THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

19.2 Recording Equipment 571In the following sections of this chapter, we outline the principles of soundrecording and playback, concluding with a prognosis of future developments.19.2 Recording EquipmentRecording of sound occurs in different ways and it can be defined as the storingof information of how the amplitude of sound varies with time. The stored informationcan be retrieved by playback at a later time through control of a sourceof sound waves. The twentieth century has seen the evolution of the phonographcylinder into the 78-rpm recordings which in turn gave way to the 45 rpm and33-1/3 rpm vinyl disks. These mechanical types of recordings use wavy grooveson a surface that guide a stylus to replicate the signals that were recorded. Reelto-reeltape recorders have been used since the 1940s to make recordings, andmany great performances have been archived on this medium. These master tapesare used to make vinyl disks and eventually CDs. While tapes carry the advantageof being relatively easy to edit, they do lack the random accessibility of anytype of disks. Tapes solely for playback are now principally in the compact cassetteformat.Magnetic RecordingFigure 19.1 shows a fundamental record-play system using magnetic recording.A close-up of the interior construction of the magnetic head is also included inthe figure. Tape D is made of a smooth, durable plastic such as polyester (Mylar)and is coated on one side with a magnetizable material, most commonly a driedsuspension of acicular gamma ferrite particles about 0.25 μm long in a lacquerbinder. 1 The thickness of the magnetic layer is of the order of 12 μm or less. Thetape unreels from supply reel A and threads over a magnetic head B that suppliesa magnetic field at recording gap C. The magnetic field varies in its strengthaccording to the sound being recorded. The now magnetized tape is pulled pastthe head by capstan roller E and pinch roller F and spooled onto a take-up reel G.In rewinding the tape, the recording head is deenergized. For playback, the tapeleaves the supply reel, as before, but the winding on head B is connected to anamplifier that, in turn, connects to a loudspeaker. The magnetized elements inducea voltage in the head winding, which translates into a replication of the gap fieldvariations during recording. The recording may be erased by an additional headH that produces a strong, steady AC field. It is not necessary to erase the tape ifit is being recorded over, because the erase head is automatically energized whenrecording, thus removing previously recorded material.1 After the tape has been coated and while the suspension is still damp, it is subjected to a magnetic fieldthat aligns the magnetic particles in order to increase recording signal strength and minimize noise.Other alternative magnetic materials include chromium dioxide and cobalt-coated gamma ferrite. Itis obvious that tapes have to be manufactured under stringent clean-room conditions because theslightest contaminants can cause impaired recordings.