LP's, 45's and the old 78's all record information in the form of a waggly spiral groove on each surface of the disc. This way of storing information uses
analog signals. As the record rotates, the groove moves a stylus from side to side in a pattern which is (or should be!) the same as the pattern of
pressure waves that make up the music. The record player turns these movements into a similar pattern of voltage variations which mimic (or are an
'analog of') the original sound pressure variations. This voltage pattern is then amplified and used to drive speakers which rattle the air in the room with
the same pattern. This recreates the sound of the music recorded on the LP.
The “45” and the “78” got their names from the rate at which they rotate — 45 and 78 revolutions per minute, repectively. Each could play for about three to five minutes per side. The “LP” or Long Playing record rotated at 33 and a third revolutions per minute and was called that because it could play for 20 or 30 minutes per side. The LP was therefore ideal for classical music, and the 45/78 for single popular songs. Although many hi-fi fanatics still insist that LPs “sound better” than CD's, the CD has essentially replaced the LP as a music recording medium.
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