Quick update, managed to get a gps lock using the focusrite Scarletts. My only problem now is that my exciters are different models (rvr Tex and rvr TEX-lcd) and I cannot get the audio to sync on air. Will probably try to get two identical transmitters to solve this, since no matter the delay adjustments I'm doing in uMPX nothing seems to make them sync up.
I doubt that that's the transmitters. Have you also connected a loopback so MicroMPX can measure the exact delay that the OS and sound card introduce?
It might help (but it's not really necessary) to use identical hardware on all the decoders. (Same PC, same OS, same sound card). But the loopback connection should take care of this.
Yes everything is connected properly, loopbacks are all ok and everything. The only difference in the computers is the cpu. The rest is all the same(motherboard etc).
At this point the only thing that is not the same is the transmitters, so I concluded that it would be my problem. Is there something else I can try? Should slightly different volume levels play a part in this somehow? Because both transmitters have analog dials I can't get them to match exactly.
When you say you cannot get them to sync on air, what exactly do you mean? Do you hear a difference in timing? Or do you mean that the combined signal doesn't receive properly?
We have seen before that there can be *very* small (but constant) differences in timing, of maybe 2-3 microseconds, which you would need to compensate for manually. You can't hear it (it's less than 0.1 sample at 48 kHz), so to see these very small differences you would need to measure it. But these differences should be small enough to not interfere with reception in the center between the two transmitters (it could move the center by a few hundred meters though). More importantly, if the overlap area between the transmitters is not in the center, you need to compensate for the speed of light (I think it's 3.336 microseconds per kilometer). It's easiest to first verify that the decoders are in sync in one location (I can't think of any reason why they wouldn't be, if the loopback is connected and used, but it's always good to verify things), then move them to the target locations and adjust for the distance.
For the transmitters: For optimal results, the modulation should be exactly the same. So, ideally, the transmitters themselves should be identical, and also locked to GPS. But even without that, just syncing the MPX signal should already improve the reception.