In a mechanical or electromechanical recording. The track inscribed in the recording medium by the cutting or embossing stylus. In a stereo disc, the two walls are cut at 45″ angles from the perpendicular. Each recedes or advances independently of the other in sympathy with the recorded sound, the outer carrying the right-hand and the inner carrying the left-hand signal. When the signal in both channels is equal and in phase, the groove moves laterally, but when they are out of phase it moves vertically. Two generators in the pickup are so mounted as to respond only to one plane of motion of the stylus. Groove width thus varies, but it should not be less than a specified 1 mil. In practice 2 mil is the observed minimum to avoid groove-jumping with cheaper equipment.
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