Q: Why is one channel always higher in level than the other?
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Flashback to the 1970s: Bob takes his recently-acquired girlfriend, Carol, out for a meal. He brings her home to his apartment where they both have the intention of 'getting to know each other better'.
Bob puts a record on the record player - Barry White of course - and they settle on the sofa. Uh-oh - twenty minutes later the needle is in the run-out groove, irritatingly clicking away. Bob jumps up and turns the record over.
Twenty minutes later, the same thing happens and Bob has to choose another album.
The evening is punctuated every twenty minutes or so, and although Bob and Carol are enjoying each others' company, somehow they just can't get really comfortable.
Anyone who was in this situation in the 1960s or 1970s will remember it vividly. Compact disc came as a godsend, lasting up to 74 minutes or more without having to change sides, but of course Bob and Carol were married by then and they were re-collecting all their old albums on CD so they could enjoy the occasional nostalgia trip.
The question of course is why must the duration of a vinyl record be so short? The principal limiting factor is the level of the audio. The record has to be cut at as high a level as possible to make sure the signal is well above the noise, to give it a good signal-to-noise ratio. But the higher the level, the greater the 'excursion' of the groove. The excursion is the extent to which the groove 'wiggles' from side to side.
The greater the excursion, the wider the spacing between the grooves has to be, and therefore the shorter the duration of the record. Twenty minutes is about the maximum duration of a side without lowering the level to a point that is less-than-optimum. Records have been made with durations of up to 40 minutes a side, but only at the expense of low levels, and therefore significant background noise.
(By the way, Bob and Carol's son Ted has now grown up. When he brings his girlfriend Alice home, he puts on an MP3 CD that he compiled himself. Just the one disc lasts 12 hours. Ted and Alice are very happy).