Most trunked radio systems have the channels they use spread out over a couple MHz worth of spectrum. In order to be able to record more than one broadcast at once, you pretty much need to have an SDR that has enough bandwidth to cover the lowest channel in the system and the highest on at the same time. For the DC Fire & EMS system, the lowest channel is at 854.86250 MHz and the highest is at 860.98750 MHz. This means that your SDR needs to have 6+ MHz of bandwidth. This isn’t too much for most modern SDR, like HackRF, BladeRF or one of the Ettus. These are great SDR, but they cost atleast $300.
The lowly RTL-SDR is a $20 SDR that is just a repurposed USB TV dongle. While it makes for an OK SDR, it only has ~2MHz of bandwidth. When I re-wrote Trunk Recorder, I designed it to support using multiple SDR, but I never really tried it out, so I decided to give it a try.
One of the reasons I was interested was to see if I could get better CPU performance. There is a lot of CPU time dedicated to cutting down the full bandwidth that is coming from the SDR into the small sliver that is interesting. This has to be done for each of the “recorders” that is actively trying to capture a channel. When I was using a single SDR, each Recorder had to take in the full 8MHz and pull out the small 12.5KHz that was interesting. The end results is that I could only record about 3 channels at once before the CPU got overloaded. Since that control channel was going at the same time, that was the equivalent of about 32MHz of bandwidth to process.
With the RTL-SDR, each Recorder only has to look at 2MHz, which puts a lot lighter load on the CPU. Roughly speaking, having 3 Recorders active, plus the control channel would mean that only a total of 8MHz was being processed. As you can see, this means that it scales much more efficiently.
The new code for multiple SDRs has been merged in, give it a try!
A couple notes on my setup…
First, start out with an RTL-SDR dongle that has a TXO. This helps make sure the tuning is pretty accurate and will not drift as it gets warmer. These dongles from rtl-sdr.com have been great and the antenna they come with seems to be pretty good.
Also, make sure you assign your dongles each a unique serial number. You can do this with the rtl-eeprom -s command. Using this serial number, you can define different error corrections values for each dongle. I have notice they will very by a few hundred hertz between dongles. There is an example of how to do this in the multi-sdr config file.
The other thing I got was a USB 3 hub. Having more than 2 USB 2 dongle connected to a USB 2 hub might max out the bandwidth. I am not sure if this is necessary, but I wanted to limit the places where things could get screwed up.