Adding some details for others who may be interested: The above is a good thing to keep in mind! And I was a little worried things wouldn't work at first..
However, I've found that unless you want to receive 100% of transmissions, even when they are concurrently transmitting at near opposite ends of the system frequency range, a single SDR will still yield great success! My RTL-SDR V3 is stable at 2.8M sample rate (a bit more than the 2.4M I see most places), so if we assume we are centered at control channel 853.925MHz, the tuner window is around:
853.925-(2.8/2) ~ 853.925+(2.8/2) -> 852.525MHz ~ 855.325MHz
In practice, here is a range of voice channels my receiver is hearing:
The system's range appears to be roughly:
855.4625−851.15 -> 4.3125MHz
This is larger than the approximate 2.8MHz bandwidth available from the tuner. But the op25 software seems to shift the tuner window to listen to voice channels, while trying to keep the control channel inside the window. I have noticed that while listening to a transmission on 851.15MHz, a higher-priority talkgroup starting at a much higher frequency successfully preempts the current transmission. It must be still receiving control data, despite listening to the low end frequency!
How is it doing this? If we're listening to 851.15MHz, the lowest observed system frequency, we don't need to center on that frequency. It just has to be inside the tuner window. Perhaps the software shifts the range down to roughly:
851.15 ~ 851.15+2.8 -> 851.15MHz ~ 853.95MHz
If my research is correct, P25 Phase 1 channels are 12.5kHz bandwidth. So it appears the 853.925MHz control frequency is still inside the above window. We're not missing control data, even though we're listening to a voice channel at the low end of the system range! We certainly could be missing transmissions on a different talkgroup, happening at the opposite (high) end of the system. But for my purposes, that is totally fine. I'm really only interested in ensuring it can hear the control channel at nearly all times, so my talkgroups of interest can be heard. I have also whitelisted my talkgroups and applied priorities, so there is less chance of missing what I want to hear.
Of course all of this is subject to change, depending on which control frequency happens to be active: If the control channel was 853.9625MHz instead of 853.925MHz, we would be missing control data while listening to the lowest frequency 851.15MHz. The only thing left to help is that op25 makes use of trunking data transmitted inside the voice channels. I imagine that's not perfect, so perhaps I will add a second SDR to use multi_rx.py in the future.
Anyways, I'm still quite new to all of this, so I welcome any corrections. It's so nice to hear crystal clear Raleigh chatter again!
