STRUCTURE
The
normal commercial disc is engraved with two sound bearing concentric
spiral grooves, one on each side of the disc, running from the
outside edge towards the centre. Since the late 1910s, both
sides of the record have been used to carry the grooves. The
recording is played back by rotating the disc clockwise at a
constant rotational speed with a stylus (needle) placed in the
groove, converting the vibrations of the stylus into an electric
signal, and sending this signal through an amplifier to loudspeakers.
The
majority of records are pressed on black vinyl. The colouring
material used to blacken the transparent PVC plastic mix is
carbon black, the generic name for the finely divided carbon
particles produced by the incomplete burning of a mineral oil
based hydrocarbon. Carbon black increases the strength of the
disc and renders it opaque.
Some
records are pressed on coloured vinyl or with paper pictures
embedded in them ("picture discs"). These discs can
become collectors' items in some cases. 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 and has succeeded in keeping
7" singles a viable format.
Vinyl
record standards for the United States follow the guidelines
of the Recording Industry Association of America (RIAA).[3]
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
300 mm, 250 mm and 175 mm.
There
is an area about 6 mm (0.25?) wide at the outer edge of the
disk, called the lead-in where the groove is widely spaced and
silent. This section allows the stylus to be dropped at the
start of the record groove, without damaging the recorded section
of the groove.
Between
each track on the recorded section of an LP record, there is
usually a short gap of around 1 mm (0.04") where the groove
is widely spaced. This space is clearly visible, making it easy
to find a particular track.
Towards
the label 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 Spice by Eon), 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 raises the arm and moves it out
of the way of the record.
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