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Video Recording

Basic Principles Of Video Recording, Recording Techniques, Frequency Modulation, Video Systems, Digital RecordingVideo formats, VHS format, Betamax format, Video-format (8 ), VHS-C format

The term "video recording" refers to storing a video signal (information designed to specify a moving image) in a recording medium such as magnetic tape, optical disc, or computer memory. Video signals have much larger bandwidths &NA;65 MHz) than do audio signals (&NA;20 kHz), and thus involve a more complex recording and playback technology.

Transverse recording

The transverse recording technique is based upon the concept of rotation of the head simultaneous with transverse movement of the tape over the head. The head rotates at a speed of 14,400 revolutions per minute, recording a track that zigzags along the tape and gives an effective writing speed of 38 meters per second. In this method, a single image is divided into 16 segments. All these segments are then recorded linearly onto the magnetic tape, in Figure 2. (A) A schematic representation of the rotating drum and moving tape. (B) A schematic diagram of a magnetic tape that has recorded signals. Illustration by Hans & Cassidy. Courtesy of Gale Group. parallel. This requires a great deal of horizontal synchronization while reproducing the video signal.

Helical recording

Helical recording enables the linear speed of the tape itself to be reduced while increasing the writing speed. Instead of a single recording head, two heads are set diametrically into a small rotating drum. The magnetic tape wraps around the drum as it moves forward, thus both the head and the tape are moving in the same direction. This drum is tilted at an angle, which causes the heads to traverse the magnetic tape in slanted tracks. Again a track length that is much longer than the tape length is achieved.

For maximum utilization of the magnetic tape, at least two heads are essential. The two heads are set in the drum so that their gaps are at an angle of 6° plus or minus from the "zero" position. The "zero" position is defined as the right angle to the direction of the rotation. This angle is called the azimuthal angle, and this type of recording is also referred to as azimuth recording. The plus and minus sign in the angles ensure that the two heads identify their own tracks while reproducing the video signals. Unlike transverse recording, the picture field is divided into two segments and each segment is recorded by each head. Thus in one rotation of the drum one picture field is written completely.

The width of these magnetic tracks is 0.049 mm and the total width of the tape is 12.65 mm. In addition to the video tracks, the tape has two other tracks, the sound track and a control track for synchronizing tape speeds. The latter two tracks are stored in a linear fashion.

The video signal is recorded using the helical scanning technique. Use of different azimuthal angles for recording leads to different video formats.

This is one of the most commonly used video formats. The azimuthal angle is +6 or -6 degrees. The writing speed is usually 4.85 m per second, while the linear speed of the tape is 23.99 mm per second. The video track width is 0.049 mm and the actual tape width is 12.65 mm.

The azimuthal angle for this format is +7 and -7 degrees. The linear speed of the tapes is 18.7 mm per second slower than the VHS tape speed, although the writing speed is 5.83 m per second. The video track width is 0.0328 mm on a tape 12.7 mm wide.

This format uses azimuthal angles of +10 and -10 degrees. The standard video tracks are 0.0344 mm wide on a tape that is 8 mm wide. The writing speed is 3.12 m per second with a linear tape speed of 20.051 mm per second. The drum size used in this format is smaller than those used in VHS or Betamax format.

VHS-C stands for VHS compact. This format is widely used in video recording cameras and is fully compatible with the standard VHS format. The tape width is same as in VHS, but the drum is 41.33 mm instead of 62 mm in VHS. The other difference is that VHS-C uses four-head helical scanning in order to produce the same magnetic pattern in VHS.

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