Sound Recording and Reproduction

I

Introduction

Sound Recording and Reproduction, conversion of the vibrations caused by sound waves into a permanent record that can be manipulated or played back in its original form.

The most common method of sound recording is to use magnetic media such as tape to store the music, speech, or other sound being captured. Using this approach, sound waves are transformed via a microphone into electrical signals which are then used to magnetize a plastic recording tape coated with metal oxide. The magnetization varies with the frequency and intensity of the sound being recorded—so the tape, usually in the form of an audio cassette, carries a permanent record of the sounds that were picked up by the microphone. As sound varies with time, some form of mechanical movement of the recording medium is required to provide a sequenced record that reflects the sequence of the original sound vibrations. This is familiar to most people who have watched a cassette run from beginning to end.

II

Mechanical Recording

The operation of a sound recording system can be most easily understood by looking at the now obsolete method of mechanical recording. In this method, sound waves are used directly to activate a stylus or cutter that engraves a pattern on a disc or cylinder that corresponds to the sounds being recorded. This process, with improvements, was used for many years in the production of gramophone records. In the direct method of mechanical recording, sound waves make impact with a very light diaphragm and cause it to vibrate in sympathy with the pitch and intensity of the sound. Attached to the diaphragm directly or indirectly is some form of cutter that vibrates with the diaphragm. Under the cutter is a disc of wax or some other substance that is moved past the cutter to allow a continuous recording. The result can be seen in the groove in a long-playing record. Under a microscope, it is clear that this groove varies in width and depth along its length. In this way the sound waves are captured.

The limitations of mechanical recording can be demonstrated by looking at what really happens in the above process. For instance, to record a musical tone of A in the treble clef, which oscillates at a frequency of 440 hertz (Hz)—that is, 440 cycles per second—the cutter oscillates 440 times a second. If the record is moving at a rate of 10 cm/s, the groove will be cut with a pattern of 44 cycles along every centimetre. To reproduce the sound, a needle attached to a diaphragm is lowered and placed in the groove and the record turned at a rate of 10 cm/s. The crests and troughs in the groove cause the needle to oscillate at 440 cycles/s, thus causing the original sound to be reproduced. As the whole process relies heavily on the precision of the mechanical components, there has always been a limit on affordable quality.

III

Optical Recording

In the optical method, sound waves are transformed by a microphone into equivalent electrical impulses that can then be amplified and made to drive a device that changes the intensity of a light beam. This process is known as modulation—using one oscillating signal (the sound) to change another oscillating signal (the light).

The resulting light beam is focused on a moving film used to give a photographically recorded track in much the same way as for mechanical recording. To reproduce the sound, a light source is focused on the film and a photoelectric cell placed behind it. The variations produced in the light as the film passes between the light and the cell generate a varying electrical signal that can be used (after amplification) to drive a loudspeaker. This process has long been used to record and reproduce the sound accompanying cinema films.

IV

Electromagnetic Recording

In audio tape recording, sound waves are converted into electrical impulses before being recorded on to a magnetized plastic or paper tape. Tape recording works by using an electromagnetic head to print a varying magnetic image of the sound on to the tape as it moves under the head. One of the attractions of this form of recording is that tapes can be reused to make another different recording, in contrast with the first two recording processes explained above.