Making a master disc
To make a master disc for reproduction, the sound is picked up by a number of microphones and the electrical signals are recorded in from 2 to 48 separate tracks on magnetic tape.
The tape is then edited on a complex electronic mixer on which the recording technician can modify the tonal quality and loudness of each track.
The producer may wish to increase the volume of a particular instrument, for example. In this way, the multi-track recordings are mixed to produce a two-track master tape with the sounds blended and balanced for the best effect from the left and right stereo channels.
The master disc is then cut with electrical signals fed from the master tape.
On the disc-cutting lathe, a chisel-shaped cutting stylus vibrates and cuts a wavy groove in the recording surface (generally a layer of lacquer on a flat aluminium disc), in a spiral from the edge to the centre. The disc is spun at precisely 33.3 revolutions per minute (rpm) to make a long-playing record, or 45rpm for a short single record, but the speed of the cutting head varies with the strength of the signal, and is faster when the signal is louder.
The stereo signals force the stylus to vibrate so that it cuts a different pattern in each wall of the V-shaped groove. The groove walls are at 45 degrees to the disc surface but at right angles to each other.
There may be up to 350 grooves to 1in (140 to lcm) - the number varies according to the loudness of the recording. A loud passage needs more space because the stylus vibrates more, so produces fewer but wider grooves.
After the sound groove has been cut in the lacquer, the master disc is electroplated with nickel and processed to eventually produce a very thin reverse-image nickel disc, called the stamper, which is the mould for making records for sale.
The records you buy are moulded PVC (polyvinyl chloride). Granules of PVC are simultaneously pressed and heated between two separately recorded stampers one for each side of the disc - then cooled. Each long-playing record pressing takes about 25 seconds.