Video Recording

Magnetic tape is still the most common method of storing video signals, whether analog or digital. In analog magnetic recording, a thin layer of metallic material (e.g., iron oxide) on some moving substrate (e.g., a tape being wound from one reel to another, or a rotating disc) is magnetized under the control of an oscillating electrical signal (the video signal). The video signal is controls passed to a recording head, which consists of a coil of wire wound around a core made of ferrite (iron-based) material. When this signal is passed through the recording-head core, which is Ω-shaped, a magnetic field arcs across the gap in the Ω.

As the video signal goes through a positive-negative oscillation, the polarity of the two ends of the core changes, reversing of the direction of the magnetic flux. The intensity of the video signal determines the strength of the magnetic flux. This flux impresses a magnetized area on the flexible tape or other magnetic medium, which is moving past the recording head. That is, the field produced by the recording head forces atoms in the medium's coating to shift their alignment; this alignment remains fixed even after the recording head is no longer in the vicinity, producing a weak, permanent magnetic field on the surface of the medium.

As the video signal oscillates, a linear series of such magnetized areas are produced on the recording medium, the magnetic-field directions and strengths of these areas correspond to the polarity (positive or negative) and strength of the original video signal at each moment. These magnetized areas comprise the recording of the analog video signal.

In order for the video signal to be recorded properly, the medium (usually a tape) has to move at a constant and sufficient speed across the end gap of the head. This leads to magnetization of the tape according to the signal content at each moment of time.

Although a digital video signal has a very different electrical structure—a series of sudden flips between a high level and a low level, rather than a smoothly varying level—recording of a digital signal on a magnetic medium works much the same way as for an analog signal. The major difference is that a digital magnetic recording consists of a series of discrete microregions of tape (or disc surface), each one of uniform field strength, rather than a smoothly varying continuum of magnetized particles.

Figure 1. A schematic diagram of a recording head. Illustration by Hans & Cassidy. Courtesy of Gale Group.