Don't forget to give redsea something to decode!  For output from a USB 
software-defined radio stick, you may want to consider installing 
comms/rtl-sdr or another SDR tool of your choice that can feed raw I/Q 
data on stdout.  

You can also provide redsea with hex data from RDS Spy, raw ASCII 
bitstream, or an audio file containing a multiplexed FM signal.

Fuller details are contained in the redsea wiki:
https://github.com/windytan/redsea/wiki/Input-formats