Please note: this is describing the original (2011-2014) site which monitored the analog SmartNet-based city of Santa Clara radio system.
The original Super Trunking Scanner production backend used a USRP1 with a DBSRX2 daughterboard for radio reception. This photo shows that unit with some extra antennas installed for another daughterboard.
This is all running on a normal (but beefy) Linux box. A fair amount of CPU power is required to keep up with the decoding work.
The backend was a custom project built on top of GNU Radio to decode the control channel and drive audio logging handlers. It took the raw I/Q data arriving from the USRP (about 115 Mbps, 24 hours a day) and saved it to separate calls.
Unlike ordinary scanners, this system receives all channels simultaneously and can record all of them in parallel if necessary. Even if multiple talkgroups are active -- police and fire, for example -- you'll still hear every call.
The development of this project was documented in a series of web log posts. Start here for the details.