4 ms·
> (1) A mono needle would not have been able detect the vertical movement, resulting in a missing channel. (2) A stereo needle able to play such a record would
by timmg 4y ago
> (1) A mono needle would not have been able detect the vertical movement, resulting in a missing channel. (2) A stereo needle able to play such a record would not have been able to play a classic mono record, because the mono signal would have only been translated into a single channel
I think the way they do it with FM radio is: one channel is A+B and the other is A-B. The A+B channel is the "mono" channel for backward compatibility. (Then they add/subtract to get left/right: (A+B)-(A-B)=B; (A-B)+(A+B)=A.)
Pretty elegant solution, IMHO.
- phkahler 4y agoI don't recall why or how, but the A-B channel is of lower bandwidth, or otherwise more likely to drop out. Hence when you're starting to lose FM, you may find it dropping to mono.
- timmg 4y agoI think the reason is less interesting: when you start to go out of range, your radio thinks there is only a mono signal, so it switches to mono mode. Same reason "static" on your TV is black and white. Your TV didn't detect a color signal, so it is looking for the black and white one. (For anyone old enough to remember static on their tv :)
- trelane 4y agoIndeed: https://en.wikipedia.org/wiki/FM_broadcasting https://en.wikipedia.org/wiki/FM_broadcasting "For this reason, the left (L) and right (R) channels are algebraically encoded into sum (L+R) and difference (L−R) signals. A mono receiver will use just the L+R signal so the listener will hear both channels through the single loudspeaker. A stereo receiver will add the difference signal to the sum signal to recover the left channel, and subtract the difference signal from the sum to recover the right channel." Other systems were considered by the FCC, but this one won (see article for the details) If you're interested in this sort of thing, the book Defining Vision is a great discussion of the history of HDTV.