Gramophone Record - Structure

Structure

The normal commercial disc is engraved with two sound-bearing concentric spiral grooves, one on each side, running from the outside edge towards the centre. The last part of the spiral meets an earlier part to form a circle. The sound is encoded by fine variations in the edges of the groove that cause a stylus (needle) placed in it to vibrate at acoustic frequencies when the disc is rotated at the correct speed. Generally, the outer and inner parts of the groove bear no intended sound (an exception is Split Enz's Mental Notes).

Increasingly from the early 20th century, and almost exclusively since the 1920s, both sides of the record have been used to carry the grooves. Occasional records have been issued since then with a recording on only one side. In the 1980s Columbia records briefly issued a series of less expensive one-sided 45 rpm singles.

The majority of non-78 rpm records are pressed on black vinyl. The colouring material used to blacken the transparent PVC plastic mix is carbon black, which increases the strength of the disc and makes it opaque. Polystyrene is often used for 7-inch records.

Some records are pressed on coloured vinyl or with paper pictures embedded in them ("picture discs"). Certain 45 rpm RCA or RCA Victor "Red Seal" records used red translucent vinyl for extra "Red Seal" effect. During the 1980s there was a trend for releasing singles on coloured vinyl—sometimes with large inserts that could be used as posters. This trend has been revived recently with 7-inch singles.

Since its inception in 1948, vinyl record standards for the United States follow the guidelines of the Recording Industry Association of America (RIAA). The inch dimensions are nominal, not precise diameters. The actual dimension of a 12-inch record is 302 mm (11.89 in), for a 10-inch it is 250 mm (9.84 in), and for a 7-inch it is 175 mm (6.89 in).

Records made in other countries are standardized by different organizations, but are very similar in size. The record diameters are typically nominally 300 mm, 250 mm and 175 mm.

There is an area about 6 mm (0.25 in) wide at the outer edge of the disk, called the lead-in, where the groove is widely spaced and silent. The stylus is lowered onto the lead-in, without damaging the recorded section of the groove.

Between tracks on the recorded section of an LP record there is usually a short gap of around 1 mm (0.04 in) where the groove is widely spaced. This space is clearly visible, making it easy to find a particular track.

Towards the centre, at the end of the groove, there is another wide-pitched section known as the lead-out. At the very end of this section the groove joins itself to form a complete circle, called the lock groove; when the stylus reaches this point, it circles repeatedly until lifted from the record. On some recordings (for example Sgt. Pepper's Lonely Hearts Club Band by The Beatles, Super Trouper by Abba and Atom Heart Mother by Pink Floyd), the sound continues on the lock groove, which gives a strange repeating effect. Automatic turntables rely on the position or angular velocity of the arm, as it reaches these more widely spaced grooves, to trigger a mechanism that lifts the arm off the record. Precisely because of this mechanism, most automatic turntables are incapable of playing any audio in the lock groove, since they will lift the arm before it reaches that groove.

The catalog number and stamper ID is written or stamped in the space between the groove in the lead-out on the master disc, resulting in visible recessed writing on the final version of a record. Sometimes the cutting engineer might add handwritten comments or their signature, if they are particularly pleased with the quality of the cut. These are generally referred to as "run-out etchings."

When auto-changing turntables were commonplace, records were typically pressed with a raised (or ridged) outer edge and a raised label area, allowing records to be stacked onto each other without the delicate grooves coming into contact, reducing the risk of damage. Auto-changers included a mechanism to support a stack of several records above the turntable itself, dropping them one at a time onto the active turntable to be played in order. Many longer sound recordings, such as complete operas, were interleaved across several 10-inch or 12-inch discs for use with auto-changing mechanisms, so that the first disk of a three-disk recording would carry sides 1 and 6 of the program, while the second disk would carry sides 2 and 5, and the third, sides 3 and 4, allowing sides 1, 2, and 3 to be played automatically; then the whole stack reversed to play sides 4, 5, and 6.