Tape Recorder

Two heads — or even three — are better than one when it comes to understanding how tape recorders exploit electromagnetic induction.

The earliest magnetic recording device was built in 1898 by Danish engineer Valdemar Poulsen. His telegraphone was capable of magnetically recording sound on a steel piano wire, but never achieved widespread use. A few decades later, a device for recording audio signals via magnetized tape, similar to what we know today, was invented, making its debut in 1935. By the late 1960s, magnetic tape cassettes and cartridges with pre-recorded music were common in American households, largely supplanting phonograph records. A key advantage of magnetic tape is its ability to be easily erased, recorded on, and instantly played back, all processes that require a single device generally known as a tape recorder, pictured in the tutorial below.

Located inside of a tape recorder are multiple heads. Each head is a small electromagnet consisting of a ring of ferromagnetic material around which a coil of wire is wound. At the base of each ring is a small gap positioned directly over the tape, a plastic film coated with minuscule magnetic particles. Current passing through the coil generates (per Faraday’s Law of electromagnetic induction) a magnetic field in the ring to make it an electromagnet. The gap in the ring makes this a kind of horseshoe magnet, with the magnetic field crossing from the north pole to the south. This field impinges on the magnetized tape, and vice versa. This is at the heart of how a tape recorder works. Our illustration depicts three heads with different but related purposes suggested by their names: the Erase Head, the Record Head and the Playback Head.

For music or other sound to be properly reproduced on magnetic film, any signal already present on the film must be removed. That’s accomplished by the first head encountered by tape passing through a recorder, the erase head. A high amplitude, high frequency signal is sent via the wire to the erase head; the resulting magnetic field induced in the head jumbles up the magnetic particles in the tape below, in so doing erasing any sound that had been preserved within the way those particles were arranged on the tape.

To get a bit more technical, the orientation of the Magnetic Domains (depicted as white arrows on the tape) within the magnetic particles are randomized by the erase head. Magnetic domains are tiny areas inside the particles in which the atoms (themselves tiny magnets) are aligned in the same direction, in the same magnetic field. These domains can be made by outside magnetic fields to change direction, aligning with that external field.

This erasing process eliminates most signals that may have been present on the tape; however, some magnetization remains. To overcome this Hysteresis (the tendency of magnetized materials to retain their magnetization), an additional step termed Biasing is taken. This involves applying a high frequency Bias Signal to the tape that mixes up any magnetic domains still under the influence of hysterisis.

Biasing takes place at the record head, in tandem with the recording of music (or other sound). The sound waves being recorded are combined with the biasing signal, then transduced into electrical signals sent to the record head. The amplitude and frequency of the audio signal vary, of course, resulting in a corresponding change in the magnetic field they (after being converted into electrical signals) induce. Consequently, the magnetic particles on the tape are affected differently, depending on the point at which they are exposed to the varying field. Their magnetic domains shift in alignment with whatever field they pass beneath, thus imprinting a kind of magnetic picture of the audio signal on the tape.

Immediately after a signal is recorded onto tape, it can be played back. In many tape recorders the record head also serves as the playback head, but they are shown separately in the tutorial for illustration purposes. When recorded tape scrolls under the playback head, the moving magnetic fields induce a varying current in the head (again, as per Faraday’s Law). This voltage produces an electrical representation of the magnetic signal on the tape. This is then passed through an equalization circuit and amplification/speaker system (not shown) so that the recorded music is audible.

Last modified on 17 June 2019
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