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Disc cutting

The signal from a tape player (or from a recording console, in case of direct-to-disc recordings) is fed to the disc cutter and modified to the RIAA recording characteristic. It is pre-sampled 1.1 seconds ahead of the main playback head to provide a control signal for the cutter-head radial drive, or the main cutter signal may be delayed. By this means groove spacing can be varied according to the amplitude of the signal, thereby achieving maximum playing time. Spacing can vary from 130 to 390 grooves per inch. A heated triangular chisel, actuated by two helium-cooled coils. cuts the groove in a lacquer-coated aluminum disc 14 inches in diameter for a 12 inch record and 10 inches for a 7 inch.