Above: Mono groove, magnified 150 times.
Above: Stereo groove, magnified 150 times, displaying the same piece of music.
Above: Human hair, magnified 150 times.
Putting it on Record The sound that is recorded on this tape, in terms of magnetic impulses, must now be transferred to record. It is passed through amplifiers, curve benders and other exotic electronic devices to the cutting stylus of a vertical lathe. A soft lacquer disc, 14 inches in diameter for the production of a 12-inch record, is placed on the turntable. The cutting stylus is heated, and, as it vibrates in response to the signal from the tape, it cuts a groove in the surface of the disc. This groove itself is a miracle of engineering. Half a mile long in the case of a 12-inch record, and narrower than the thickness of a human hair, its minute indentations, or "squiggles", contain all the information needed for a pick-up head to reproduce every nuance of the orchestral performance.
Spacing between grooves is not uniform, A loud passage on the tape causes the stylus to produce wider squiggles (both wider and deeper in the case of stereo), and these could impair neighbouring grooves-causing echo and even breakdown of their walls-if they came in too close proximity to them. To cater for the irregular volume of sound, another electronic device scans the tape ahead of the cutting stylus, at a distance of about one complete revolution, and warns the stylus to anticipate what is to follow. The technique of variable groove spacing, as this process is called, makes the most economical use of the space available on the record by keeping grooves closer together during quiet passages and increasing the distance between them to accommodate loud ones.
