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.
The problem is that if you mis-tune the radio by 100 kHz or so, the FM detector will give you an output, but it will be distorted. The issue is that the FM detector is linear over a small range, but outside that range, you get non-linear side lobes. So if you tune to a side-lobe frequency, the radio will lock onto the frequency, but the output will have harmonic distortion. In this case, the IF frequency is way off from 70 kHz, enough that the delayed signal and the inverted signal don't match at all, so the correlation fails and mutes the audio. Then you'd re-tune and find the right frequency.
[1] See Figures 8-12. Link: https://www.tel.uva.es/personales/tri/radio_TDA7000.pdf
Wow, I always thought the background noise was just natural when it was off-station, like on a TV.
kens•6mo ago
magnat•6mo ago
wkat4242•6mo ago
CamperBob2•6mo ago
Muting the audio would make more sense -- and would certainly have been familiar to the CB[1] radio operators of the day in the form of a squelch effect -- but this chip was targeted at consumers who expected it to behave like a conventional FM radio.
1: An early incarnation of social media, for better and worse
wkat4242•6mo ago
But I didn't think of this because it's an analog receiver. I thought it would just receive noise in the absence of a signal like its older brethren did.
CamperBob2•6mo ago
It's a much more interesting chip than it initially appeared to be, that's for sure.
kens•6mo ago
magnat•6mo ago
CamperBob2•6mo ago
rep_lodsb•6mo ago
contingencies•6mo ago
kens•6mo ago