It sort of depends on what your frame of reference is. L+R and L-R makes sense if you are referencing to horizontal/vertical motion. The walls of a stereo recording are at 45 degrees to horizontal. If you reference to the slope of the walls it appears as if you get each wall moving back and forth for each channel.
Running one channel with reversed phase provided 2 benefits. Horizontal motion was automatically mono. Vertical motion cancelled out common signal. Bass tends to be mixed centre, and if it was so mixed, would not act to bounce the stylus out of the groove.
There became something of a chicken and egg here. Bass was mixed to the middle, and so music reproduction systems started to assume all bass information was intrinsically common. But if you say look at a symphony orchestra, the bass drums and tympani are off to the sides. A rock band might have the drummer in the middle. But there is more music than that to be recorded.
I’m not sure when automatic record changers (the metal stick in the middle of the turntable, on which you could stack several records, and they would drop one by one when the record playing currently finished), but mutiple-record sets/albums often had their sides numbered (e.g. a two-record set would have: record 1: sides 1 and 4; record 2: sides 2 and 3), so that you could stack the records and have them play the “sides” in order (assuming it mattered), and then flip the stack over and play the remaining sides.
As a youth in the 70’s, a member of the kiss army, and a wanna be guitar player, the 16 speed was the best thing that happened to me. My parents got a new stereo and I took the old one. It allowed me to slow the Destroyer album to half speed to learn the cool Ace Frehley licks!
My folks had one of those - probably purchased in the 1960s (but maybe before) and still working into the 1980s at least
(P.S. Anyone remember “Close and Play” - the record-player for kids without the manual dexterity to drop the needle onto the record? (the needle was built into the player’s cover - close the player and the record started))
Because of the way stereo records are made, a mono record can be played on a stereo system, with a stereo cartridge. The two signals from the two groove walls are identical, so each can be fed into its own channel, and you’ll get the same sound from each speaker.
Suppose a stereo record worked as you claim, with one groove wall carrying L+R, and the other carrying L-R. You’d get the left channel by adding the two signals together: L+R+L-R = 2L, and you’d get the right channel by subtracting them: L+R-(L - R) = 2R. With a mono record, though, the signal (let’s call it S) is identical for both groove walls, so if you played the record with this type of stereo system, the signal for the left channel would be 2S, and the signal for the right channel would be 0.
That’s not what happens. The recording system for stereo records was designed so that older mono records could be played with stereo equipment, and the same sound would come out of both speakers. That’s why a stereo record has each channel isolated to its own groove wall, with the modulation at 45 degrees.
You can view this system as having horizontal and vertical modulation. If you look at it that way, the horizontal modulation carries the signal L+R, and the vertical modulation carries the signal L-R. It would be possible to make a stereo cartridge with one magnet/coil that picked up only horizontal motion, and another that picked up only vertical motion, then added the signals to get the left channel and subtracted them to get the right channel. No one has ever done that to my knowledge, because it’s too complicated and has no advantages. All stereo cartridges have magnets and coils at 45 degrees, one for each channel.
Please watch the Technology Connections video that markn_1 posted. It does a good job of explaining how stereo records work.
In addition to the spoken word records, “Highway Hi-Fi” records–which were meant to be played on special record players (with damping system designed by Peter C. Goldmark of LP fame) in Chrysler cars–operated at 16 2/3 RPM. So if you had those records for your car, and you wanted to play them at home, you would’ve needed that setting on your turntable.
The records in the Seeburg 1000 Background Music System also played at that speed–however, those were for commercial use, so it is unlikely that the average consumer would’ve gotten their hands on those, back then (these days, you can probably find them on eBay).
Both those formats were used for music, and AFAIK, they worked just fine for that purpose.
