Magnetic Recording/Audiocassette

Information becomes stored on magnetic tape as it passes by the so-called recording head, which is a small electromagnet. There must be a narrow gap in this electromagnet so that its magnetic field will extend over the nearby section of tape.

The signal coming from the audio input is an alternating, back-and-forth current. An audio sound with a frequency of 1,000 cycles per second, for example, reverses its electric current direction every one-thousandth of a second. When the current is reversed, the North and South poles of the recording head electromagnet are interchanged. Consequently, the nearby magnetic particles embedded in the tape will become reoriented in the opposite direction.

When a loud sound is being recorded, the current to the electromagnet is large and its magnetic field will be relatively strong. Therefore a large number of magnetic particles in the tape will become aligned. A soft sound produces a weak field, so only a small fraction of magnetic particles will be affected.

For audiocassette players, the tape is designed to move at a standard speed of 1 7/8 in (4.8 cm) per second. During one cycle of a 1,000 cycle note, the tape moves Figure 1. Recording head. Illustration by Hans & Cassidy. Courtesy of Gale Group.
only about l/500th of an inch (0.005 cm), which is a distance smaller than the diameter of the period at the end of this sentence. Several magnetic particles in a row must fit into such a short distance on the tape.

The human ear normally can hear sounds up to about l5,000 cycles per second. To record such a high frequency without distortion requires extremely tiny magnetic particles. The magnetic material must be easy to align and should retain its pattern of orientation indefinitely. Better quality audio tapes use very fine grains of chromium dioxide instead of iron oxide. Inexpensive tapes are adequate for recording the spoken voice because its frequency range is much less than for music.

An audiocassette has a built-in erase head to remove previously recorded information. The tape has to be blank before it can be used again to make a new recording. The erase head normally is an electromagnet that operates at an ultrasonic frequency, much higher than the human ear can hear. It effectively randomizes the alignment of magnetic particles. Audiocassettes are designed so that the tape passes by an erase head just before the recording head. Sometimes musicians at a recording studio want to record a second sound track over the first one. In that case the erase head has to be deactivated, so the original sound is not lost.