Clearing the fog

A primer on High Definition video and what it means to the systemsintegrator and his customers.

SMPTE 240M defines "High Definition" (HD) as video that has a widthof 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 specificationthat is used as the baseline of HD quality. Additionally, SMPTE 260Mdescribes the digital implementation of the 240M standard. HD issubsequently defined as "an image having approximately double thenumber of horizontal and vertical lines as current broadcast televisionwith approximately the same frame rate." The line rate is approximatelydoubled (from about 15 kHz to about 34 kHz), and the image must alsohave a 16:9 aspect ratio. The net result is high-quality and life-likeimagery, better sharpness and detail, a truer reproduction of color, afilm-like appearance, digital storage and transmission, variable bitrates for optimal balance of quality to medium, compatibility with filmand computer image formats, global compatibility, and, perhaps mostimportant, lower costs for production, post producti on anddistribution.

Further, within the scope of HD, there can be various levels ofquality and different configurations. Although it was initiallydeveloped as an advancement of television, its quality and featureshave brought HD to the attention of other media industries. Filmproduction, computer graphics, special effects, digital cinema,multimedia artists, medical imagery, post production and others havebegun to make the transition to HD.

HD may have segmented configuration standards designed for specificapplications. Within this article, HD refers to any High Definitionvideo imagery. HDTV, on the other hand, is the specific subsetconfiguration that is part of a broadcast television system. HDTV (HighDefinition Television), because of its quality and limitation on datarate, is more of a consumer-oriented product. HD can be used atwhatever quality and data rate is desired. Thus, its flexibility (andthe increased costs) make HD more of a professional medium.

HDTV

At the end of 1995, after many false starts and modifications, theAdvisory Committee on Advanced Television Service submitted its finalreport on advanced television (ATV). In this report, the committeerecommended that the ATSC digital television standard be adopted as theUnited States ATV broadcast standard. The ATSC established thatbroadcast HDTV systems must be digital and part of the ATSC's ratifiedformats, use video compression syntax that conforms with MPEG-2 at abit rate of approximately 18.9 Mbps, provide a signal quality equal orsuperior to SMPTE240M/274M, not take up more bandwidth than theexisting 6 MHz of normal television channels, and use Dolby AC3 foraudio at a nominal data rate of 384 kbps.

In 1994, the Moving Pictures Experts Group defined a standard forthe digital coding and handling of moving pictures and audio. Improvingon an earlier standard, (MPEG-1, ISO/IEC-11172), MPEG-2 (ISO/IEC 13818)was designed to be flexible, expandable, scalable and have a higherquality suitable for the broadcast industry. The original design ofMPEG-2 was founded upon the requirements of normal video, but it soonexpanded to include increased requirements of HD. Although MPEG-3 wasbeing developed for HD, it was realized that MPEG-2 could be modifiedto include HD, and MPEG-3 was dropped.

MPEG-2 has various profiles and levels that are set up to segmentand classify the configuration, bit rate and resolution of the video.In this way, the most appropriate and least expensive encoding systemscan be used while maintaining compatibility with the MPEG-2 standard.Table 1 shows the different profiles and levels in MPEG-2 video. NormalDTV 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 beused to distribute the encoded image data. SMPTE 310M is a transportstream based on MPEG-2 that is used for the ATSC 19.4 Mbps HDTVstandard. The DVB transport protocol used for satellite, cable andEuropean digital video transmission is also based upon the MPEG-2transport stream structure.

HD signal flow

HD begins life as either film transferred to HD, a productioncaptured with HD equipment, computer animation rendered as HD orstandard video unconverted to HD. Within the production andpost-production environment, HD is usually transferred betweenequipment as an SDI-HD (serial digital interface-HD, SMPTE 292M, 1.5Gbps) signal. SDI-HD has become the predominant standard foruncompressed digital HD signals.

Editing and post production can take place with uncompressed HD, butunless quality matters more than money, MPEG-2 encoding usually happensfirst. 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 canor may need to be set for the specific use. In this case, professionalencoding services should be consulted and used, especially for thefirst few projects.

Given good computing algorithms and enough processing power, thesignal that goes in does not have to equal the signal that comes out.Although it is always true that you cannot completely recreate missingresolution, I have seen some extremely good attempts. Some of theNTSC-to-HD upconverters on display at NAB this year produced outputthat would be acceptable to all but the most critical viewer.

The MPEG-2 encoders' output can be set to any number of establishedstandards. For DTV in the United States, the standard is anATSC-compliant MPEG-2 transport stream (SMPTE 310). Some encoders canchange bit rates by taking MPEG-2 in, processing it and outputtingMPEG-2 at a different rate. This data stream output can be stored ordistributed and broadcast as desired. The particular methods of storageor distribution can dictate the format and bit rate from the decoder.It should be noted that the many different ways of handling these largeamounts of data has helped contribute to the confusion of HD.

The telecommunications industry has always been in the business ofcontrolling the synchronized distribution of large amounts of data. Theequipment, terminology and data rates they established were folded intothe early architecture of digital HD. Image data streams, unlike filesor e-mail, work best with network protocols that maintain constantpacket order and signal flow, like ATM's point-to-point rather thanTCP/IP's send-it-everywhere-and-see-whatever-gets-there-whenever. Thedata rates for the various types and combinations of networkconnections became the bit rate for many MPEG-2 transport streamstandards.

Storage

Once you have encoded your HD content, you can store it, stream itor decode it. In the computer world, HD is usually stored on harddrives, digital liner tape (DLT) and DVD-ROMs. At a data rate of 19.4Mbps (2.425 MBs) for ATSC-compliant HDTV MPEG-2 files, it takes 145.5Mb a minute and 8.73 GB per hour to store. As the bit rates rise, thestorage requirements for HD grow quickly. Compressed but high-qualityHD can have 64 Mbps to 120 Mbps (8 MBs to 15 MBs). An uncompressed HDdata stream requires approximately 11.25 GB a minute to store.

In the video world, tape is the preferred medium with Panasonic'sD-5 format and Sony's HDCam being the most popular. Incidentally, an HDversion of the DVCPRO with 100 Mbps was demonstrated at NAB. Designingan HD VCR on a lower cost platform like DVCPRO will bring more peopleinto the world of HD production.

For storage and playback, tape has the clear economic andflexibility advantage. Hard drives are better suited to nonlinearaccess 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 systemsintegrators were offered methods of using some kind of data network asa distribution pathway for streaming media. Data networks and theirinterconnective, open nature are currently being introduced as anupward migration path from traditional analog baseband and RFdistribution. Analog is far from dead, but it is no longer the only wayto deliver content. Real-time HD, much less regular video, is not yetrealistic over the public network, but it is clear that the directionhas been established. Within a private network, paying for theconnection is the only real limit that you will find in streaming.

There are some protocol issues to keep in mind with streaming MPEG-2content over the Internet. The TCP/IP data packets and MPEG-2 datapackets are not the same size. This means that when MPEG-2 is sent overthe Internet, there is unused space in some of the TCP/IP packets.Also, the Internet was never designed for maintaining sequential packetdelivery. Between fractionating MPEG-2 packets and Internet latency,reliability and transfer rates can suffer. To solve this issue, manycompanies provide software to pre- and post-process MPEG-2 to slipefficiently across the Internet.

Decoding

How the data stream is decoded depends upon its ultimatedestination. If it is going into a digital device of some sort, thentranscoding or bit rate changing may be required. If analog signals arenecessary, then an analog decoder is needed. In either example, thesolution is a piece of interface equipment. Not too long ago, findingSDI-HD-to-analog decoders was extremely difficult because they wereexpensive and not readily available. Now, high-quality, inexpensivedecoders are coming to market from many vendors.

PCs even offer solutions. Some computer graphics cards haveinexpensive DTV tuners as options. With such a card, a PC and a TVantenna, watching HDTV on your computer will be about as easy aswatching a DVD-video. The quality may not be at the presentation level,but as an inexpensive way to begin with HDTV, it simply cannot bebeat.

Viewing HD signals

To display HD, the requirements are fairly straitforward. Anydisplay device (CRT, projector or flat panel) that can accept an imagewith a 16:9 aspect ratio and 1,920 infinity 1,080 active pixels comingin at around 34 kHz horizontal and 60 Hz vertical can potentially showHD. Most XGA-compatible or higher displays can show HD, although aspectratio and color distortion may occur. Variable-resolution displays willmost likely do better than those with fixed resolution, unless thescaling devices on the fixed-resolution displays have an understandingof HD. For testing and occasional watching, a good multi-frequencycomputer monitor that can show 1,600 infinity 1,200 seems to work justfine. Using a display designed for HD should work better with lessconnection complications, but be prepared for a hefty price premium.For the near term, video projectors with HD-compatible inputs willprobably be the most popular method of displaying HD signals.

Additionally, certain hardware is necessary to view HD signals. Atthe consumer and prosumer level, HDTV systems need to have a tuner ordemodulator to separate the digital signal from the analog carrier wave(8VSB) on which they are transmitted, a demultiplexer to separate theaudio and video portion of the ATSC transport stream, an MPEG-2 videoand audio decoder, and an HDTV-compatible display. All of thesefunctions, except the display, are performed within an ATSC DTVreceiver. All of the DTV receivers on the market take in the ATSCMPEG-2 transport stream and produce analog outputs. The outputs rangefrom NTSC composite to high-frequency component (RGB or YPbPr) atSMPTE-240M. Every manufacturer seems to have its own opinion on whatconnectors will have which output signals, and some DTV receiversproduce unique outputs designed primarily for specific televisionmodels.

Because many displays accept high-frequency RGB, not YPbPr, you mayneed either a DTV receiver with selectable outputs or a YPbPr-to-RGBconverter. This brings up the ongoing issue of component video's havinga number of different meanings. RGB, RGBS, RGsB, RGBHV, YPbPr, YR-yB-y,YCrCb, YUV and Y-C are all called component video by knowledgeablepeople from different industries. The first four are red, green andblue signals with different configurations of sync. The next four areluminance (Y) and various methods of calculating color space differenceencoding. The last one is simply composite video with separate chromaand luminance channels, and hopefully, it will have nothing to do withHD. Given a choice, I always recommend RGBS as the means ofestablishing connections between devices.

HDTV content is already out there in a few different ways. HBO andShowTime currently broadcast HDTV content via satellite, but acompatible HDTV satellite receiver is required to view it. Most largercities have at least one television station that performs some HDTVtransmissions. Content can be put on an HD playback device and usedlike any other controlled video playback equipment. Such companies asVisual Circuits, Electrosonics, Videon Central, Alcorn Mcbride,Sencore, Quvis, Pluto, Panasonic and Sony offer equipment to store andplayback HD and HDTV content. Some have 8VSB output and require a DTVtuner, some digital (SDI-HD or a DVB transport stream) and some analogcomponent. It is best to make sure you understand the input and outputformat for the equipment you are using. It is really easy (and somewhatfrustrating) to get everything together only to discover that you haveincompatible pieces of equipment and that you cannot get the converterfrom what you have to w! hat you need.