 | What is HDTV? |
Technical Answer:
HDTV is a new television standard that is based upon special broadcasting methods and special display hardware. The HDTV standard models the human eye's capabilities more closely than other standard formats such as PAL or NTSC. The HDTV aspect ratio of 16:9 is much closer to the human eye's field of vision at comfortable viewing distances than the standard 4:3. Thus, a higher percentage of visual field is contained by the image on the screen. This results in a more natural appearance of events on the screen. HDTV also has a higher resolution than standard television: 1050 * 600 pixels vs. standard television's 525 * 427. This is twice the resolution in the horizontal plane, and about one-third more resolution in the vertical plane. The HDTV acronym (High Definition Television) is derived from this higher resolution, despite the fact that HDTV's purpose is to better fill the viewers' field of view.
The ideas for HDTV was born with wide-screen movies. Movie producers were editing movies with high-definition television sets as early as 1984, based upon 1970's technology. These television had detail rivaling 35 mm film, so the producers could record, edit, and move to film with a minimum of delay. The electronic nature of the medium allowed for special effects that were difficult or impossible to reproduce with film alone. As great as these methods were for producing films to be seen in theaters, home viewers suffered from chopped images (because of the difference in aspect ratio) and low resolution images. As the push to bring wide-screen HDTV sets to the home market continues, several problems arise.
One of the first problems realized by HDTV advocates was bandwidth. Standard television resolution of 525*427 pixels refreshing at 29.97 Hz require a bandwidth of 3.35 MHz, which is well with the 6 MHz allocated by regulating agencies. HDTV broadcasting at a resolution of 1050*600 at the same refresh rate requires over 18 MHz of bandwidth! This presents engineers and regulating agencies with three options: 1) To increase the channel allocation from 6 MHz to 20 MHz, or 2) Use signal compression to fit inside the current 6 MHz bandwidth allocation, or 3) Allocation of multiple channels for HDTV. As a reallocation of channels is not compatible with NTSC service and signal compression won't bring 18 MHz down to 6 MHZ, only the third option remains viable. One channel could be used for transmitting NTSC signals and the remainder for augmenting HDTV signals. In a broadcast of this type, one existing VHF channel would be associated with two UHF channels, whereby the VHF channel would carry the NTSC signal.
Another problem is that of broadcasting methods. Should HDTV broadcasts be satellite-only, as in Japan (the only country in the world currently broadcasting HDTV), or should they be compatible with existing terrestrial equipment? As previously stated, regulating agencies only allocate 6 MHz per channel. However, they have no jurisdiction over satellite broadcasts or cable transmissions, thus a satellite operator could provide the 20 MHz necessary for uncompressed HDTV transmissions. Satellites are also well suited to delivery of content over a large geographical area. However, designing television sets specifically for these systems discounts the possibility of using existing terrestrial transmission methods. The television would be incompatible with existing NTSC standards- in violation of a 1987 FCC ruling that required compatibility.
The problem of interlacing is less of a hurdle. Actual observed resolution is much lower than the maximum vertical resolution that any television system can deliver, HDTV or otherwise, because there are pixels that do not lie on the scanned lines. Progressively scanned images provide about 70% of the maximum resolution, as does interlacing for stationary objects. Moving objects in interlaced images return a resolution that is about half of the maximum. Interlacing also produces misaligned frames and flicker to the horizontal edges of moving objects. Thus, many HDTV proposals favor progressively scanned images.
The 'Grand Alliance' is a group of television makers that have vowed to work together to design and implement an HDTV standard that the FCC could and would accept for US broadcasts. In response the the FCC's ruling that HDTV standards would be compatible with NTSC, over twenty proposals were presented to the commission within two years. Subsequent FCC decisions that HDTV would be broadcast simultaneously with NTSC and that the standard would be all digital led to the alliance of AT&T, GI, MIT, Philips, Sarnoff, Thomson and Zenith. The Grand Alliance's purpose was to combine the best qualities of each of the HDTV standards proposals and to make them compatible with the FCC rulings. The standard that the alliance subsequently presented was unique in the fact that it was not designed to be compatible with existing standards, rather, it was digital in nature and designed to be compatible mainly with computers. It is a compressed standard, utilizing a discrete cosine transform (DCT) algorithm as well as motion compensated algorithms. As MPEG-4 syntax was not yet widely available at the time of implementation, MPEG-2 was chosen. Full surround sound will be provided under the Dolby AC-3 digital audio compression that is part of the standard. Delivery will be via a switch packet system, where packet headers consist of the first four bytes, and the remaining 184 bytes reserved for the data word. Individual packets are capable of transmitting either video, audio, or auxiliary information such as subtitles. A program clock reference in the transport stream will be used for synchronization. Each stream will carry a presentation time stamp for video/audio sync. A proposed terrestrial broadcasting system utilizes an eight-level vestigial sideband (VSB) technique. Trellis coding/ uncoding enables the 8 levels to be broadcast over four-level AM VSB signals. This system requires much processing power to implement, so the focus on computerized applications is clear. As MPEG-2 is widely used in today's computing applications, standard computer programs will be able to record, edit, and playback HDTV signals.
Short Answer:
HDTV is a television format that offers a larger, ...
Long Answer:
HDTV, or High Definition Television, is a new television format that aims to bring the rich exper...