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Comment by kens | Hacker News Reader

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

Hey Ken, great read as always. I wonder if in future you would consider doing an overview of the various early radio chips and their evolution. I recall recently reading some HAM projects and understanding that a lot of the later radio chips were clones of earlier designs. Given your suggestion that this earlier period of integrated radio innovation is 'low hanging fruit' in terms of RE-friendliness, it should be an interesting read and I'm sure a very large number of radio enthusiasts would love to see your insights.

It would be cool to look at more early radio chips, but I have a lot of other projects to do first...

The separate noise source is a bit of surprise here. Why is it necessary? Wouldn't RF noise produce same results?

It depends, if the RF frequency you use has a signal on it then it won't be random so it's not really noise. I wonder why they need a noise generator in a receiver chip though.. They're usually used for crypto stuff.

It's to provide "comfort noise" when the correlator indicates a missing or mistuned signal.

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

Haha yes I know CB radio. I used it for many years and I'm still a ham. It was a lot like social media yes. Never thought of it that way. Also, BBS'es. And the combination thereof which was packet radio.

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.

You would hear the usual white noise between stations, just by virtue of FM reception relying on high IF gain ahead of a limiting stage. But judging from the description of the block diagram, I suspect the noise you'd hear when tuning in a station would be very unfamiliar, maybe a lot of distorted tones and whistles. I can't believe they'd have gone to so much trouble to hide it otherwise.

It's a much more interesting chip than it initially appeared to be, that's for sure.

I'm not sure what the FM demodulator produces when it's mistuned, but I'm guessing that you'd get pretty much no output, rather than white noise (since there's no frequency for the demodulator to lock onto). The problem for the user is that you wouldn't know if your batteries are dead or if you just haven't found the station. By adding a "hiss" between stations, the radio has better usability

If RTL-SDR is a good reference - when demodulating FM it produces pretty much the same noise you'd expect from a mistuned oldschool radio.

In a conventional radio, yes, but I'll bet this approach would sound incredibly awful if mistuned.

I wondered about this too, but from the linked articles it seems to be designed that way in order to make it more user friendly: when not correctly tuned to a station, it outputs the artificial white noise instead of a possibly distorted signal from a nearby frequency (or just silence if the demodulator can't lock on to anything).

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