Playback of the Recorded IQ Data

Yesterday I described how to record the IQ data from the USRP focusing on generating unique file names every time the GNU Radio / GRC script is executed. Playing the recorded IQ data is really simple: We take the corresponding receiver application and replace the USRP source with a file source and a throttle block.

The recorded IQ data is a simple binary file containing nothing more than the received samples. There is no information about frequency or sample rate which is why we need a throttle block to maintain a constant sample rate in the receiver.

My experimental application was forĀ  reception of APT signals from the NOAA weather satellites so my playback application is a WFM receiver. You can get the example .grc file here.

GNU Radio Companion flow graph for playing back recorded IQ data

And here is a video showing the playback in action. The receiver can be used exactly the same way as when it receives the samples from the USRP. A really great and very useful feature in GNU Radio!

Author: Alexandru Csete

Embedded software engineer in the satcom industry during the day. Radio amateur and SDR hacker during the night.