Video recording

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 ?65 MHz) than do audio signals (?20 kHz), and thus involve a more complex recording and playback technology.

Basic principles of 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 ?-shaped, a magnetic field arcs across the gap in the ?.

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.

Recording techniques

The baseband (original) frequencies contained in a video signal lie between 10 Hz and 5 MHz. For a given bandwidth, assuming a simple, linear motion of the tape over a stationary recording head, the speed of the tape would be given by:

Speed of tape (meters per second) = width of gap (meters) × 2 × frequency (Hertz).

For a 5-MHz bandwidth and gap width of 1 × 10-6 meters, the speed of the tape required would thus be 10 m per second, and 36,000 meters (over a mile) of tape would be needed to record a one-hour program. In practice, a linear tape speed for recording video signals of 24 mm per second can be achieved using various techniques. Some of the recording techniques used in the past and others currently in use are discussed in the following sections.

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 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.

Frequency modulation

The wide bandwidth of a video signal poses a problem, due to the way that the inductive impedance of the recording head (i.e., resistance of the recording head to rapid changes in current flow) rises with the increase in the frequency. For normal recording a thousand times more head voltage will be required for a 5-MHz signal than for a 30-Hz signal. To avoid this problem, the wide-width luminance (brightness) signal is not recorded directly, but is instead recorded using a process called frequency modulation (FM), where the original signal is used to modulate (vary) the frequency of a high-frequency carrier signal. This effectively increases the ratio of the lowest and highest frequencies but does not reduce the bandwidth. FM signals give a better signal-to-noise performance and are less sensitive to unwanted interference .

Video systems

Video-recording systems are dependent, in their mechanical and electrical details, on the format of the television signals to be recorded. These signals vary in different parts of the world. For example, electrical-power standards vary from region to region, with two of the most common power frequencies being 50 Hz and 60 Hz. In order to match the frequency of the power supply, the television signals have been adapted to these standards. In countries with 60-Hz power, like the United States, Canada, and Japan, 30 video frames per second are transmitted, while in countries with 50-Hz power, like Australia , India, and some European countries, 25 frames per second are transmitted.

A T.V. picture consists of a series of dots. For a black and white picture the dots are black, gray, and white; in a color picture, they are usually red, green, and blue. These dots are usually very small and, if viewed from more than a meter or so away, invisible to the human eye . A series of these dots are synchronized in the form of horizontal lines and vertical lines, forming the image. The structure of the image lines also determine the bandwidth of the television signal, and, as with electrical power, the number of horizontal lines varies from country to country. Countries with 60-Hz power use 525 horizontal lines, while others use 625 lines. There is no world standard yet for these horizontal lines and the number of frames for transmission. This has led to the development of various incompatible video recording systems. There are three major types of video recording: NTSC (National Television System Committee); PAL (phase alternating line); and SECAM (from the French for "sequential color with memory"). The major differences in the three systems are:

Horizontal lines: NTSC = 525, PAL = 625, SECAM = 625. Fields per second: NTSC = 60, PAL = 50, SECAM = 50. Frames per second NTSC = 30, PAL = 25, SECAM = 25

Besides these major differences, there are variations in their subcarrier frequency, luminance, and chrominance bandwidths. (PAL and SECAM differ in these variables, not in their arrangemtns of lines, fields, and frames per second.) There are also variants of these video systems, the differences between these minor variants being mainly in FM bandwidth.

Video formats

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

VHS format

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.

Betamax format

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.

Video-8 format

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 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.

Digital recording

Most of the current video recording devices are still analog recorders, but this is changing rapidly as hand-held digital video cameras, digital video discs (DVDs), and other digital video technologies capture increasingly large segments of the consumer market. The broadcast television market is also shifting rapidly to digital signal standards. Digital recording requires high-density recording technology due to the large bandwidth—125–270 million bits per second—but has many advantages over analog recording, including greater reliability, low cost (given recent advances in digital-signal-processing technologies), higher resolution, and greater color accuracy. Digital video has the additional virtue of transferability, as it may be recorded on any medium capable of storing digital data: computer hard drive, digital videotape, optical disc, or other. Given contemporary standards for memory and processing speed in affordable desktop computers, both professional and amateur video users can now upload digital video into working computer memory and edit it at will. There is little doubt that analog television signals, both for broadcast and recording, will be a thing of the past within some 10 or 20 years; indeed, in May, 1997 the U.S. Federal Communications Commission (FCC) mandated that U.S. broadcasters begin to phase out NTSC in favor of digital television.

Unfortunately, there is presently even more global disparity among digital television signal types than among analog signal types. The most pervasive—used on DVDs, streaming video on the Internet, and in broadcast—is probably that which exploits the type of data compression termed MPEG. This acronym is itself compression of "motion-JPEG," where JPEG stands for Joint Photographic Experts Group, the name of the body that designed this compression algorithm . Data compression enables the number of bits in a video frame to be reduced by as much as 75% without (hopefully) compromising the image quality. Image compression can, however, degrade image quality if misapplied. MPEG is actually two standards: MPEG-1 for low-quality video (e.g., streaming video on the Internet), while MPEG-2 is for broadcast-quality video.

Resources

books

Marsh, Ken. Independent Video: A complete Guide to thePhysics, Operation, and Application of the New Television for the Student, the Artist, and for Community TV. Straight Arrow Books, 2001.

other

"Digital Television Frequently Asked Questions." U.S. Federal Communications Commission, 2002 [cited February 7, 2003]. <http://www.fcc.gov/mb/policy/dtv/>.

Satyam Priyadarshy