I want to call out two minor points from this discussion.
The main point is to distinguish an SDR's sample rate from it's usable bandwidth. They are not one to one. Using the Realtek SDR running at 2.4 Msps as an example given earlier, there is aliasing and band pass filter roll-off that occurs at the edges of the SDR's sampled spectrum. For the typical SDR, that means the top ten percent and bottom ten percent of spectrum are not usable. If you take eighty percent of 2.4 Msps, you get a net usable of 1.92 MHz. When figuring out how many SDRs to stack for full site coverage, you need to consider each SDR's usable bandwidth.
This next point comes from boatbod's comment:
Fundamentally, P25 passes signaling on both the control channel and voice channels, so no matter where the radio is tuned, it can keep and eye on all voice activity and retune if a higher priority call is identified.
This will work on small sites (say 5 channels) but not so on 20+ channel sites. P25 link control messages are confined to a very small portion of the voice channel's data bandwidth. If you pick an arbitrary voice channel and monitor all of the link control activity, you will miss some of the events published on the control channel. Having this extra information certainly helps - particularly if you're on a budget and want to run only one SDR - but not to the same degree as monitoring the control channel full time.
Here are some numbers to help understand. The control channel publishes up to about 40 events per second. During an active call, a P1 voice channel puts out one link control message every eighteen IMBE voice cells. Each cell is 20 milliseconds so that's one message every 360 milliseconds or less than three messages per second.
If a call comes through on a more important talkgroup while you are monitoring a different call, the soonest the voice channel can notify you is 180 milliseconds later. It might take longer or not happen at all.
I don't want to diminish a very useful feature provided by op25, DSD+ and other programs. I just want to clarify what's possible on a P25 system.
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