Audio recording Systems

Charles Cros, a French scientist, produced a theory concerning a phonograph, but he didn’t manufacture a working model. It was Thomas Edison who produced a working model. Edison conceived the principle of recording and reproducing sound in 1877 as a byproduct of his efforts to “play back” recorded telegraph messages and to transmit them by telephone.

Edison’s early phonograph recorded onto a tinfoil sheet phonograph cylinder using up-down motion of the stylus. The tinfoil sheet was wrapped around a grooved cylinder, and the sound was recorded as indentations into the foil. Edison’s early patents show that he also considered the idea that sound could be recorded as a spiral onto a disc, but Edison concentrated his efforts on cylinders, since the groove on the outside of a rotating cylinder provides a constant velocity to the stylus in the groove.

Alexander Graham Bell and his two associates took Edison’s tinfoil phonograph and modified it considerably to make it produce sound from wax instead of tinfoil. They began their work at Bell’s Volta Laboratory in Washington, D.C., in 1879 and continued until they were granted basic patents in 1886 for recording in wax.

It took many years and further improvements before the recording industry became a major factor in home entertainment. Disk recording is neither better nor worse than cylinder recording in potential audio fidelity. But there were commercial advantages to a disk system since the disk could be easily mass produced by molding and stamping and it required less storage space for a collection of recordings.

For a long time hi-fi recordings have been produced on vinyl gramophone records. Records use an analogue recording system, which stores patterns by cutting a continuous groove in a vinyl disk. The sound can be reproduced by spinning the record and using the movement of a metal needle in the groove to produce varying magnetic fields. These magnetic fields are then processed to produce the sound. A typical LP (long-playing record) has a recording capacity of about 45 minutes.

A digital recording system, known as a compact disc (CD) system, was introduced in 1982. This uses a laser optical mechanism in which a laser beam reads marks on the surface of a specially prepared perspex disk. It gives near-perfect reproduction of sound and the sound quality does not deteriorate with use. Some of the problems associated with vinyl records are eliminated such as “crackle” caused by dust and static, and “jumping”, due to scratches on the recording surface.

In a CD system, a recording is made by electronically sampling the sound 44, 100 times every second. The electronic samples are used to control a laser beam, which makes a pattern of very small pits in the surface of the plastic disc. The audio pattern is represented by the length of the pits and the distance between them. The pits are arranged in circular tracks. A typical CD has about 20, 000 circular tracks and a maximum recording capacity of 74 minutes.

To play back the recording, the disk is made to revolve at a constant speed and a laser beam is directed at its surface. The varying reflection of the laser beam is fed into a digital-to-analogue converter (DAC). This produces the electronic signals, which are amplified to drive a loudspeaker.

In 1989, sales of compact disks (CDs) exceeded sales of long-play albums (LPs) for the first time. By 1990, CD sales were more than double those of LPs. Cheaper CD players and the introduction of mid-price and budget-price discs have been partly responsible for the increase in CD sales.