Alec Watson of Technology Connections took an amusingly forthright look at how vinyl records hold sound and how a single stylus can create two separate channels for stereophonic sound. Watson explains the history of early recordings, how we wound up with pressed discs rather than cylinders, and how the stylus’s movement affects the record’s imprinted sound.
Stereo records do contain a combination of vertical and lateral movement in their grooves, but it’s a bit more complicated than
“…the left channel is vertical movement and the right channel is lateral movement.” That would work, but not very well. …To detect this two-dimensional movement, a stereo phonograph cartridge has two pickups placed at right-angles to one another. Both pickups share the single stylus, and this whole contraption is set at a 45 degree offset with respect to the groove. With this arrangement, each pickup is affected by both vertical and horizontal movement.
While acceptable, Watson states that this setup is not perfect, particularly in this digital age of lossless audio.
Now, this method is not perfect. Complete stereo channel separation just isn’t possible when your sound is coming from physical movements. Even if you nailed the angles just right and the record was mastered perfectly, some vibrations from one of the pickups are liable to make their way through the cartridge and into the other. So no matter how staunchly someone defends the sound of vinyl, records are objectively pretty bad at isolating the two channels.