authorization. The 4402X doesn't have a seperate video output for monitoring with diagnostic OSD screens, it just has a general video outputto connect to the modulator.
