> When the chip is not tuned to a station, the chip replaces the audio with a white noise source.

Wow, I always thought the background noise was just natural when it was off-station, like on a TV.

Author here for if you have questions on this chip...

The correlator is interesting. I don't see how it works. In the perfectly-tuned case, how does delaying the signal by half an (IF?) period and inverting it yield a match for the original signal? Inversion isn't the same as a delay.

I guess the idea is that the 70 kHz IF is effectively sampled at 2x the necessary Nyquist cutoff needed for 15 kHz baseband audio. So the signal content at half the period can be relied upon to match after an inversion and delay, assuming it was (a) band-limited at the source (or by the clever deviation-reduction scheme), which it would be; and (b) tuned correctly.