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Vinyl Recording Process


The basic principle of making records is simple. The movement of a microphone diaphragm is transformed into variations in the groove on a moving piece of vinyl. During playback a stylus tracing the variations exactly reproduces the motion of the diaphragm at the time the recording was made.

Electricity is really incidental to this process, which is essentially mechanical. Early gramophones were wind-up and used a cone to amplify the sound of the record vibrating as the stylus moved acroos it. Electricity merely serves as a convenient link between the microphone and cutter or speaker and pickup.

Most of the development in record technology has been devoted to getting as much music as possible onto a single record. The obvious approach, slow speed and a narrow groove, reached a practical limit in the middle of the century with the 33 1/3 rpm microgroove record. At that speed, (9 inches per second in the inner part of the groove) a 20 khz signal has a wavelength of .0004 inch. It is very difficult to manufacture a stylus that would handle wavelengths smaller than that.

The major consumer of space on the record is the low frequency content. This is because the amplitude of the electrical signal produced is proportional to the side to side velocity of the stylus. Given equal velocities, a low frequency variation will swing wider than one of high frequency because at low frequencies the cutter will not turn around as often as it does at high frequency. To counteract this effect, the low frequency content of the record is deliberately reduced, and this low end rolloff has to be corrected by a bass boost in the playback system.

The high frequency content is given a treatment opposite to that of the lows. High frequency information is emphasized during recording, and reduced during playback. This is an attempt to reduce the noise generated by the roughness of the vinyl. That noise is white noise, and as such sounds like a high frequency phenomenon. When the playback system reduces the high frequency content to its proper level, the noise in that range is reduced by the same amount.

The combination of bass roll-off and treble boost is called the recording characteristic, and the complementary response of the playback system is called RIAA equalization after the manufacturer's association which standardized this feature in 1956.

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