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Dosta ljudi u zadnje vreme pita o cemu je ustvari rec kad se kaze HDTV. E pa evo, cisto radi znanja kod nas se rade filmovi, i to oni ozbiljniji, sa ovakvim kamerama! Ko procita tekst, shvatice i zasto.
High Definition Television
Also known as advanced television (ATV), extended-definition television (EDTV), and improved-definition television (IDTV), HDTV is an improved television system with at least double the horizontal and vertical resolution, wider aspect ratio, and superior audio when compared to the current television broadcast standards, e.g., NTSC and PAL.
With approximately twice as many scan lines as current television systems, a larger screen with a wider aspect ratio, and six-channel, compact-disc-quality, surround sound, the HDTV experience will approach projected 35mm film. According to CCIR Report 801, HDTV is described as able to replicate reality when the viewer is seated three screen heights away from the display. Higher resolution, better color reproduction, separate color and luminance signals, a wider and perhaps larger screen, and life-like audio will all be combined to make the HDTV experience larger than life, especially when compared to the current NTSC system. HDTV also has professional and business applications beyond television entertainment. Some suggested applications for this new technology include; telemedicine, computer design, and teleconferencing. Yet another suggestion is that HDTV will finally make possible a concept sometimes referred to as electronic cinema. The concept is to create a network of video theaters with distribution by direct-broadcast satellite. This approach would provide an alternative to traditional film print distribution.
Major players in the race to bring HDTV to market have been the global economic superpowers: Japan, the United States, and to a lesser degree, the European community. The Japanese, who began working on HDTV in 1969, have been delivering a domestic HDTV service via their multiple sub-Nyquist sampling encoding (MUSE) system since 1991. And while the US has focused on terrestrial broadcast of HDTV signals (due to concern for local broadcasters' interests), Japan has moved ahead with DBS delivery systems. But even after several years of trial delivery, only 25,000 Hi-Vision sets were sold due to high cost.
Despite Japan's worldwide dominance in consumer electronics hardware and the US's role as the world's chief supplier of programming, the European community has been determined to be a participant in the development of HDTV standards which will impact on their electronics and broadcasting industries. The European market had been at odds with the Japan Broadcasting Corporation (NHK) system for some time due to its incompatible frame rate (1125 scan lines; 60 fields per second). Europe is on a 50 hertz, 25 frame system with its PAL and SECAM systems. Converting from or to a 30 frame system is both costly and a technical compromise, according to European sources. In fact, Europe's tentative development in 1987 of its own HDTV system, Eureka 95/HD-MAC, along with the International Telecommunication Union's (ITU) decision in 1986 to delay a vote on an HDTV standard, thwarted Japan's hopes for a world-wide standard.
In the United States, HDTV by the late 1980s garnered the attention of industry, government, military, education, and research institutions. While Japan took an early lead with its analog HDTV system, the US was in a debate about what approach it should take to join the race. In the economic, political, and business arenas, three scenarios for transition from NTSC to ATV were being debated: the one-step, two-step, and leapfrog approaches. One-step proponents argued for a quick and final decision on an HDTV delivery system so that the US could get on with the business of making the transition. Two-step advocates believed that it was too early to make a final decision as technology was changing so fast. Instead, they argued, the US should introduce limited and NTSC-compatible improvements now with the goal of achieving full HDTV technology a few years down the road. Leapfrog strategists argued for a stay of all current research and a jump to a fully digital technology. This approach would allow the US to leap ahead of Japanese and European systems. Proponents of the leapfrog approach included the computer and telephone industries and the Department of Defense (DOD). Of course, broadcasters were not about to sit by and watch other delivery systems bypass them. In 1987, broadcasters called for the formation of the Advisory Committee on Advanced Television Service (ACATS) to look out for their interests.
Another interest to be protected was that of the consumer. In 1990, the FCC has determined that no matter what HDTV system was adopted, it would have to be compatible with the current NTSC system. Current thinking on HDTV can be divided into three interrelated but separate areas of concern: production, distribution, and display.
Production
Production and transmission need not share the same technical system. In fact, as long as a production standard is readily convertible to the transmission standard, it makes a great deal of sense to use two different systems, according to many HDTV experts. For years, broadcast television has used 35mm film as its acquisition format and as a source for transfer to NTSC video for post-production and distribution. Despite the availability of HDTV production technology since the mid 1980s, 35mm film remains the premier worldwide acquisition standard for high-quality television. In fact, all the talk of HDTV may have resulted in the promotion of film as a production format. Due to all the uncertainty as to which HDTV transmission system will finally prevail, many producers feel that the safest route is still to shoot on film; they reason that they will eventually be able to transfer the film images to whatever HDTV system wins out. Currently, the closest thing that the video community can promote as a worldwide production standard is D-1, which records both 525- and 625-line systems. The NHK 1125/60 system was being promoted as a worldwide standard with the assumption that once the material has been recorded and edited, it can be down-converted to either NTSC or PAL, or even transferred to film. And once the HDTV distribution systems have been standardized, the 1125/60 video could be converted to whatever HDTV transmission system is required. The unknown variable here is the quality and cost of the conversion.
The 1125/60 Standard
The current NHK standard of 1125/60 (1125 scan lines and 60 fields per second, 2:1 interlaced) was the first system used for both production and transmission. In the spring of 1991, NHK reported that more than 600 programs had been produced in the 1125/60 HDTV system. In what many consider to be a very controversial move, the Society of Motion Picture and Television Engineers (SMPTE) in 1988 approved a slight modification of the 1125/60 system as the HDTV production standard for the US, officially referring to it as 240M. It is interesting to note that another standards-setting organization, the American National Standards Institute (ANSI), at first concurred with the SMPTE but later withdrew its approval after an appeal by Capital Cities/ABC. With an aspect ratio of 16:9, 1125/60 is definitely widescreen TV. However, it requires the use of 30 MHz of bandwidth for recording. To achieve these frequencies, some HDTV recorders incorporated a modified 1-inch machine that runs at twice normal speed. The 1125-line HDTV system has been in use in the United States since the mid 1980s. Rebo Productions, 1125 Productions, and David Nile's New York-based production company are just a few of those who pioneered shooting in the 1125/60 HDTV format. Standard 240M, however, is an analog system. In 1992 a digital version of the same system was submitted and approved by the SMPTE as standard 260M.
In the spring of 1988, CBS selected NHK's 1125/60 system for principal photography of the film Innocent Victims, the first US made-for-TV movie produced using HDTV technology. By taping the program in HDTV instead of 35mm film, CBS estimates that it saved 15% in production costs. Made-for-television movies may appear to be a natural choice to begin the transition from 35mm film to HDTV for production, but what about films that will be released in theaters? The world's first high-definition theatrical film, Julia and Julia, was produced by the Italian network RAI. Another early HDTV production was a 14-part Canadian series, Chasing Rainbows, which was produced by the Canadian Broadcasting Corporation (CBC). The first theatrical feature film shot in the United States on HDTV, Razbijac in the Mirror, starred and was directed by Robby Benson. Razbijac in the Mirror was shot in HDTV by Rebo Productions and was transferred to 35mm film for release. All of these productions have been simultaneously praised and assailed on the basis of picture quality, production ease, and overall effectiveness.
Two glitches in the HDTV production process are still being resolved. One is constructing an imaging device that has both the resolution and the sensitivity necessary to produce an image suitable for HDTV pictures. Tubes, which were quickly replaced by CCDs in almost every other production environment, have disappeared more slowly from HDTV camera. Tubes still have an edge in resolution, and resolution is, of course, central to the whole idea of HDTV. The tubes used in some HDTV cameras were high-gain, avalanche rushing amorphous photoconductor (HARP) tubes. Unfortunately, resolution is achieved at the expense of sensitivity. The smaller the focus of the electron beam, the higher the resolution and the lower the sensitivity, thus requiring more light on the set. Especially when compared to the newer and faster 35mm film stocks, HDTV production using tube cameras required extra lighting, which in production means more fixtures and increased setup time. Another complication has involved achieving the necessary optical resolution for the lenses used with HDTV cameras. While lenses for high-quality 35mm film production have evolved to become high quality imaging tools, the history of video production has not, until HDTV, required similar performance.
Distribution
The problem of production is not nearly as complicated as that of distribution-in particular, terrestrial broadcasting. One of the principal problems is that of spectrum scarcity. As a general rule, the better an HDTV system's performance, the greater its bandwidth needs. The NTSC video signal requires 6 MHz of spectrum space for terrestrial broadcast, and the full-bandwidth, uncompressed HDTV signal requires 30 MHz. (By using multiplexing and digital compression, researchers have been able to reduce digital HDTV to fit within the same 6 MHz band.) Because the FCC is the regulatory body that controls the use of spectrum, their approval is necessary before an HDTV transmission standard can be adopted for use in the US. The 1988 draft statement on HDTV by the FCC stated that any system to be considered must be compatible with the existing NTSC system. This means that the HDTV system to be adopted for US broadcast must be able to transmit an NTSC-compatible signal within a bandwidth no wider than 6 MHz. One way to get around this requirement would be to use two channels. This additional spectrum would come from existing unallocated and "taboo" channels within the VHF and UHF spectrums and would coexist with current NTSC allocations. The controversy is compounded by the conflicting interests of several groups: notably, broadcasters, cablecasters, and, more recently, the phone companies. Fiber optics, direct-broadcast satellites, and videocassettes are other technologies with the capability for the delivery of HDTV signals into the home. Terrestrial broadcasters are greatly concerned that HDTV pictures delivered by cable, satellite, or videocassette will make their pictures look bad by comparison. Analogies have been drawn comparing the transition from NTSC to HDTV to the rise of FM radio at the expense of AM. Broadcasters and cable operators are similarly concerned that the introduction of fiber optics by cable television and phone companies may allow the delivery of additional and competing services into the home.
To protect owners of existing receivers, the FCC asked proponents of ATV systems to ensure NTSC compatibility. Two approaches were considered: either the NTSC signal would be augmented by a secondary signal that is broadcast on another frequency to complete the ATV information, or the two signals would be simulcast. In the first case, the ATV receivers combine both signals to construct the ATV image. In the second scenario, NTSC receivers continue to receive the NTSC broadcast while ATV receivers tune into the simulcasted ATV signal. In March 1990, the FCC indicated their clear preference for the simulcast rather than the augmentation approach. To enable broadcasters to simulcast NTSC and ATV, the FCC announced their intention to assign to each NTSC broadcaster a second 6 MHz channel for the new ATV service. These channels would come from currently unused and taboo channels. In the plan approved by the FCC in 1992, once the FCC approves a standard and allots channels, stations would have six years to implement ATV service. Simulcasting ATV and NTSC would continue until fifteen years had passed and then the NTSC channel allocation would be surrendered to the FCC. Broadcasters, especially small stations, have argued that even this schedule may be too demanding and may result in some stations going out of business. The cost of passing an ATV signal, with no local production capability, is estimated at $1.8 million. As of the spring of 1995, over 1,600 existing NTSC broadcast stations have received an additional 6 MHz channel allocation for the purpose of ATV transmission.
FCC Testing and Approval
In 1989 the FCC Advisory Committee on Advanced Television Service set up a testing schedule for the proponents of what were then several analog NTSC-compatible advanced television systems who wanted to be considered for approval. Testing took place at the Advanced Television Test Center (ATTC). Initially, testing was to have begun in the spring of 1990 and have continued through the fall of 1991. However, last-minute conversion of several proposals to all-digital systems resulted in a delay in the testing schedule.
It was a game of high stakes, beginning with the testing procedure. Each of the system proponents had to pay a $175,000 testing fee just to reserve a position. The initial round included: Advanced Television Research Consortium (ATRC was composed of the David Sarnoff Research Center, NBC, Thomson Consumer Electronics, and the North American Philips Corporation), Zenith Electronics Corporation and AT&T, Japan Broadcasting Corporation, and the American Television Alliance (ATA), which was made up of two formerly separate proponents: the Massachusetts Institute of Technology (MIT) and General Instruments Corporation. The four proponents submitted six different systems for consideration by the FCC (ATRC and ATA each submitted two proposals).
Perhaps the most striking development in the quest for an improved broadcast television standard was the transition from analog to digital HDTV proposals. General Instruments' DigiCipher HDTV proposal generated interest in June 1990 when it proposed the first all-digital system. In quick succession the other proponents took an all-digital approach. The Advanced Television Research Consortium announced its all-digital system in November 1990, and Zenith-AT&T switched over to an all-digital system in December 1990. After a short time as the only proposal based on analog transmission technology, NHK pulled out of the race altogether.
When testing was completed, the decision by the FCC was that no one system was a clear winner and that there were elements in each proposal which were worth preserving. The recommendation was that the proponents join efforts to create a "grand alliance" for the purpose of developing an HDTV transmission system.
The Grand Alliance
The Grand Alliance was formed in May 1993 by seven organizations. The former competitors turned allies were: AT&T Corporation, General Instrument Corporation (GI), Massachusetts Institute of Technology (MIT), Philips Consumer Electronics, David Sarnoff Research Center, Thomson Consumer Electronics, and Zenith Electronics Corporation. Their mission was to evaluate technologies and to decide on key elements that will be at the heart of the best of the best HDTV system.
Broadcast and cable carriage of digital HDTV signals were tested under field conditions in Charlotte, North Carolina in the summer of 1995. On November 28, 1995, the FCC Advisory Committee on Advanced Television Service recommended that the FCC adopt the ATSC Digital Television Standard as the ATV broadcasting standard. On May 9, 1996, the FCC issued its fifth Further Notice of Proposed Rule Making regarding adopting the ATSC digital television (DTV) standard for terrestrial digital television broadcasting in the United States. And on June 19, 1996, the FCC awarded the first experimental HDTV license to Raleigh, North Carolina television station WRAL-TV.
Specifics of the proposed ATSC DTV Standard include:
Video compression technology based on the MPEG-2 video and systems syntax, including B-pictures, using a motion compensated discrete cosine transform (DCT) algorithm.
Audio encoded using Dolby's Digital Audio Compression (AC-3) Standard. This is a variation of Dolby's Surround Sound used for theatrical film projection. AC-3 allows up to 5.1 audio channels: left, right, center, left surround, right surround and a 0.1 channel for a subwoofer signal. Multiple audio channels permit the inclusion of multiple languages or services for the visually or hearing impaired.
Packetized data transport system allowing for the transmission of virtually any combination of video, audio, and data, and facilitates interoperability with other delivery and imaging systems. Data packets would be 188 bytes long, with 4 bytes of header/descriptor and 184 bytes of payload.
Terrestrial broadcast transmission using 8-VSB (Vestigial Sideband) to minimize potential interference with other service, especially NTSC transmissions. This will deliver an effective payload of approximately 19.28 megabits per second (Mbps). Extensive error correction is employed to counteract the harsh operating conditions encountered in terrestrial broadcasting. For cable delivery, a 16-VSB mode provides twice the capacity of the 8-VBS terrestrial broadcast mode.
Support for both interlaced and progressive scanning modes.
The last two design aspects, progressive scan and square pixels, are important for the "interoperability" of HDTV with computers, telecommunications and other media and applications.
Aspect Ratio
An attribute of HDTV closely related to screen size is aspect ratio. The aspect ratio of most HDTV systems is considerably wider than NTSC television. In fact, the wider aspect ratio is considered by manufacturers to be one of the most important attributes of HDTV. According to experts, for consumers to spend several thousand dollars on a new television receiver there must be several visible and striking differences between the new technology and the old. The aspect ratio of the picture is one such difference. However, the perceived advantage of a widescreen experience comes with the disadvantage of receiver incompatibility.
If the HDTV image is produced in a 16:9 aspect ratio, there would have to be some sort of aspect ratio accommodation to display the image on a 12:9 NTSC television receiver. The difference between 35mm theatrical film and NTSC television aspect ratios is solved in one of several ways. These including the letterbox approach, which preserves the integrity of the film's aspect ratio but introduces black areas at the top and bottom of the television frame, and the pan and scan approach, which compromises the film's aspect ratio but maintains a normal television image. If and when HDTV broadcasting is fully implemented, the aspect ratio problem could become an issue once again. This time, archival NTSC video would be too narrow to fill the HDTV frame and cropping the top and bottom of the frame would be an unlikely solution. While the 16:9 aspect ratio is all but certain, it should be noted that a group of cinematographers has proposed abandoning the 16:9 aspect ratio in favor of 2:1, at least for transmission. Their rational is that 2:1 is a good compromise if all of the various film formats are taken into account.
Interlaced vs. Progressive
One important consideration for HDTV picture creation and display is whether to select interlaced rather than progressive, or sequential, scanning. This debate has pitted broadcaster against computer manufacturers. Interlaced scanning, which is currently used by all worldwide television systems including NTSC, uses two fields to make up each frame of video. The effect is higher dynamic (motion) resolution while conserving precious bandwidth. Each field of video has only half the resolution of the entire frame, but because each field is replaced at twice the frame rate, the movement appears more fluid, and less flicker results. Sequential scanning systems do not divide the frame into two or more fields but rather increase the frame rate to reduce flicker and other motion artifacts. Sequential scanning more accurately approximates the motion picture imaging process. Although this increases the bandwidth requirements of the sequential scanning systems, the dynamic (moving) and temporal (static) resolution are improved over the interlaced systems. Most experts agree that the ideal is a progressive scanning system that has high spatial resolution if the system's bandwidth or data rate can support it. Zenith and AT&T have proposed a progressively scanned 787.5 line, 59.94 field system. The scanning formats selected by the Grand Alliance are focused primarily on computer-friendly progressive scanning, while offering an interlaced mode important to broadcasters. Both interlaced (1440 x 960 x 30 Hz) and progressive (1280 x 720 x 60 Hz) modes will be supported.
Quality or Quantity
Some broadcasters, with Fox TV leading the charge, are especially concerned about the audience's lack of appreciation for the improved quality of HDTV, and would like the option provided by a policy of "flexible use." Reed Hundt, FCC Chairman, has expressed willingness to allow TV broadcasters to use the new spectrum for broadcasting multiple standard-definition television channels, or perhaps even the transmission of data and other non-broadcast services. The Grand Alliance transmission system, although designed initially to deliver HDTV, can generate a 19 megabits-per-second (Mb/s) data stream. With digital compression, one 6 MHz channel for HDTV could instead by used for as many as four or five digital standard-definition television (SDTV) signals. The flexibility of the Grand Alliance DTV standard allows for dynamic scalability, e.g., broadcasters could offer high definition service for movies, sports and during prime-time programming and then switch to multiple standard-definition signals for other day parts. Datacasting, e.g., delivery of paging service, computer data, and other non-television services, could provide an additional revenue stream for broadcasters and serve as a stepping-stone for the transition to digital high definition television service. The decision to allow flexible use of the new 6 MHz spectrum is closely tied to the debate over the process and conditions of spectrum allocation. Legislators and consumer groups have questioned whether the allocation of new spectrum should include requirements on content, e.g., political and children's programming, and the type of services provided. Others, citing concerns about the budget deficit, are calling for the spectrum to be auctioned to the highest bidder.
Display
One interesting thing about HDTV is that tests have shown that the average consumer does not notice much improvement over NTSC when viewing the images on a small display. Futuristic scenarios depict wall-sized, flat-panel LCD displays of HDTV images, but unfortunately, these are still some years away. The choices today include direct-view cathode-ray tubes (CRTs) and projection televisions. For people to fully appreciate HDTV's advantages, a screen size of 36 inches is required, with greatest benefit realized with a screen size exceeding 48 inches. But at several hundred pounds for a cathode-ray tube of that size and a price tag of nearly $5,000, size and cost become important factors for most consumers. Despite the current growth in "home theater" systems, electronics industry experts argue that until the price drops below $500, HDTV will face slow adoption rates by consumers. Even if you have the money to spend, there may be another hitch: most rooms and entryways are too small for a direct-view monitor of this size. A projection television may be easier to get into your house, but everyone has seen poorly aligned (and even properly aligned) projection TVs that displayed a poor image. Light output and resolution will have to increase before projection televisions are widely accepted.
This article was written by Samuel Ebersole for the Focal Encyclopedia of Electronic Media.. Permission for electronic posting courtesy of Focal Press, a division of Butterworth Heineman.
To contact the author, e-mail him at samuel.ebersole@colostate-pueblo.edu
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