A tape player's ability to make music starts with the device that stores the musical information: the cassette tape itself.
An audio cassette is a relatively simple machine. It consists of two spools seated inside a hollow plastic casing. Between these spools is wound a length of flat plastic tape which is coated with a layer of ferric oxide, a metal substance which, once magnetized, remains that way for the rest of its life. These mechanisms are protected by a plastic outer case (the word “cassette” actually refers to this).
When sounds are recorded onto a cassette tape, the process involves the impression of an electromagnetic signal onto the ferric oxide coating on the plastic tape. Though the coating is permanently magnetic, the signal itself is not and can be overwritten with each new recording.
The translation of sound into electrical signals occurs within the structures of a microphone.
Channels and Stereo
The sound for sides “A” and “B” of a cassette are recorded on the left and right sides of the strip of audio tape, rather than the top and bottom. Laid side-by-side across the width of the tape are four channels of audio signal, designed for stereo sound reproduction. The left and right audio channels of side “A” are recorded onto one side of the tape while the left and right audio channels of side “B” are recorded onto the other.
Electrical signals are transferred onto the audio tape as it is stretched between the capstan (a metal spindle) and rubber rollers. This occurs at the bottom of the cassette where the tape is stretched between the two main spindles.
A motorized mechanism turns the spools of the tape by turning spindles which are threaded through the main spools. This motor must have a constant rotation speed in order accurately record and playback sounds as they occur. Other mechanisms and motors within the tape player maintain the tension of the stretched tape. The cassette (plastic outer shell) and its mechanisms are made to fit within the mechanical workings of the tape player/recorder.
The playback process happens in a manner very similar to the recording process, except in this case, the player is taking the electromagnetic signals from the cassette player and converting them into amplified audio sound. In this case, the electromagnetic recording on the oxide coating transmits electrical signals rather than receiving them.