How vinyl works.
Until very late in the 1950s, all records were mono (one channel). On a mono record, the stylus tracks lateral movement (side-to-side squiggles) only — the groove is a consistent depth, representing the sound wave. When record companies saw a market for two-channel stereo recordings, the vertical dimension of the groove (up-and-down hills and valleys) represents the most obvious solution — to use “lateral” as one channel and “vertical” as the other — presented at least two problems. For one, it would make stereo records incompatible with mono turntables (you’d only get one side instead of a summation of both). Another challenge: Large vertical excursions (hills) make for difficult tracking. The last thing we want is a ramp that launches the stylus right out of the groove.
Ultimately, a brilliant solution was conceived, albeit one requiring a bit of clever mathematics. With a little pencil-pushing, “left” (L) and “right” (R) channels can be rearranged as “mid” (M) and “side” (S).
“Mid” contains everything that’s the same in both L and R, and “side” contains everything that’s different between them. To encode into mid-side: (L+R)/2 = M and (L-R)/2 = S. To decode back into stereo, M+S = L and M-S = R.
If the stereo signal is converted into mid/side for cutting, both aforementioned problems are solved. The “mid” channel moves in the lateral side-to-side dimension, resulting in perfect compatibility on a mono player. Remember: M = (L+R)/2. The “side” signal moves in the vertical dimension, and as long as bass frequencies are kept in-phase and close to the middle of the stereo field, there will be few large vertical excursions
REFERENCES:
Butcher. M (2018). How does vinyl work? Retrieved from: https://www.londonsoundacademy.com/blog/how-does-vinyl-work
Williams.B.A (2017). Vinyl: How It Works (and What That Means for You). Retrieved from https://flypaper.soundfly.com/produce/vinyl-how-it-works-and-what-that-means-for-you/
Ediriwira. A (2014). Incredible photos of recording grooves under an electron microscope. Retrieved from https://thevinylfactory.com/news/incredible-photos-of-record-grooves-under-an-electron-microscope/
Noise above our heads. 