Having got to the point where I can listen to the output of the receiver in real-time, I decided to spend some time trying to improve the quality of the received audio. Here's a recording of the result: receiver-test3.mp3

This is noticeably better than the output I shared previously. I think most of the improvement comes from using a better antenna, though this is still just a random wire. I also added about 6 dB more gain (to the ADC buffer stage) in order to use more of the available ADC resolution. (I might be using something like 7 bits now rather than 6.) It's also possible that I did a better job of setting the various DC offset adjustments of the receiver than I did before.

However, there's still room for improvement. In particular, there's some background hiss and a distinctive periodic noise pattern, both of which can be heard in the quieter section of the recording above. I'm not currently using a de-emphasis filter, which I assume would help with the hiss. Beyond that, I don't yet know which aspects of the receiver I need to improve to further improve the audio quality. Maybe someone reading this will recognise the periodic noise as being indicative of a particular problem, and point me in the right direction!