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Video Recording | Article View | |
| I. | |
Introduction |
Video Recording, process of recording still or moving images electronically, rather than photochemically as in photographic film.
| II. | |
Analogue Recording |
| A. | |
Videotape |
The techniques used to record images on videotape are similar to those used in sound recording and reproduction. Electrical signals from a television camera (or from a television camera via a television receiver) are stored as patterns of magnetized regions of iron oxide on magnetic tape. When the recorded tape is played back, the original signals are generated. These signals can then be disseminated by broadcast antenna or by cable to television receivers that translate the signals into images and sounds.
Videotape recorder/playback systems for domestic use are connected directly to a television receiver. Unlike motion picture film, videotape does not require processing, and so may be played back immediately. This makes possible the instant replay common to televised sporting events.
Audio signals have a bandwidth, or frequency range, of about 20,000 Hz and can be recorded on magnetic tape that passes over the record/playback head relatively slowly. Video signals, on the other hand, such as the signal that modulates the carrier wave of a television transmitter, have a bandwidth as high as 6 MHz and therefore require a much higher scanning speed to accommodate the 300-fold increase in information to be recorded or played back. The first magnetic video recorders, built in the early 1950s, relied on very fast tape speeds—up to 914 cm (360 in) per second—to record and reproduce images of an acceptable quality. Such recorders were not really practical, and the television industry continued to use motion picture film for recording live television programmes. These were referred to as kinescope recordings.
| B. | |
Quadruplex |
However, in 1956 a transverse scanning system of video recording and playback was developed by engineers of the Ampex company in the United States. In this system, called quadruplex, four record/playback tape heads are mounted on the circumference of a drum that rotates rapidly (14,400 rpm) at a right angle to the direction of tape movement. In this manner, the heads scan the video tracks on the tape with head-to-tape speeds equivalent to 3,810 cm (1,500 in) per second. The tape itself travels slowly: either 38 cm (15 in) or 19 cm (7 ½ in) per second. The sound track and the picture control track run linearly near the edges of the tape and are scanned by fixed heads. The picture control track generates signals that serve to adjust the speed of the rotating drum so that each head is aligned directly over the correct part of each recorded video track. The tape used in quadruplex systems is 50 mm (2 in) wide, and a 25-mm (1-in) length of tape contains 64 transverse tracks—enough for two frames of 525 lines each. This system came to be generally used in the television industry from the end of the 1950s.
| C. | |
Helical Scan |
In the 1960s a variant of this scanning principle was developed, called helical scan (or slant-track scan). At first this had a much slower head-to-tape speed than the quadruplex system, and the maximum bandwidth was only 3 MHz. Picture resolution was therefore inferior to that of quadruplex.
In helical scan, one or two record/playback heads are mounted on the circumference of a drum that rotates rapidly in the same direction as the tape transport. The tape is wrapped round the drum in a helical manner. The wrap angle (that is, the angle between initial and final contact with the drum) is anywhere from 180° to 360°, depending on the make of the recorder. The early helical scan systems, developed for domestic or professional use, employed tape widths of 13 mm (½ in) or 19 mm (¾ in) housed in tape cassettes.
At the end of the 1970s new models of helical scan recorders using 25-mm (1-in) tape and higher tape speeds were developed that could finally equal the quality of quadruplex machines. Manufacturers eventually standardized on two variants called “B format” and “C format”, with the latter proving more popular with television broadcasters as the new standard recording device that would displace quadruplex machines.
Video cassette recorders (VCRs) became relatively inexpensive by the early 1980s and were commonly sold for domestic use. Watching videos changed trends in television viewing: by the early 2000s the number of households in the UK owning a VCR had peaked at 87 per cent.
The first domestic videotape to be introduced onto the market was Betamax, by the Sony company in 1975; it was followed a year later by VHS, developed by JVC. For several years the two video formats (both 13 mm/½ in) were in competition until VHS finally became the industry standard in the 1980s. U-matic (¾ in), first developed by Sony in 1971, became a widely used format for corporate productions and ENG (electronic news gathering) in the 1970s and 1980s. Sony’s Betacam (13-mm/½-in) system, a professional counterpart of Betamax, later became the standard production format used in broadcasting after its launch in the early 1980s.
From the late 1980s onward, 8-mm (3/10-in) magnetic tapes also became much more common for use in domestic camcorders (combination video cameras and tape recorders). The smaller tape cassette size and long recording time of two hours made the 8-mm format attractive for use in lightweight, portable equipment.
Following their introduction, both the ½-in and the 8-mm formats were augmented by improved versions—Super-VHS and Hi8, respectively—that could handle greater bandwidths. The result was better picture definition or detail approaching that of professional video recorders.
| III. | |
Digital Recording |
| A. | |
Digital Videotape |
In digital video recording, the moving pictures and sounds of the television signal are represented by sequences of binary numbers (zeros and ones). The main advantages of digital videotape are much higher picture and sound quality, and the ability to copy material without degradation.
Starting in the late 1980s, new helical scan recorders that record the television signal in digital form began to be introduced by manufacturers. At the highest-quality end the first agreed international standard was the D1 format. D1 was ideal for television studio production and postproduction because of its extremely high quality, but its suitability for general production was limited by its high cost, and it has largely been superseded by more economical formats. It was followed by many other varieties from different manufacturers, including D2, D3, D5, and D9. All of these use tape in cassettes, with widths of 19 mm (¾ in) in the first two cases, and 12.7 mm (½ in) in the latter three. Initially, as analogue television cameras were still the norm for production, the analogue signal was digitized by an analogue-to-digital converter, and then further encoded for error correction, before the signal was recorded on to digital tape, and the process then reversed to give an analogue signal for broadcasting. However, in the 1990s a large-scale shift occurred in the television industry as digital became the standard for the complete production process, from shooting to editing and postproduction. Digital Betacam, a digital version of the earlier ½-in analogue Betacam format, was introduced in 1993, and is now widely adopted for location and studio recording, and in postproduction and telecine (transfer from film to video). (See also Digital Broadcasting.)
Other semi-professional formats have also emerged since the mid-1990s, which are close to broadcast quality, such as DVCPRO and DVCAM (both utilizing 6-mm/¼ in tape). For domestic camcorder use, from the late 1990s, analogue 8-mm video began to be superseded by the 8-mm digital formats miniDV and Digital8.
| B. | |
High-Definition Video |
As high-definition television (HDTV) came closer to reality, professional HD recorders began to be introduced, beginning with Sony’s HDCAM in 1997. Various other models followed, such as HDCAM SR, D5-HD, HDV, DVCPRO-HD, and D9-HD. HDTV technology produces greatly improved picture quality, clarity, and colour definition by increasing the resolution from the standard 625 lines of conventional television to 720 or 1,080 lines.
The first consumer high-definition camcorder was launched by JVC in 2003. High-definition cameras have also been developed for the shooting of cinema films on digital video as opposed to conventional film. Two of the earliest films to utilize this technology were Star Wars Episode II: Attack of the Clones (2002) and Star Wars: Episode III: Revenge of the Sith (2005) by George Lucas.
For a detailed discussion of the techniques of digital cinematography and digital non-linear editing see Cinematography III. Digital Cinematography and VI. Digital Editing.
| C. | |
Videodiscs |
Videodiscs were developed as a response to the high cost of magnetic tape in the 1960s and early 1970s. Non-magnetic discs most commonly allowed only the reproduction of pre-recorded material, but the advantages they had over domestic video cassette recorders were higher picture quality and rapid access to any part of the recording.
The only remotely commercially successful system, called LaserVision and now largely abandoned, was developed by engineers of the Philips company in the Netherlands, and became available at the beginning of the 1980s. The discs were of moulded plastic, with a diameter of 30.5 cm (12 in). The large bandwidth of the video signal was accommodated by disc rotation speeds of up to 1,800 rpm. Soon after, this system was adapted for sound recording by Philips and Sony, using smaller compact discs of 12 cm (4 in) in diameter, that work in essentially the same way.
In the LaserVision system the original video signals were first encoded by the manufacturer in the same way as for digital videotape recording. The resulting string of bits was burned by a laser into the surface of a rotating glass master disc covered with a special coating. The result was a series of minute elliptical depressions, or “pits”, in the surface of the disc. This information was arranged in a single long spiral, like the groove of a record. The master disc was given a thin silver coating, followed by a thick one of nickel, and then this double coating was stripped off. From it a series of identical metal discs were made by further coating and stripping. These “stampers” were used as masters in presses to mould the final plastic discs that were to be bought by the consumer.
The undersurface of the clear plastic discs produced by injection moulding were coated with a very thin reflective aluminium layer by vacuum deposition, and then an even coating of clear plastic was applied over the surface of the disc with the pits in it to protect it. Finally, two such discs were glued back to back to make the final product. This gave a two-hour playing time that could accommodate feature films by playing the two sides in succession.
To play back the images, the track was scanned by a very narrow laser beam. Light from the laser beam, as modified by the pits, was converted back into the original patterns of electrical signals that were then fed by cable into a standard television receiver. There were two kinds of disc and recording used with the system. In one, the disc rotated at constant speed, which gave a picture recording duration of 36 minutes. In this case it was possible to freeze the image completely at any instant, or to view it in slow or fast motion.
In the second kind of laser videodisc, which was usually used for making a recording of a complete feature film, the disc rotated at speeds that varied between 1,500 rpm when the track was being scanned at its inner end, to 500 rpm at the outer end. This arrangement meant that the track of pits was passing the laser scanning head at constant linear speed, and gave a running time of one hour per side; variable speed effects, however, were not possible. Sales of videodisc players are now virtually non-existent.
| D. | |
DVD |
Another laser-dependent home entertainment technology is the Digital Video Disc (DVD), sometimes called Digital Versatile Disc. Digital technology is now commonplace with DVD films, television programmes, concerts, educational programming, and games all widely available. The technology again depends on a laser to “read” the entirely microscopic “pits” or indentations in the plastic layer of the DVD. Most DVD players will also play CDs, the earlier “sound-only” digital disc technology. A CD can accommodate around an hour of recorded music (to a maximum of about 74 minutes), while the much greater capacity of a DVD can comfortably handle a two-hour film and more. DVDs have the same dimensions as CDs (diameter: 12 cm/4 in).
As previously explained, DVDs store data in microscopic grooves running in a spiral around the disc. All the various DVD versions use a laser beam to scan along these grooves: minuscule reflective bumps (properly called “lands”) and non-reflective holes (the “pits”) aligned along the grooves representing the zeros and ones of digital information.
DVDs use much narrower tracks (0.74 microns wide, compared to 1.6 microns on a music CD) as well as improved modulation and error correction methods. These technologies allow them to store data seven times as large as that of a CD. The narrow tracks require special lasers—which cannot read CD-ROMs, CD-Rs, CD-RWs, or audio CDs. DVD drive manufacturers now tend to make drives with wide compatibility between formats; in other words, it is usual for a standard, inexpensive DVD player to also read music CDs.
The end result of this technology is much-improved video quality, about twice the resolution of a conventional video cassette. Besides improved on-screen resolution, the DVD (or CD) is not physically touched while it spins around in the drive. The laser beam focuses on the groove, but does not actually touch the surface, ensuring long life and no physical wear on the disc.
The original DVD formats are officially licensed by the industry consortium, the DVD Forum. An alternative, but similar, set of disc standards were also developed by the DVD+RW Alliance of manufacturers. More recent developments include dual-layer DVDs, and double-sided DVDs.
DVD Forum formats:
DVD+RW Alliance formats:
Domestic DVD recorders began to be available to consumers from 1999, and non-professional camcorders that can record direct to DVD have been available since 2000.
| E. | |
HD-DVD and Blu-Ray |
High Definition DVD and Blu-Ray are “next generation” DVD products, and depend on somewhat different technologies that mean they are not compatible. They superficially look the same as standard DVDs, but while DVD uses a red laser light measuring just 650 nanometers in diameter, an HD-DVD uses a blue/violet laser light, which can focus at the much narrower 405 nm. A human hair is much, much bulkier, at around 100,000 nm in diameter.
HD-DVD can handle 30 gigabytes of data, and thus easily cope with high-definition versions of films or concerts. Gone With the Wind, a long film that runs 233 minutes, for example, can easily fit on a single side of an HD-DVD disc that is handling data at 25 Mb/second, using MPEG2 compression.
Blu-Ray discs, like HD-DVD, were developed to handle high-definition and thus much higher data rates. A single-layer Blu-ray disc can hold 25 gigabytes, which can be used to record over two hours of high-definition television or more than 13 hours of standard-definition TV. There are also dual-layer versions of the discs that can hold 50 gigabytes.
HD-DVD and Blu-Ray formats were initially developed for the Japanese market, and during 2005 players started to appear internationally, in particular in the United States.
Both the HD-DVD and Blu-Ray systems are claimed to be backward compatible with existing DVD system, provided the manufacturer of the players builds in such compatibility. In essence, however, industry observers expect most of the Hollywood studios to ensure that their product is widely available whichever format emerges. The first-ever Blu-Ray HD film to be released (November 2005) was Charlie’s Angles: Full Throttle (Sony Pictures). In time, versions for home-recording of HD content will be available.
| F. | |
TiVo/PVR/DVRs |
While DVD and CD sales remain buoyant (along with other packaged media such as console games) there has been a dramatic shift since 2004 in sales of Digital Video Recorders, notably TiVo-type devices in North America, Sky+ Personal Video Recorders in Britain, and similar devices in mainland Europe.
There are some subtle differences. For example, TiVo can be used as a stand-alone recorder, hooked up to an ordinary TV set. Other versions of TiVo are integrated into more sophisticated satellite or cable systems. Sky+, developed by technology company NDS (who also encrypt DirecTV and BSkyB’s satellite signals), can only be used to receive satellite signals in the UK.
These PVR/DVRs use multiple tuners to record content onto a computer-type hard disk. The multiple tuners allow viewers to select various “trick” modes, fast-forwarding, and replaying while at the same time recording a signal from another channel. Some manufacturers are building units with four—and more—tuners, and ever-larger hard drives to store all this recorded content. The multiple tuners have another advantage, which is to feed programming to different rooms in the home. While the primary TV set, in the sitting room, might house the main TV screen, multiple tuners could feed channels to a bedroom, or child’s room. Most industry observers see these sophisticated models leading on to so-called “home servers”, devices with much greater, terabyte capacity.