Video Recording
Video Recording
Digital recording and storage disparity
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 (from 1 to 270 MHz or larger) than do audio signals (≥20 kHz), and thus involve a more complex recording and playback technology. Video technology, first developed for television, is being used in such forms as through the Internet, as streaming media clips on computers, and in DVDs, used in unison with televisions and computers.
Several digital storage formats for video recording include DVD (digital versatile disc, sometimes called digital video disc), MPEG-4 (moving picture experts group-4), analog videotapes, VHS (video home system), and Betamax. Three-dimensional-video (3D-video) is also a recent addition to video recording, where several cameras with real-time depth measurements are used to record three-dimensional videos. In unison with digital storage technologies, digital television (DVT) is quickly becoming a standard for television video.
Video display standards
In the 2000s, the following are some of the most popular video display standards in digital and analog. Some of the digital standards are: ATSC (Advanced Television Systems Committee, used in North America), DVB (Digital Video Broadcasting, in Europe), ISDB (Integrated Services Digital Broadcasting, in Japan). Several of the older analog standards that are still in use are: MUSE (analog HDTV [high-definition television]), NTSC (National Television System(s) Committee, in North America and Japan), PAL (phase-alternating line, Europe, Asia, and Australia), and SECAM (sequential color with memory, in Russia, central Africa, and France).
Magnetic tape video storage
Magnetic tape is a 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 passed to a recording head, which consists of a coil of wire wound around a core made of ferrite (iron-based) material (Figure 1). 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.
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/sec, 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/sec 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 m/sec. 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 (Figure 2). 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 through NTSC (National Television System(s) Committee). While in countries with 50-Hz power, like Australia, India, and some European countries, 25 frames per second are transmitted through SECAM (sequential color with memory) and PAL (phase-alternating line). (Frame rate is the number of still pictures that pass per unit of time on video. A minimum of about ten frames per second is required to perceptually change a still picture into a moving picture)
A television 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. The major differences in the three main systems (NTSC, PAL, and SECAM) 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 arrangements 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 a commonly used video formats, although one that is decreasing in popularity. The azimuthal angle is +6 or -6 degrees. The writing speed is usually 4.85 m/sec, while the linear speed of the tape is 23.99 mm/sec. 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/sec with a linear tape speed of 20.051 mm/sec. The drum size used in this format is smaller than those used in VHS 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
Digital recording and storage involves an analog signal being transmitted from an input device (such as a microphone) to an analog-to-digital converter, commonly abbreviated a ADC device. The ADC device converts the analog signal into a series of binary numbers (zeros and ones). This digital signal is then transmitted to a storage device, such as a DVD, a computer hard drive, or other such storage device.
Some of the current video recording devices are still analog recorders, but handheld digital video cameras, digital video discs (DVDs), and other digital video technologies are increasingly capturing 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 to 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 the decade of the 2000s; 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. Since then, the FCC has mandated that all television sets include digital tuners by 2007.
Optical disc video storage includes digital versatile disc, sometimes called digital video disc, (DVD). It is used for data storage including movies with video and audio. They are similar to CDs (compact discs) with respect to size but are encoded with a different format and with a much higher density. The four basic types of DVDs are: single sided/single layer, single sided/dual (double) layer, double sided/single layer, and double sided/dual layer. DVD disc capacity for 12-cm discs is: for a single sided/single layer, 4.7 GB (gigabytes); double sided/single layer, 9.4 GB; single sided/double layer, 8.5 GB; and double sided/double layer, 17 GB. Eight-centimeter discs are also available in the following four types: single sided/single layer, 1.4 GB; double sided/single layer, 2.8 GB; single sided/double layer, 2.6 GB; and double sided/double layer, 5.2 GB. Some of the more popular formats of DVD include: DVD-R (DVD recordable), DVD+R, DVDRW (DVD rewriteable), and DVD+RW.
DVD also comes in high density DVD, sometimes called high-definition DVD): HD-DVD (by Toshiba). It is used specifically for high-definition video and data storage. It uses a blue laser to store data. Some of the commercial products using DVD technology include video games by Sony PlayStation and Microsoft Xbox. HD-DVD is a supposed successor to the DVD format because it will be able to handle high-definition video, which will be compatible with the HD-TV (high-definition television). HD-DVD currently use 30 GB dual-layer discs.
Blu-ray Disc (BD, by Sony and Panasonic) is a high-density optical disc storage technology for high-definition video. It uses a blue-violet laser to store data on a disc. Because it uses a shorter wavelength (405 nm) than the DVD format (650 nm, red laser), it is able to store more information on the same sized disc. BD technology uses 50 GB and 200 GB capacity discs.
The Holographic Versatile Disc (HVD) is an optical disc technology that is emerging into the video recording and storage industry as of November 2006. It uses collinear holography, which is a process where a red laser and a blue-green laser work together (are collimated) into a single beam for the purpose of recording and storing video data. (A hologram is the image produced by holography, which uses optical phase information to produce a three-dimensional image with the use of a pattern of 1s and 0s [binary numbering system].) These disks, by Hitachi Maxell, hold 3.9 terabytes (TB) of data on one disc, which is over 150 times the capacity of a Blu-ray Disc. A release of this technology is expected in late 2006.
Digital recording and storage disparity
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.
The present and the future
DVD is the standard for recording and storing video data in the United States and Europe. As of the last quarter of 2006, there are many hopeful successors to DVD in the marketplace. Sony and Panasonic’s BD, Toshiba’s HD-DVD, and Hitachi Maxell’s HVD are three such hopeful technologies. Currently, it is unclear whether any of these technologies will replace the DVD in the near future. For the most part, digital video storage has become the dominant way to record and store video data.
Resources
BOOKS
Beiser, Leo. Unified Optical Scanning Technology. New York: Wiley-Interscience, 2003.
Block, Bruce A. The Visual Story: Seeing the Structure of Film, TV, and New Media. Boston, MA: Focal Press, 2001.
Jones, Frederic H. How to Do Everything with Digital Video. Berkeley, CA: McGraw-Hill/Osborne, 2002.
Marsh, Ken. Independent Video: A complete Guide to the Physics, Operation, and Application of the New Television for the Student, the Artist, and for Community TV. Straight Arrow Books, 2001.
Ozer, Jan. Guide to Digital Video. Indianapolis, IN: Wiley, 2004.
Rubin, Michael. The Little Digital Video Book. Berkeley, CA: Peachpit, 2002.
Sadun, Erica. Digital Video!: I Didn’t Know You Could Do That. San Francisco, CA: Sybex, 2001.
Wright, Steve. Digital Compositing for Film and Video. Oxford, UK: Focal, 2006.
OTHER
“Digital Television Frequently Asked Questions.” U.S. Federal Communications Commission, 2002. <http://www.fcc.gov/mb/policy/dtv/> (Feb. 7, 2003).
Satyam Priyadarshy
Video Recording
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 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.
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
Video Recording
Video recording
Video recording is the process by which visual images are recorded on some form of magnetic recording device such as tape or a video disc. In magnetic recording, an unrecorded tape is wrapped around a rotating drum that carries the tape through a series of steps before it leaves as a recorded tape.
The actual recording of the tape occurs on a cylindrical device known as the head. The head consists of a coil of wire wrapped around a core made of ferrite (iron oxide). When a camera is focused on a scene, the visual images it receives are converted to an electrical signal within the camera. That electrical signal passes into the recording head.
When the electrical signal reaches the recording head, it passes through the wire coil. When an electrical current passes through a metal coil, it creates a magnetic field. The strength of the magnetic field (called a flux) created depends on the strength of the electric current passing through the coil of wire. The strength of the electric current, in turn, depends on the intensity of the light received by the video camera. If the camera sees a bright spot, it produces a strong electric current, and the strong electric current produces a strong magnetic flux. When the camera sees a dim spot, it produces a weak electric current, and that weak electric current produces a weak magnetic flux.
The strength of the magnetic flux produced by the head is recorded on the magnetic tape that passes over it. The magnetic tape consists of millions of tiny pieces of iron oxide, like very tiny specks of flour. When the tape passes through a magnetic field, the iron oxide particles line themselves up in the direction of the magnetic field. If the field is very strong, all of the particles will line up in the same direction. If the field is very weak, only a small fraction of the particles will be aligned in the same direction. The brightness or dimness of the scene being photographed, then, is eventually translated into many or few iron oxide particles being lined up on the recording tape.
Video disk recording
Magnetic tape is satisfactory for recording visual images under most circumstances. However, it does have certain disadvantages. One disadvantage is the time it takes to locate and play back any given portion of the recorded image. An alternative to using tape for recording images is a video disk.
A video disk is similar to a sound recording disk. It is a round, flat object covered with a thin layer of iron oxide. An incoming electrical signal is fed into recording heads posed above the rotating disk. As the signal is recorded on the disk, the recording heads move outward in a series of concentric circles away from the middle of the disk. Recording continues until the disk is filled.
[See also DVD technology; Magnetic recording/audiocassette ]
Videocassette Recorder
VIDEOCASSETTE RECORDER
VIDEOCASSETTE RECORDER (VCR) is a device that records, stores, and plays back television programs on magnetic tape; VCRs are also used to play prerecorded commercial cassettes. Videocassette recorders for consumers evolved from models used by broadcast professionals. By the early 1970s, two competing types of VCRs were available: the Betamax format, produced by the Sony Corporation, and the VHS format, produced by the Matsushita Corporation. Although Betamax was widely acknowledged to be technologically superior, the consumer market ultimately abandoned this format because it lacked several desirable features, such as recording times that could accommodate movies and longer programs on a single tape.
The introduction of the VCR not only transformed the television-viewing habits of Americans by allowing them to tape programs and time-shift their viewing; it also shook the roots of the powerful movie industry. Initially, industry leaders feared that the availability of recorded movies to be watched at home would cause Americans to leave theaters in droves. But their fears proved to be unfounded, and over the next decades the revenue from videocassette sales and rentals became a significant portion of the profits from Hollywood films. The introduction of inexpensive camcorders—movie cameras that use videotape recording—gave another boost to the popularity of VCRs as families used them to make home movies that they could later view on their television sets and share with relatives and friends.
Videocassette recorders gave Americans new flexibility, privacy, and control in viewing television programs and movies. They were perhaps the first technology that enabled consumers to personalize their viewing experiences, a trend that continues to grow in importance.
BIBLIOGRAPHY
Dobrow, Julia R., ed. Social and Cultural Aspects of VCR Use. Hillsdale, N.J: L. Erlbaum Associates, 1990.
Gaggioni, H. P. "The Evolution of Video Technologies." IEEE Communications Magazine 25 (Nov. 1987): 20–36.
Graham, Ian. Television and Video. New York: Gloucester Press, 1991.
Wolpin, Stewart. "The Race to Video." American Heritage of Invention and Technology 10 (Fall 1994): 52–63.
Loren ButlerFeffer
See alsoElectricity and Electronics ; Film .
videocassette recorder
vid·e·o·cas·sette re·cord·er (abbr.: VCR) • n. a device that, when linked to a television set, can be used for recording on and playing videotapes.
videocassette
vid·e·o·cas·sette / ˌvidēōkəˈset/ • n. a cassette of videotape.