Clearing the fog
May 1, 2000 12:00 PM, Josh Kairoff
A primer on High Definition video and what it means to the systems integrator and his customers.
SMPTE 240M defines "High Definition" (HD) as video that has a width of 1,920 active pixels, 1,080 lines, a 60 Hz or 59.94 Hz frame rate, and 30 MHz of bandwidth. This is the original analog HD specification that is used as the baseline of HD quality. Additionally, SMPTE 260M describes the digital implementation of the 240M standard. HD is subsequently defined as "an image having approximately double the number of horizontal and vertical lines as current broadcast television with approximately the same frame rate." The line rate is approximately doubled (from about 15 kHz to about 34 kHz), and the image must also have a 16:9 aspect ratio. The net result is high-quality and life-like imagery, better sharpness and detail, a truer reproduction of color, a film-like appearance, digital storage and transmission, variable bit rates for optimal balance of quality to medium, compatibility with film and computer image formats, global compatibility, and, perhaps most important, lower cos! ts for production, post producti on and distribution.
Further, within the scope of HD, there can be various levels of quality and different configurations. Although it was initially developed as an advancement of television, its quality and features have brought HD to the attention of other media industries. Film production, computer graphics, special effects, digital cinema, multimedia artists, medical imagery, post production and others have begun to make the transition to HD.
HD may have segmented configuration standards designed for specific applications. Within this article, HD refers to any High Definition video imagery. HDTV, on the other hand, is the specific subset configuration that is part of a broadcast television system. HDTV (High Definition Television), because of its quality and limitation on data rate, is more of a consumer-oriented product. HD can be used at whatever quality and data rate is desired. Thus, its flexibility (and the increased costs) make HD more of a professional medium.
HDTV
At the end of 1995, after many false starts and modifications, the Advisory Committee on Advanced Television Service submitted its final report on advanced television (ATV). In this report, the committee recommended that the ATSC digital television standard be adopted as the United States ATV broadcast standard. The ATSC established that broadcast HDTV systems must be digital and part of the ATSC's ratified formats, use video compression syntax that conforms with MPEG-2 at a bit rate of approximately 18.9 Mbps, provide a signal quality equal or superior to SMPTE240M/274M, not take up more bandwidth than the existing 6 MHz of normal television channels, and use Dolby AC3 for audio at a nominal data rate of 384 kbps.
In 1994, the Moving Pictures Experts Group defined a standard for the digital coding and handling of moving pictures and audio. Improving on an earlier standard, (MPEG-1, ISO/IEC-11172), MPEG-2 (ISO/IEC 13818) was designed to be flexible, expandable, scalable and have a higher quality suitable for the broadcast industry. The original design of MPEG-2 was founded upon the requirements of normal video, but it soon expanded to include increased requirements of HD. Although MPEG-3 was being developed for HD, it was realized that MPEG-2 could be modified to include HD, and MPEG-3 was dropped.
MPEG-2 has various profiles and levels that are set up to segment and classify the configuration, bit rate and resolution of the video. In this way, the most appropriate and least expensive encoding systems can be used while maintaining compatibility with the MPEG-2 standard. Table 1 shows the different profiles and levels in MPEG-2 video. Normal DTV video would be MP@ML, and ATSC HDTV 1,080i would be MP@HL.
MPEG-2 also defines a data transport stream structure that can be used to distribute the encoded image data. SMPTE 310M is a transport stream based on MPEG-2 that is used for the ATSC 19.4 Mbps HDTV standard. The DVB transport protocol used for satellite, cable and European digital video transmission is also based upon the MPEG-2 transport stream structure.
HD signal flow
HD begins life as either film transferred to HD, a production captured with HD equipment, computer animation rendered as HD or standard video unconverted to HD. Within the production and post-production environment, HD is usually transferred between equipment as an SDI-HD (serial digital interface-HD, SMPTE 292M, 1.5 Gbps) signal. SDI-HD has become the predominant standard for uncompressed digital HD signals.
Editing and post production can take place with uncompressed HD, but unless quality matters more than money, MPEG-2 encoding usually happens first. MPEG-2 encoding is logistically simple but technically complex. Functionally, an engineer simply hooks the SDI-HD up to the encoders' input, selects the output format and runs with it. Technically, however, there is a wide variety of settings and adjustments that can or may need to be set for the specific use. In this case, professional encoding services should be consulted and used, especially for the first few projects.
Given good computing algorithms and enough processing power, the signal that goes in does not have to equal the signal that comes out. Although it is always true that you cannot completely recreate missing resolution, I have seen some extremely good attempts. Some of the NTSC-to-HD upconverters on display at NAB this year produced output that would be acceptable to all but the most critical viewer.
The MPEG-2 encoders' output can be set to any number of established standards. For DTV in the United States, the standard is an ATSC-compliant MPEG-2 transport stream (SMPTE 310). Some encoders can change bit rates by taking MPEG-2 in, processing it and outputting MPEG-2 at a different rate. This data stream output can be stored or distributed and broadcast as desired. The particular methods of storage or distribution can dictate the format and bit rate from the decoder. It should be noted that the many different ways of handling these large amounts of data has helped contribute to the confusion of HD.
The telecommunications industry has always been in the business of controlling the synchronized distribution of large amounts of data. The equipment, terminology and data rates they established were folded into the early architecture of digital HD. Image data streams, unlike files or e-mail, work best with network protocols that maintain constant packet order and signal flow, like ATM's point-to-point rather than TCP/IP's send-it-everywhere-and-see-whatever-gets-there-whenever. The data rates for the various types and combinations of network connections became the bit rate for many MPEG-2 transport stream standards.
Storage
Once you have encoded your HD content, you can store it, stream it or decode it. In the computer world, HD is usually stored on hard drives, digital liner tape (DLT) and DVD-ROMs. At a data rate of 19.4 Mbps (2.425 MBs) for ATSC-compliant HDTV MPEG-2 files, it takes 145.5 Mb a minute and 8.73 GB per hour to store. As the bit rates rise, the storage requirements for HD grow quickly. Compressed but high-quality HD can have 64 Mbps to 120 Mbps (8 MBs to 15 MBs). An uncompressed HD data stream requires approximately 11.25 GB a minute to store.
In the video world, tape is the preferred medium with Panasonic's D-5 format and Sony's HDCam being the most popular. Incidentally, an HD version of the DVCPRO with 100 Mbps was demonstrated at NAB. Designing an HD VCR on a lower cost platform like DVCPRO will bring more people into the world of HD production.
For storage and playback, tape has the clear economic and flexibility advantage. Hard drives are better suited to nonlinear access and repetitive play, but with high bit rates or long content, they can become prohibitively expensive.
Streaming
This year at NAB, broadcasters, content creators and systems integrators were offered methods of using some kind of data network as a distribution pathway for streaming media. Data networks and their interconnective, open nature are currently being introduced as an upward migration path from traditional analog baseband and RF distribution. Analog is far from dead, but it is no longer the only way to deliver content. Real-time HD, much less regular video, is not yet realistic over the public network, but it is clear that the direction has been established. Within a private network, paying for the connection is the only real limit that you will find in streaming.
There are some protocol issues to keep in mind with streaming MPEG-2 content over the Internet. The TCP/IP data packets and MPEG-2 data packets are not the same size. This means that when MPEG-2 is sent over the Internet, there is unused space in some of the TCP/IP packets. Also, the Internet was never designed for maintaining sequential packet delivery. Between fractionating MPEG-2 packets and Internet latency, reliability and transfer rates can suffer. To solve this issue, many companies provide software to pre- and post-process MPEG-2 to slip efficiently across the Internet.
Decoding
How the data stream is decoded depends upon its ultimate destination. If it is going into a digital device of some sort, then transcoding or bit rate changing may be required. If analog signals are necessary, then an analog decoder is needed. In either example, the solution is a piece of interface equipment. Not too long ago, finding SDI-HD-to-analog decoders was extremely difficult because they were expensive and not readily available. Now, high-quality, inexpensive decoders are coming to market from many vendors.
PCs even offer solutions. Some computer graphics cards have inexpensive DTV tuners as options. With such a card, a PC and a TV antenna, watching HDTV on your computer will be about as easy as watching a DVD-video. The quality may not be at the presentation level, but as an inexpensive way to begin with HDTV, it simply cannot be beat.
Viewing HD signals
To display HD, the requirements are fairly straitforward. Any display device (CRT, projector or flat panel) that can accept an image with a 16:9 aspect ratio and 1,920 infinity 1,080 active pixels coming in at around 34 kHz horizontal and 60 Hz vertical can potentially show HD. Most XGA-compatible or higher displays can show HD, although aspect ratio and color distortion may occur. Variable-resolution displays will most likely do better than those with fixed resolution, unless the scaling devices on the fixed-resolution displays have an understanding of HD. For testing and occasional watching, a good multi-frequency computer monitor that can show 1,600 infinity 1,200 seems to work just fine. Using a display designed for HD should work better with less connection complications, but be prepared for a hefty price premium. For the near term, video projectors with HD-compatible inputs will probably be the most popular method of displaying HD signals.
Additionally, certain hardware is necessary to view HD signals. At the consumer and prosumer level, HDTV systems need to have a tuner or demodulator to separate the digital signal from the analog carrier wave (8VSB) on which they are transmitted, a demultiplexer to separate the audio and video portion of the ATSC transport stream, an MPEG-2 video and audio decoder, and an HDTV-compatible display. All of these functions, except the display, are performed within an ATSC DTV receiver. All of the DTV receivers on the market take in the ATSC MPEG-2 transport stream and produce analog outputs. The outputs range from NTSC composite to high-frequency component (RGB or YPbPr) at SMPTE-240M. Every manufacturer seems to have its own opinion on what connectors will have which output signals, and some DTV receivers produce unique outputs designed primarily for specific television models.
Because many displays accept high-frequency RGB, not YPbPr, you may need either a DTV receiver with selectable outputs or a YPbPr-to-RGB converter. This brings up the ongoing issue of component video's having a number of different meanings. RGB, RGBS, RGsB, RGBHV, YPbPr, YR-yB-y, YCrCb, YUV and Y-C are all called component video by knowledgeable people from different industries. The first four are red, green and blue signals with different configurations of sync. The next four are luminance (Y) and various methods of calculating color space difference encoding. The last one is simply composite video with separate chroma and luminance channels, and hopefully, it will have nothing to do with HD. Given a choice, I always recommend RGBS as the means of establishing connections between devices.
HDTV content is already out there in a few different ways. HBO and ShowTime currently broadcast HDTV content via satellite, but a compatible HDTV satellite receiver is required to view it. Most larger cities have at least one television station that performs some HDTV transmissions. Content can be put on an HD playback device and used like any other controlled video playback equipment. Such companies as Visual Circuits, Electrosonics, Videon Central, Alcorn Mcbride, Sencore, Quvis, Pluto, Panasonic and Sony offer equipment to store and playback HD and HDTV content. Some have 8VSB output and require a DTV tuner, some digital (SDI-HD or a DVB transport stream) and some analog component. It is best to make sure you understand the input and output format for the equipment you are using. It is really easy (and somewhat frustrating) to get everything together only to discover that you have incompatible pieces of equipment and that you cannot get the converter from what you have to w! hat you need.
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