stereo records

[Fred Flin]

a stereo record works like this - horizontal motion on a stylus provides a signal for one speaker. vertical motion on a stylus provides a signal for the other speaker.

a mono record - this only has horizontal motion and the signal is sent to both speakers.

how does a record player know if you're playing a mono record, or a stereo record with a silent bit on one channel.

[kempie]

The description you give for how the channels are split on a stereo record is incorrect. That system was proposed but never adopted; instead the Westrex 45/45 single groove system was utilised.

http://en.wikipedia.org/wiki/Gramophone_record#Stereo_and_beyond