Home Theater Video Processors
Home Theater Video P rocessors
Introduction T o Video Scanning P rocessors ------------------------------------- Understanding P rogressive Scanning --------------------------------------------- Variable Line Multipliers ------------------------------------------------------------- Popular Line Doubler and Scalars ---------------------------------------------------

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ur current NTSC color video standard is very old. The original B&W portion of the signal was developed during the 1930s when the largest CRT displays were less than 10 in diameter. The engineers involved at the time probably never thought that many years later the country would have thousands of home theaters with 100/120-size video images using the same basic signal -a signal that has been blown up so large that the scan lines that make up the picture are distractingly obvious. Today, electrical engineering has advanced to a point where it is possible to reduce the visibility of the picture scan lines using sophisticated digital processing techniques. The general term for this process is called line doubling and line quadrupling. It is very popular for high end home theaters because it makes a video image look smooth and film-like. Line doubling/quadrupling is an amazing process, but it requires an investment in two
items: a line doubler/quadrupler and a data grade (or graphics grade) projector. Newer units called scalars, which optimize any source for your projectors optimum scanning rate are beginning to appear on the market. How A Video Image Is Made A video image is made by sweeping an electron beam across phosphors (chemicals that glow when electrically excited) and changing the intensity of the beam to paint the picture with light and dark areas. If you look closely at the picture on a television set, you will see the scan lines and colored phosphor stripes that make up the image. The standard video signal in North America is referred to as NTSC Video or Composite Video. This signal consists of 262.5 horizontal scanning lines per video field, two video fields per video frame (thus 525 lines per frame) and there are 30 video frames scanned per second. The
diagram on the next page shows a simplified scanned image with two video fields combining to make one video frame. This is called interlaced scanning. There are three primary phosphor colors (red, green and blue) used in color video display devices and by combining the excitation of the three different phosphors, a complete spectrum of colors can be reproduced. With a CRT based projection television, the primary colors are projected on top of each other to produce the full spectrum of broadcast colors. How Line Doublers/Quadruplers Work As we mentioned before, the present NTSC 525 line format was developed as a black and white standard in the early 1940s and color was added in 1953. When the standard was originally conceived, the electrical engineers chose 525 lines so that the average viewer would not see the scan lines making up the image. They succeeded in this respect, but, as we mentioned, the picture tubes of the time were considerably smaller than what we have today. Line doublers are really just signal processors that take an NTSC video signal and convert it into a doubled scan rate video signal. A line doubler allows the display of 525 (instead of 262.5) distinct lines every 1/60th of a second, thus reducing line visibility. The missing lines are generated two ways. If there is no motion in the picture, the missing lines are generated from the previous field. If there is motion in the picture. the

How video projectors make full color images
missing line are generated by interpolation the present upper and lower lines.The secret of a well-designed line doubler is the motion detector which allows a choice between the two modes of operation. The line doubler is far from simple. It requires sophisticated digital signal processing algorithms that function in real time to create seamless action with no artifacts. Line Doublers Improve The Picture In Other Ways Too Elimination of color blurring: Because of the limitations of the human visual system, humans cannot see sharp details in colored images. TV engineers exploited this phenomena when the NTSC standard was being developed. As a result, the NTSC video signal has severe chroma (color) bandwidth restrictions. The result is blurry, smeared colors. This effect is further aggravated by storage media, such as VHS tape, that further degrade the chroma. (ever notice that highly saturated reds always look smeared on VHS tape? This problem results from lack of chroma bandwidth.) A solution is circuitry that uses
The Circuitry in the Faroudja 200 Series Line Doublers
How Video Scanning P rocessors Integr ate Into Typical Home Theaters
graphic appears in the video image. It is especially obvious when a color bar test pattern is displayed. Dot crawl is a rapid upward movement of of colored dots on vertical transitions in the graphic. The other artifact, hanging dots, lie underneath all the colored horizontal transitions. Engineers refer to both of these phenomena as the artifacts of of cross-luminance interference. They appear from an imperfect color decoding process. Again, we have techniques that that can minimize these effects. Note the absence of dot crawl and hanging dots in the image above when our improved decoder circuitry is used. It should be noted that these techniques are also being applied to Pal and SECAM video signals. (The NTSC format is used in North America and Japan. PAL is used in most of Europe, Asia and Africa. SECAM is used in France and the countries that comprised the former Soviet Union.) In particular, the color-transition sharpening circuitry, the digital adaptive comb filtering and other chroma decoding techniques are useful for PAL enhancement. The line doubling technology that produces additional scan lines can be applied to SECAM and PAL displays. In Europe where MAC (Multiple Analog Components) transition schemes are being used, line doubling and some of the signal enhancement methods can also be used to improve the the decoded RGB video signal. How They Do It In a line doubler/quadrupler, the processing takes place digitally, so the input analog NTSC video signal is demodulated into red, green and blue signals and then immediately digitized. Then the signal is scan doubled, motion corrected, sharpened, all in the digital domain, and converted back to an analog RGB signal. This signal now scans at 31.5 kHz, twice the NTSC frequency, and is connected to a data grade or graphics grade projector. For quadrupling, four times the NTSC frequency or 63Khz is needed.

Before

Minimizing Dot Crawl with accurate Y/C decoding
the sharper B&W transitions in the signal to create a correction signal to sharpen the color transitions. Eliminating rainbow patterns: Have you ever watched the detail in a hounds tooth sports jacket ripple with colored rainbows as the camera zooms in? If so, you have seen a good example of cross color interference. This annoying artifact is caused by the imperfect separation of the color and the B&W information by the color decoding circuitry. In the past there was little that could be done about this problem. Today we can use digital adaptive comb filter techniques that dont get fooled by areas in the video image that have fine detail. When the techniques are properly executed, the color rippling caused by cross-color interference can be essentially eliminated. Minimizing dot crawl and hanging dot structure: This phenomena is easily seen when a large stationary colored
Elimination of Dot Crawl and Hanging Dots

Before Processing

After Processing
Understanding Progressive Scanning
the electron gun, and head straight for individual phosphor patches deposited on the face plate. After impact, the phosphors glow, for a brief moment, and then extinguish. The key to making a complete video image with this system is to scan all phosphor patches across the face plate repeatedly. And this is where raster scanning comes into the story. Looking straight at the face of a picture tube, the raster scanning process starts in the upper left hand corner. The electron beam is positioned here, electromagnetically, by the deflection yoke assembly. Scanning starts when the beam is rapidly swept from the left side of the tube over to the right, again, electromagnetically. As it runs across the tube face, the electron beam varies in intensity and causes the phosphors to glow in differing amounts. This first completed sweep becomes one thin slice of a complete video image. Next, the beam is then blanked (turned off) and "flys back" to the left hand side of the tube, and then the whole process begins again. Scan.flyback.scan.flyback. this procedure occurs until the scanning reaches the bottom of the tube and one pass is completed. The electron beam is now blanked again, this time for a longer period, and the vertical section of the deflection yoke lifts the electron beam up to the left-hand top of the tube where the next pass begins.
Now that we have illustrated how one complete pass is completed, let's look at how others are added. This can be accomplished in two Raster Scanning 101 ways; either by "interlacing" the scans, or simply writing the entire Raster scanning is the image at once; "progressively". As standard process by which it turns out, you have probably CRT-based display devices seen both methods in use. create video images. There are How picture tubes Interlaced scanning is the other ways to derive images technique utilized by all standard produce light from CRT displays, such as television receivers. It is called vector-based methods (used in interlacing because incomplete "A some air traffic control displays and military applications), fields" are displayed first and then "B fields" come along but by far the most common method used is raster and interlace between the lines. The diagram on the next scanning. Raster scanning refers to the method by which page illustrates this. In case you think this is an odd way video images are actually "assembled" on the face of the to create video images, you're right. But there's a good CRT. But before we dig into the principals of scanning, reason for it, and that is to conserve bandwidth. By using let's consider how standard picture tubes actually scans that interlace, the resultant television signal is half generate light. the size (in frequency) as a progressively scanned one, and in the telecommunications world, bandwidth is scarce. It starts with a device located deep in the neck of all There is only so much bandwidth (frequency spectrum) to picture tubes called an electron gun. Electron guns are go around, so engineers are constantly finding ways to assemblies that are designed to emit, focus and control maximize the amount of information they can fit into a streams of electron particles. They are connected to allotted frequency slots. In the all-analog world of the external high voltage power supplies which generate a 1930s, interlacing was the technique chosen to keep the tremendous potential (27 to 32 Kilovolts) between the size of the signal manageable, and as a side benefit, it electron gun and shadow mask/face plate assemblies. made the receivers less expensive to produce (more on The result is that electrons fly off the cathode surface of this later).

ince debuting in the late 1930s, television receivers and the images they display, have evolved continuously and prodigiously. From small, marginally acceptable, B&W affairs television images have morphed into enormous, full color, theater-like displays. And this remarkable change can be attributed to the unrelenting R&D efforts on the parts of hundreds of video technology companies, and individuals, all in pursuit of progress and "competitive advantage". Yet despite the magnitude of this effort, and major advancements in componentry, such as transistors, integrated circuits and microprocessors, some aspects of today's video displays remain firmly rooted in the past. And one of these is the very basic format by which standard video images are created; via interlaced raster scanning techniques.
Progressive scanning is another way to generate and display video images. Instead of transmitting interlacing A & B fields, a complete video image is transmitted all at once. The computer industry long ago decided that progressive scanning was the technique of choice for them. Since they are not constrained to narrow terrestrial broadcast channels, the computer manufacturers went for maximum image quality. Progressive scanning is a requisite for this.
interlacing brings to us is a reduction in resolution that occurs when fine detailed images move up and down. What happens is that when objects move at exactly the right rate, one video field captures the movement of the object as it scrolls vertically, and the other does not. The effect is to cut the vertical resolution in half because only one field is used to transmit the image. Unfortunately, this often occurs when credits scroll at just the right speed and the result is poor legibility

The Evils of Interlace

What can be done, besides just talking about it?
Not only does the concept of interlacing video images On standard NTSC seem odd, it also produces television receivers, not odd artifacts. The much. Interlacing, and it's engineers that designed attendant artifacts, are the system long ago were simply a way of life. It's well aware of these been that way since the artifacts, but weren't beginning of television bothered because they broadcasting. But don't were considered lose sleep over this, How Raster Scanning Works: imperceptible on the small interlacing artifacts are Scan.Flyback.Scan.Flyback. 5 to 9" B&W displays rarely perceptible on common at the time. And smaller displays (under 50 today? Well, we have inches or so). They really displays over ten times are more of an academic that size and, as a result, interlacing artifacts can problem, and only occasionally seen in significantly larger sometimes be seen. For example: images. But you say you want to build a home theater with a 100" front projected display? Then, there is one device 1) Interline Flicker. Video consists of a rapid series of that can help: a line doubler. images or frames displayed one after another. They occur so rapidly that the human visual system integrates them Line doublers are signal processing devices that take into a continuous moving image. However, if the frequency standard NTSC video, adds some image enhancement, of frames slows down, you will see the video image and converts the signal to progressively scanned 31.5Khz flickering, just like in an old B&W movie. This critical video. Because the output of these devices is "flicker frequency", as measured by countless progressively scanned, the artifacts we illustrated before psychoperceptual studies, occurs somewhere below 50-60 are not seen. (It is impossible to get a 30 hz flicker in a times per second (it depends on the person observing, 60hz progressively scanned image because every single some people are more perceptible to flicker than others.) pixel is refreshed at a 60 hz rate.) But note: because the Now this is not a problem with larger objects being line-doubled output signal is a higher scan rate than displayed because both the A and B fields contain sections NTSC, it must be displayed by a data or graphics-rate from the same image. However, if the image is made up of display device, typically a front projection monitor. These fine horizontal lines, some of the information may not be are more expensive than standard video-grade monitors. averaged over different fields. It will show up in specific fields, either all the A fields, or the B fields, and because Grand Illusions these are drawn 30 times per second, you are bound to see interline flicker. Engineers sometimes refer to this The reason discussions of interlace vs progressive problem as "venetian blind flutter" because venetian blinds scanning are becoming so common these days is because are one of the most common objects demonstrating the of the new digital television standard being developed. phenomena. It occurs when the venetian blind is just at This new standard, DTV (previously referred to as "HDTV" the right size so that each blade of the blind is scanned in and "ATV" ), is almost certainly going to incorporate both the same field. The result is the entire blind pulsates at 30 types of scans. You would think with a new, state-of-thehz. Our diagram shows how this could happen. art, digital television standard about to appear, that interlaced scanning as a technique would be relegated to 2) Reduction of Vertical Resolution. Another artifact that the video history books. However, this is not the case, and

there are several reasons for it. It starts with the Grand Alliance. This consortium of key industry groups, including AT&T, General Instruments, MIT, Philips, Sarnoff Labs, Thompson and Zenith, was allowed by the FCC to combine forces and help define the final digital television standard. Incorporating the desires of the television broadcast industry, the computer industry, and international groups, the Grand Alliance has suggested four main "modes" for the digital television signal format. The chart to the below illustrates the modes suggested as this magazine goes to press (there are already rumors that it may change in the interim). As you can see, three of the modes can be displayed in interlaced form. The lowest resolution mode, 640 x 480, allows four different vertical rates, and one of them is interlaced. The reason for the incorporation of this particular specification is for backward compatibility with existing sets. This format will be able to be utilized by conventional NTSC television receivers after it is converted from digital to analog composite signal form. The purpose of the other interlaced scanning mode is more obtuse. Why would one want to compromise the stellar quality of a 1920 x 1080 high resolution mode with antiquated interlacing scanning? The reason is cost. Building interlaced monitors can be significantly cheaper than progressive scanned ones. Interlaced monitors run at slower horizontal scan rates, so deflection circuitry is less expensive and with interlaced monitors, the bandwidth of the video signal channel is less, so video processing and CRT drive boards are less expensive to design and build. And about the artifacts? On smaller displays artifacts are unlikely to be a problem, because they will be minor in nature and difficult to see at high resolutions. So the television broadcast industry has argued that even at the highest resolution mode, the economics of the matter decree that interlacing still has home in digital television displays. As you may know, the final specifications for DTV are still being worked out. One of the latest conflicts involves the computer industry. Certain vocal representatives are trying to get the Grand Alliance and the FCC to eliminate the inclusion of any of the interlacing formats. Their argument is that of compatibility with the all-digital computer/televisions of the future. Behind the scenes information suggests that cost may be more of an issue. Interlaced images require expensive frame storage RAM to convert the fields into frames and additional memory requirements are not a point relished by the computer industry whose profit margins are razor thin as it is. In any case, they have a valid objection, from their point of view anyway, and it has been officially tossed into the ring with all the other groups involved. We will see what happens, but, we can almost be assured of one thing; interlaced scanning, a primitive technique used almost 60 years ago to trim transmission bandwidth requirements and keep television receiver costs reasonable, continues to exist as a basic technique to create images on CRT-based video displays. It is highly probably that we will still be using it in the all-digital future of television technology.

Comparison of Interlaced and P rogressive Scanning F rames
Variable Line Multipliers
raditionally the technique to reduce the visibility of scan lines on large screen CRT-based displays has been to increase the number of horizontal scanning lines via line doublers, line triplers, and line quadruplers. The fundamental operation of these devices is simple: by increasing the number of horizontal scan lines in the image raster, the vertical line structure of the image becomes finer and significantly less visible. However, recent research into video raster smoothing techniques has revealed a superior method. As it turns out, the simple multiplication of a displays horizontal scanning frequency via integral multiples (2x, 3x, 4x), while easy to do electronically, may produce a less than optimum effect. Too little multiplication still results in some scan line visibility, and too much actually causes scan lines to overlap thereby decreasing vertical resolution. Typically, the optimum scanning frequency, where the scanning lines just blend into each other, is between two integral scanning multiples (2x, 3x, 4x) and requires a variable line multiplier to obtain. Variable line multipliers are designed so that one can dial-in exactly the right amount of line multiplication so that a video displays optimum line density is achieved. This optimum line density, which is characterized by the size of the projection CRT tubes and the size of the scanning electron beam, is the point where horizontal scan lines just blend into each other to produce a seamless, film-like image. Once a variable line multiplier is programmed properly (during set-up), it automatically calculates the with the correct scanning frequency for different aspect ratio video sources. (if you look at the diagram on the following pages you will see that different video sources have different numbers of scanning lines in the active image of the picture. This means that when you blow up that section of the image to fill a screen, a different scanning frequency is necessary to preserve the optimum line density).
Optimum Line Density Means That The Scan Lines Just Blend Into Each Other
Calibrating Variable Line Multipliers
c) Once an image is displayed, use the adjust and select buttons on the remote (see manual) to go to the display setup menu. First, enter the aspect ratio of the screen you are projecting on (4:3 or 16:9). After that, verify that the Display Lines reads < 525 >. If not, adjust it till it does (see manual).

2) Program the Transcanner for optimum line density a) Freeze-frame the video source on a white field test pattern (or a scene with a great deal of white content), then measure the height of the image. (Note: this is not the height of the screen but the actual video image that is projecting on the screen, see our diagram on the next page). b) While standing close to the screen surface, use the video projectors height control to reduce the picture height until the scan lines just begin to touch each other and produce a seamless image. Measure the height of the resultant image. c) Calculate the optimum scan line density by dividing the original height by the new height and multiplying by 525 (see diagram on next page). d) Go to the TranScanners display set-up menu and enter the number just calculated in the Display Lines field. The TranScanner is now programmed to display the projectors optimum line density.
This section is from the DWINs TranScanner Operating Instructions. It illustrates the procedure followed to find the optimum scanning density of a CRT-based video display
1) Install the Transcanner in the system a) Connect AC power to the transcanner, connect the RGBS output jacks to a multisync video display device (projection monitor) and connect a video source to one of TranScanners video inputs (if you have a laserdisc or DVD disc with a disc you can freeze-frame, use this as the source material). b) Turn the TranScanner on. If it doesnt automatically switch to the connected signal source, use the Source button on the remote control to cycle to it.
Adjusting A Variable Line Multiplier For Optimum Line Density
Popular Line Doublers and Scalars
LINE DOUBLERS: IEV TurboScan 1500 - Converts 480I to 480P NEC IPS 4000 - Converts 480I to 480P DVDO -Converts 480I to 480P SONY EXB-DS10 -Converts 480I to 480P
QUADRUPLERS: IEV TurboScan 4000 - Converts 480I to 960P
LINE MULTIPLIERS: DWIN TranScanner - Converts 480i to 960P in 200Khz increments
SCALARS: Communications Specialities Deuce -Converts 480I to 480P, 600P, 960P, 1024P Faroudja DVP-2200- Converts 480I to 480P, 600P Faroudja DVP-3000- Converts 480I to 480P, 600P, 720P, 960P, 1080i, 1080P NEC IPS 4000Q - Converts 480I to 480P, 600P, 768P, 960P

Now that we have illustrated how one complete pass is completed, let's look at how others are added. This can be accomplished in two ways; either Raster Scanning 101 by "interlacing" the scans, or simply writing the entire image at Raster scanning is the standard How picture tubes once; "progressively". As it turns process by which CRT-based out, you have seen both produce light display devices create video methods in use. Interlaced images. There are other ways to scanning is the technique derive images from CRT displays, such as vector-based utilized by all standard NTSC television receivers. It is methods (used in some air traffic control displays and called interlacing because incomplete "A fields" are military applications), but by far the most common method displayed first and then "B fields" come along and used is raster scanning. Raster scanning refers to the interlace between the lines. The diagram on the next page method by which video images are actually "assembled" illustrates this. In case you think this is an odd way to on the face of the CRT. But before we dig into the create video images, you're right. But there's a good principals of scanning, let's consider how standard picture reason for it, and that is to conserve bandwidth. By using tubes actually generate light. scans that interlace, the resultant television signal is half the size (in frequency) as a progressively scanned one, It starts with a device located deep in the neck of all and in the telecommunications world, bandwidth is scarce. picture tubes called an electron gun. Electron guns are There is only so much bandwidth (frequency spectrum) to assemblies that are designed to emit, focus and control go around, so engineers are constantly finding ways to streams of electron particles. They are connected to maximize the amount of information they can fit into a external high voltage power supplies which generate a allotted frequency slots. In the all-analog world of the tremendous potential (27 to 32 Kilovolts) between the 1930s, interlacing was the technique chosen to keep the electron gun and shadow mask/face plate assemblies. size of the signal manageable, and as a side benefit, it The result is that electrons fly off the cathode surface of made the receivers less expensive to produce (more on the electron gun, and head straight for individual phosphor this later). patches deposited on the face plate. After impact, the phosphors glow, for a brief moment, and then extinguish. Progressive scanning is another way to generate and

because of its ability to play back D-3 tapes. Currently, NBC, NHK (Japan), and PBS are the big networks using D-5. Digital Betacam - Digital Betacam was introduced by Sony in 1993 as a successor to Betacam SP. It is a component digital format using 10-bit 4:2:2 sampling. The format has been popular in film transfer because of its excellent quality and its ability to record video with a 16:9 aspect ratio. DV or DVC - This new format introduced in 1995 is the first major, high quality video format to be introduced into the consumer market. The format uses a 5:1 compression, M-JPEG algorithm. Some popular camcorders that utilize the DV format include the Sony VX-1000 and Canon XL-1. DVCPRO - The DVCPRO format was introduced by Panasonic simultaneously when the regular DV format was introduced. Panasonic has pushed the marketing for DVCPRO since it is much more affordable and possesses a quality, meeting or exceeding Betacam SP. DVCPRO is different from the regular DV format because of increased tape speed and wider track pitch. DVCPRO also uses metal particle tape compared to the metal evaporated used on regular DV. DVCAM - DVCAM was introduced by Sony as their professional DV format. The DVCAM recording format incorporates a higher tape speed compared to regular DV, but it is slower than DVCPRO. To compensate for the slower tape speed, DVCAM uses metal evaporated tape. Digital S - Digital S was a format created by JVC. Compared to DV, DVCPRO, and DVCAM, Digital S has two advantages: (1) it uses 4:2:2 sampling to record digital video (like D-1), (2) Digital S VTRs can playback S-VHS tapes. JVC claims that the Digital S format is more robust than DVC,DVCPRO, and DVCAM. Technically, Digital S is better than the DV formats which only use 4:1:1 sampling. As a result, DV does not produce sharp chroma keys. However 4:2:2 allows better color sampling and hence better keys. If tape size contributes to "robustness", then JVC takes the cake, because the format uses 1/2 inch tapes looking similar to S-VHS tapes. In addition, Digital-S is the only deck in the industry that has pre-read capabilities (the ability to record and playback at the same point on the tape track - useful for A/B rolling with only two decks) in the same price class as a high-end Beta SP deck. Currently, the FOX network and its affiliates have begun using Digital S. Betacam SX - Betacam SX was developed by Sony and introduced in 1996. When Digital Betacam was introduced in 1993, Sony believed that it would replace Betacam SPas a new digital video format. Because of forbidding tape costs, Digital Betacam was not accepted as a successor for Beta SP. As the years progressed and with the introduction of new digital formats, Sony took another stab at introducing a successor for Beta SP. Betacam SX, unlike the DV formats, uses 4:4:2 MPEG 2 sampling and 10:1 compression making the image quality close to Digital Betacam. Unlike Digital Betacam, Betacam SX allows the videomaker to playback and record on analog Betacam SP cassettes. (However, the deck can only record the digital signal on the analog cassettes.) Sony also claims that Betacam SX equipment costs much less to buy and run than analog Beta SP.

aspect ratio was officially adopted in 1917 by the Society Of Motion Picture Engineers as their first engineering standard, and the film industry used it almost exclusively for the next 35 years.
Understanding Aspect Ratios
he first thing we want to do is demystify this phrase. An aspect ratio is simply a numerical way of describing a rectangular shape. The aspect ratio of your standard television, for example, is 4:3. This means that the picture is 4 units wide and 3 units high. Interestingly, professional cinematographers tend to prefer a single number to describe screen shapes and reduce the familiar 4:3 television ratio down to 1.33:1, or just 1.33. This is most likely because they deal with a vastly larger number of screen shapes than television people do and out of necessity, long ago, jettisoned bulky fractional descriptions. The History Of Cinema Aspect Ratios
Because of the early precedent set by the motion picture industry with the 4:3 aspect ratio, the television industry adopted the same when television broadcasting began in the 1930s, and today the 4:3 aspect ratio is still the standard for virtually all television monitor and receiver designs. The same situation applies to video programming and software. Only until recently has there been any software available except in 4:3 format (letterboxed videos are the same thing electronically). There simply wasn't any reason to shoot or transfer in any other aspect ratio because of the standard 4:3 shape of the television displays. For the home theater owner, this situation means that compatibility issues are essentially nonexistent with standard 4:3 television receivers and standard 4:3 programming. They are all "plug and play", so to speak, at least when it comes to the shape of image. Getting Wide Back to our history lesson. After many years of experimentation, television broadcasting formally began on April 30, 1939 when NBC broadcasted Franklin Roosevelt's opening of the 1939 World's Fair. As you might imagine, the availability of a device that delivered sound and pictures in the home immediately concerned the Hollywood studios. After all, this medium had the potential to erode their lifeblood; their vital paying customer base. When color was introduced in late 1953, the studios stopped wringing their hands and sprang into action. The result was the rapid development of a multitude of new widescreen projection ratios and several multichannel sound formats. Today, just a few of these widescreen formats survive, but a permanent parting of the ways had occurred: film was now a wide aspect ratio medium, and television remained at the academy standard 4:3 aspect ratio. As we mentioned, the fact that film formats went wide in the 1950s never really impacted the production end of television. Everything stayed at 4:3 for them because of the uniformity of 4:3 television design. However, the transfer of motion pictures to video.that was another story. The question is: How do you make a wide shape fit into a narrow one? One way you've undoubtedly heard about "panning and scanning". This technique of

The Father Of 16:9

The most prevalent aspect ratios filmmakers deal with today are: 1.33 (The Academy standard aspect ratio), 1.67 (The European widescreen aspect ratio), 1.85 (The American widescreen aspect ratio), 2.20 (Panavision), and 2.35 (CinemaScope). Attentive videophiles may note that 1.77 (16:9) isn't on this list and may ask: "If 16:9 isn't a film format, then just exactly where did this ratio come from". The answer to this question is: "Kerns Powers". The story begins in the early 1980s when the issue of high definition video as a replacement for film in movie theaters first began to arise. During this time, the Society Of Motion Picture And Television Engineers (SMPTE) formed a committee, the Working Group On High-Definition Electronic Production, to look into standards for this emerging technology. Kerns H. Powers was then research manager for the Television Communications Division at the David Sarnoff Research Center. As a prominent member of the television industry, he was asked to join the working group, and immediately became embroiled in the issue of aspect ratios and HDTV. The problem was simple to define. The film community for decades has been used to the flexibility of many aspect ratios, but the television community had just one. Obviously a compromise was needed.
As the story goes, using a pencil and a piece of paper, Powers drew the rectangles of all the popular film aspect ratios (each normalized for equal area) and dropped them on top of each other. When he finished, he discovered an amazing thing. Not only did all the rectangles fall within a 1.77 shape, the edges of all the rectangles also fell outside an inner rectangle which also had a 1.77 shape. Powers realized that he had the makings of a "Shoot and Protect" scheme that with the proper masks would permit motion pictures to be released in any aspect ratio. In 1984, this concept was unanimously accepted by the SMPTE working group and soon became the standard for HDTV production worldwide. Ironically, it should be noted, the High-Definition Electronic Production Committee wasn't looking for a display aspect ratio for HDTV monitors, but that's what the 16:9 ratio is used for today. "It was about the electronic production of movies," Kerns Powers states, "that's where the emphasis was". Interestingly, today, there is little talk today about the extinction of film as a motion picture technology, but there is a lot of talk about delivering HDTV into the home. And, as a testament to Kern H. Powers clever solution, it's all going to be on monitors with a 16:9 aspect ratio.
Multiple Aspect Ratio Screens
ariable aspect ratio screen systems are a convenient way to add professional looking screen masking to home theater rooms. Each of the products we describe here are available in many sizes and configurations. This page is simply to illustrate the different types of variable aspect ratio screen systems that you can chose from. For further information, visit the manufacturers web sites.

Cables Consider ations For DTV
ncompressed, high definition video signals run at a data rate of 1.485 Gbps and a bandwidth of 750 MHz. It is no surprise, therefore, that cables designed to operate at 4.2 MHz for analog video have a much harder time at 750 MHz. These high frequencies require greater precision and lower loss than analog. Where effective cable distances were thousands of feet for analog, the distance limitations are greatly reduced for HD. When SMPTE first addressed this problem, they looked at the bit error rate at the output of various cables. Their purpose was to identify the "digital cliff", the point where the signal on a cable goes from "zero" bit errors to unacceptable bit errors. This can occur in as little as 50 feet.
the design, flaws in the manufacturing, or even errors or mishandling during installation of a cable. Ultimately, return loss shows the variations in impedance in a cable, which lead to signal reflection, which is the "return" in return loss. A return loss graph can show things as varied as the wrong impedance plugs attached to the cable, or wrong jacks or plugs in a patch panel. It can also reveal abuse during installation, such as stepping on a cable or bending a cable too tightly, or exceeding the pull strength of the cable. Return loss can even reveal manufacturing errors. Broadcasters are familiar with VSWR--Voltage Standing Wave Ratio, which is a cousin to return loss. For instance, SMPTE recommends a return loss of 15 dB up to the third harmonic of 750 MHz (2.25 GHz), this is equivalent to a VSWR of 1.43:1. If you know VSWR, you will recognize this as a very large amount of return. Others have suggested that 15 dB return loss is insufficient to show many circuit flaws. It is suggested that a twoband approach be taken, since return loss becomes progressively more difficult as frequencies increase. In the band of 5 to 850 MHz, a minimum of 23 dB would be acceptable (equivalent to a VSWR of 1.15:1) and from 850 to 2.25 GHz a minimum 21 dB (equivalent to a VSWR of 1.2:1). Some manufacturers are sweeping cables and showing 21 dB return loss out to 3 GHz, which is even better. So what cables should you use and what cables should
The SMPTE 292M committee cut cables until they established the location of this cliff, cut that distance in half, and measured the level on the cable. From there they came up with the standard: where the signal level has fallen 20 dB, that is as far as your cable can go for HD video. It should be apparent, therefore, that these cables can go up to twice as far as their 'recommended' distance, especially if your receiving device is good at resolving bit errors. Of course, you could look at bit errors yourself, and that would determine whether a particular cable, or series of cables, would work or not. There is one other way to test HD cable and that is by measuring return loss. Return loss shows a number of cable faults with a single measurement, such as flaws in

you avoid? Certainly, the standard video RG-59 cables, with solid insulations and single braid shields lack a number of requirements. First their center conductors are often tin-plated to help prevent oxidation and corrosion. While admirable at analog video frequencies, these features can cause severe loss at HD frequencies. Above 50 MHz, the majority of the signal runs along the surface of the conductor, called "skin effect". What you need is a bare copper conductor, since any tinned wire will have that tin right where the high-frequency signal wants to flow. And tin is a poor conductor compared to copper. Around the conductor is the insulation, called the "dielectric." The performance of the dielectric is indicated by the "velocity of propagation," as listed in manufacturer's catalogs. Older cables use solid polyethylene, with a velocity of propagation of 66 percent. This can easily be surpassed by newer gas-injected foam polyethylene, with velocities in the +80 percent range. The high velocity provides lower high-frequency attenuation. However, foam is inherently softer than a solid dielectric, so foam dielectrics will allow the center conductors to "migrate" when the cable is bent, or otherwise deformed. This can lead to greater impedance variations, with a resultant increase in return loss. Therefore, it is essential that these foam cables have high-density hard-cell foam. The best of these cables exhibit about double the variation of solid cables (3 foamed versus 1-1/2 solid), but with much better high frequency response. This is truly cutting-edge technology for cables, and can be easily determined by stripping the jacked and removing the braid and foil from short samples of cables that you are considering. Just squeeze the dielectric of each sample. The high-density hard cell one should be immediately apparent. Over the dielectric is the shield. Where a single braid was
sufficient coverage for analog video, it is not for HD. Older double braid cables have improved shielding, but the ideal is a combination of foil and braid. Foil is superior at high frequencies, since it offers 100 percent coverage at "skin effect" frequencies. Braid is superior at lower frequencies, so a combination is ideal. Braid coverage should be as high as possible. Maximum braid coverage is around 95 percent for a single braid. The jacket has little effect on the performance of a cable, but a choice of color, and consistency and appearance, will be of concern. There are no standards for color codes (other than red/green/blue indicating RGB-analog video), so you can have any color indicate whatever you want.
From Chapter Nine of The Guide To Digital Television Published by United Entertainment Media 460 Park Avenue South, 9th Floor New York, NY 10016 (212) 378-0449 For more information: www.digitaltelevision.com
Chapter Two: Understanding Digital Signals

From Chapter Two of The Guide To Digital Television
Published by United Entertainment Media 460 Park Avenue South, 9th Floor New York, NY 10016 (212) 378-0449 For more information: www.digitaltelevision.com
n order to understand digital, you must first understand that everything in nature, including the sounds and images you wish to record or transmit, was originally analog. The second thing you must understand is that analog works very well. In fact, because of what analog and digital are, a first-generation analog recording can be a better representation of the original images than a firstgeneration digital recording. This is because digital is a coded approximation of analog. With enough bandwidth, a first-generation analog VTR can record the more "perfect" copy. Digital is a binary language represented by zeros (an "off" state) and ones (an "on" state). Because of this, the signal either exists (on) or does not exist (off). Even with low signal power, if the transmitted digital signal is higher that the background noise level, a perfect picture and sound can be obtained--on is on no matter what the signal strength. The Language Of Digital: Bits & Bytes
percent white, and 3=100 percent white. As we increase the number of bits, we get more accurate with our grayscale. In digital video, black is not at value 0 and white is neither at value 255 for 8-bit nor 1,023 for 10-bit. To add some buffer space and to allow for "superblack" (which is at 0 IRE while regular black is at 7.5 IRE), black is at value 16 while white is at value 235 for 8-bit video. For 10-bit video, we basically multiply the 8-bit numbers by four, yielding black at a value of 64 and white at a value of 940. Also keep in mind that while digital is an approximation of the analog world--the actual analog value is assigned to its closest digital value--human perception has a hard time recognizing the fact that it is being cheated. While very few expert observers might be able to tell that something didn't look right in 8-bit video, 10-bit video looks perfect to the human eye. But as you'll see in Chapter 4: Audio, human ears are not as forgiving as human eyes--in audio most of us require at least 16-bit resolution--while experts argue that 20-bit, or ultimately even 24-bit technology needs to become standard before we have recordings that match the sensitivity of human hearing. Digitizing: Analog To Digital To transform a signal from analog to digital, the analog signal must go through the processes of sampling and quantization. The better the sampling and quantization, the better the digital image will represent the analog image. Sampling is how often a device (like an analog-to-digital converter) samples a signal. This is usually given in a figure like 48 kHz for audio and 13.5 MHz for video. It is usually at least twice the highest analog signal frequency (known as the Nyquist criteria). The official sampling standard for standard definition television is ITU-R 601 (short for ITU-R BT.601-2, also known as "601"). For television pictures, eight or 10-bits are normally used; for sound, 16 or 20-bits are common, and 24-bits are

There is a quip making the rounds that proclaims "compression has never been shown to improve video quality." It's popular with folks who think compression is a bad compromise. If storage costs are dropping and communication bandwidth is rapidly increasing, they reason, why would we want to bother with anything less than "real" video? Surely compression will fall by the wayside once we've reached digital perfection. Other people, like Avid Technology VP Eric Peters, contend that compression is integral to the very nature of media. The word "media," he points out, comes from the fact that a technology, a medium, stands between the originator and the recipient of a message. Frequently that message is a representation of the real world. But no matter how much bandwidth we have, we will never be able to transmit all of the richness of reality. There is, he argues, much more detail in any source than can possibly be communicated. Unless the message is very simple, our representation of it will always be an imperfect reduction of the original. Even as we near the limits of our senses (as we may have with frequency response in digital
if the starting point was 485x740 pixels, 4:2:2, 10-bit sampled, 30 frames per second (fps) pictures. If, however, the starting video was 480x640, 4:1:1, 8-bit, 30 fps, the ratio would be about 4.5:1. Lossless Versus Lossy There are two general types of compression algorithms: lossless and lossy. As the name suggests, a lossless algorithm gives back the original data bit-for-bit on decompression. One common lossless technique is "run length encoding," in which long runs of the same data value are compressed by transmitting a prearranged code for "string of ones" or "string of zeros" followed by a number for the length of the string. Another lossless scheme is similar to Morse Code, where the most frequently occurring letters have the shortest codes. Huffman or entropy coding computes the probability that certain data values will occur and then assigns short codes to those with the highest probability and longer codes to the ones that don't show up very often. Everyday examples of lossless compression can be found in the Macintosh Stuffit program and WinZip for Windows. Lossless processes can be applied safely to your checkbook accounting program, but their compression ratios are usually low--on the order of 2:1. In practice these ratios are unpredictable and depend heavily on the type of data in the files. Alas, pictures are not as predictable as text and bank records, and lossless techniques have only limited effectiveness with video. Work continues on lossless video compression. Increased processing power and new algorithms may eventually make it practical, but for now, virtually all video compression is lossy. Lossy video compression systems use lossless techniques where they can, but the really big savings come from throwing things away. To do this, the image is processed or "transformed" into two groups of data. One group will, ideally, contain all the important information. The other gets all the unimportant information. Only the important stuff needs to be kept and transmitted. Perceptual Coding Lossy compression systems take the performance of our eyes into account as they decide what information to place in the important pile and which to discard in the unimportant pile. They throw away things the eye doesn't notice or won't be too upset about losing. Since our perception of fine color details is limited, chroma resolution can be reduced by factors of two, four, eight or more, depending on the application. Lossy schemes also exploit our lessened ability to see

videocassette format, Digital Betacam, D9 (formerly Digital-S), DVCPRO50, and various implementations of Motion-JPEG are examples of post production gear using intra-frame compression. The MPEG 4:2:2 Profile can also be implemented in an intra-frame fashion. Symmetrical Versus Asymmetrical Compression systems are described as symmetrical if the complexity (and therefore cost) of their encoders and decoders are similar. This is usually the case with recording and professional point-to-point transmission systems. With point-to-multipoint transmission applications, such as broadcasting or mass program distribution where there are few encoders but millions of decoders, an asymmetrical design may be desirable. By increasing complexity in the encoder, you may be able to significantly reduce complexity in the decoders and thus reduce the cost of the consumer reception or playback device. Transforms Transforms manipulate image data in ways that make it easier to separate the important from the unimportant. Three types are currently used for video compression: Wavelets, Fractals, and the Discrete Cosine Transform or DCT. 1) Wavelets--The Wavelet transform employs a succession of mathematical operations that can be thought of as filters that decompose an image into a series of frequency bands. Each band can then be treated differently depending on its visual impact. Since the most visually important information is typically concentrated in the lowest frequencies in the image or in a particular band, they can be coded with more bits than the higher ones. For a given application, data can be reduced by selecting how many bands will be transmitted, how coarsely each will be coded and how much error protection each will receive.The wavelet technique has advantages in that it is computationally simpler than DCT and easily scalable. The same compressed data file can be scaled to different compression ratios simply by discarding some of it prior to transmission. The study of wavelets has lagged about 10 years behind that of DTC, but it is now the subject of intensive research and development. A Wavelet algorithm has been chosen for coding still images and textures in MPEG-4, and another is the basis for the new JPEG-2000 still image standard for which final approval is expected in 2001 (ISO 15444). More applications are likely in the future. 2) Fractals--The fractal transform is also an intra-frame method. It is based on a set of two dimensional patterns discovered by Benoit Mandelbrot at IBM. The idea is that you can recreate any image simply by selecting patterns
from the set and then appropriately sizing, rotating and fitting them into the frame (see figure 1). Rather than transmitting all the data necessary to recreate an image, a fractal coder relies on the pattern set stored in the decoder and sends only information on which patterns to use and how to size and position them. The fractal transform can achieve very high compression ratios and is used extensively for sending images on the Internet. Unfortunately, the process of analyzing original images requires so much computing power that fractals aren't feasible for realtime video. The technique also has difficulties with hard-edged artificial shapes such as character graphics and buildings. It works best with natural objects like leaves, faces and landscapes. 3) DCT--The discrete cosine transform is by far the most used transform in video compression. It's found in both intra-frame and inter-frame systems, and it's the basis for JPEG, MPEG, DV and the H.xxx videoconferencing standards. Like wavelets, DCT is based on the theory that the eye is most sensitive to certain two-dimensional frequencies in an image and much less sensitive to others.With DCT, the picture is divided into small blocks, usually 8 pixels by 8 pixels. The DCT algorithm converts the 64 values that represent the amplitude of each of the pixels in a block into 64 new values (coefficients) that represent how much of each of the 64 frequencies are present. At this point, no compression has taken place. We've traded one batch of 64 numbers for another and we can losslessly reverse the process and get back to our amplitude numbers if we choose--all we did was call those numbers something else. Since most of the information in a scene is concentrated in a few of the lower-frequency coefficients, there will be a large number of coefficients that have a zero value or are very close to zero. These can be rounded off to zero with little visual effect when pixel values are reconstituted by an inverse DCT process in the decoder. The Importance Of Standards The almost universal popularity of DCT illustrates the power of a standard. DCT may not be the best transform, but once a standard (either de facto or de jure) is in wide use, it will be around for a long time. Both equipmentmakers and their customers need stability in the technologies they use, mainly so they can reap the benefits of their investments. The presence of a widely accepted standard provides that stability and raises the performance bar for other technologies that would like to compete. To displace an accepted standard, the competitor can't just be better, it must be several orders of magnitude better (and less expensive won't hurt either).

can be applied to a range of compressed digital video storage and transmission applications. MPEG--MPEG has become the 800--pound gorilla of compression techniques. It is the accepted compression scheme for all sorts of new products and services, from satellite broadcasting to DVD to the new ATSC digital television transmission standard, which includes HDTV. MPEG is an asymmetrical, DCT compression scheme which makes use of both intra- and inter-frame, motion compensated techniques. One of the important things to note about MPEG is that it's not the kind of rigidly defined, single entity we've been used to with NTSC or PAL, or the ITU-R 601 digital component standard. MPEG only defines bit streams and how those streams are to be recognized by decoders and reconstituted into video, audio and other usable information. How the MPEG bit streams are encoded is undefined and left open for continuous innovation and improvement. You'll notice we've been referring to MPEG bit streams in the plural. MPEG isn't a single standard, but rather a collection of standardized compression tools that can be combined as needs dictate. MPEG-1 provided a set of tools designed to record video on CDs at a data rate around 1.5 Mbps. While that work was underway, researchers recognized that similar compression techniques would be useful in all sorts of other applications. The MPEG-2 committee was formed to expand the idea. They understood that a universal compression system capable of meeting the requirements of every application was an unrealistic goal. Not every use needed or could afford all the compression tools that were available. The solution was to provide a series of Profiles and Levels (see figure 2) with an arranged degree of commonality and compatibility between them. Profiles And Levels--The six MPEG-2 Profiles gather together different sets of compression tools into toolkits for different applications. The Levels accommodate four different grades of input video ranging from a limited definition similar to today's consumer equipment all the way to high definition. Though they organized the options better, the levels and profiles still provided too many possible combinations to be practical. So, the choices were further constrained to specific "compliance points" within the overall matrix. So far, 12 compliance points have been defined ranging from the Simple Profile at Main Level (SP@ML) to the High Profile at High Level (HP@HL). The Main Profile at Main Level (MP@ML) is supposed to approximate today's broadcast video quality. Any decoder that is certified at a given compliance point must be able to recognize and decode not only that point's set of tools and video resolutions, but also the tools and resolutions used at other compliance points below it and to the left. Therefore, an MP@ML decoder must also decode SP@ML and MP@LL. Likewise, a compliant

MP@HL decoder would have to decode MP@H14L (a compromise 1440x1080 pixel HDTV format), MP@ML, MP@LL and SP@ML. As with MP@H14L, not all of the defined compliance points have found practical use. By far the most common is MP@ML. The proposed broadcast HDTV systems fall within the MP@HL point. Group Of Pictures--MPEG achieves both good quality and high compression ratios at least in part through its unique frame structure referred to as the "Group of Pictures" or Gop (see figure 3). Three types of frames are employed: 1) intra-coded or "I" frames; 2) predicted "P" frames which are forecast from the previous I or P frame; and 3) "B" frames, which are predicted bidirectionally from both the previous and succeeding I or P frames. A GoP may consist of a single I frame, an I frame followed by a number of P frames, or an I frame followed by a mixture of B and P frames. A GoP ends when the next I frame comes along and starts a new GoP. All the information necessary to reconstruct a single frame of video is contained in an I frame. It uses the most bits and can be decoded on its own without reference to any other frames. There is a limit to the number of frames that can be predicted from another. The inevitable transmission errors and small prediction errors will add up and eventually become intolerable. The arrival of a new I frame refreshes the process, terminates any accumulated errors and allows a new string of predictions to begin. P frames require far fewer bits because they are predicted from the previous I frame. They depend on the decoder having the I frame in memory for reference. Even fewer bits are needed for B frames because they are predicted from both the preceding and following I or P frames, both of which must be in memory in the decoder. The bidirectional prediction of B frames not only saves lots of bits, it also makes it possible to simulate VCR search modes. The Simple Profile does not include B frames in its toolkit,
thus reducing memory requirements and cost in the decoder. All other profiles include B frames as a possibility. As with all MPEG tools, the use, number and order of I, B and P frames is up to the designer of the encoder. The only requirement is that a compliant decoder be able to recognize and decode them if they are used. In practice, other standards that incorporate MPEG such as DVB and ATSC may place further constraints on the possibilities within a particular MPEG compliance point to lower the cost of consumer products. Compression Ratio Versus Picture Quality Because of its unique and flexible arrangement of I, P and B frames, there is little correlation between compression ratio and picture quality in MPEG. High quality can be achieved at low bit rates with a long GoP (usually on the order of 12 to 16 frames). Conversely, the same bit rate with a shorter GoP and/or no B frames will produce a lower quality image. Knowing only one or two parameters is never enough when you're trying to guess the relative performance of two different flavors of MPEG. 4:2:2 Profile As MPEG-2 field experience began to accumulate, it became apparent that, while MP@ML was very good for distributing video, it had shortcomings for post production. The 720x480 and 720x526 sampling structures defined for the Main Level ignored the fact that there are usually 486 active picture lines in 525-line NTSC video and 575 in 625-line PAL. With the possible exception of cut transitions and limited overlays, lossy compressed video cannot be post-processed (resized, zoomed, rotated) in its compressed state. It must first be decoded to some baseband form such as ITU-R 601. Without specialized decoders and encoders designed to exchange information about previous compression operations, the quality of MP@ML deteriorates rapidly when its 4:2:0 color sampling

60 progressive frames and 4:2:2, 10-bit sampling requires just under 2.5 Gbps. Upgrade that to 4:4:4 RGB, add a key channel and you're up to about 5 Gbps. It's easy to see why standards for compressing this stuff might be useful. The MPEG-4 committee was receptive to the idea of a Studio Profile, and their structure provided an opportunity to break the MPEG-2 upper limits of 8-bit sampling and 100 Mbps data rate. The project gathered momentum as numerous participants from throughout the imaging community joined in the work. Final standards documents are expected by the end of 2000. A look at the accompanying table shows three levels in the proposed new profile. Compressed data rates range between 300 Mbps and 2.5 Gbps. With the exception of 10-bit sampling, the Low Level is compatible with and roughly equivalent to the current MPEG-2 Studio Profile at High Level. The Main Level accommodates up to 60 frames progressive, 4:4:4 sampling, and 2048x2048 pixels. The High Level pushes things to 12-bit sampling, 4096x4096 pixels and up to 120 frames per second. The draft standard is expected to include provisions for key channels, although the number of bits for them were still in question as of this writing. Although you can't have everything at once (a 12-bit, 120 fps, 4:4:4:4, 4096x4096 image isn't in the cards), within a level's compressed data rate limitations, you can trade resolution, frame rate, quantizing and sampling strategies to accomplish the task at hand. Like all MPEG standards, this one defines a bitstream syntax and sets parameters for decoder performance. For instance, a compliant High Level decoder could reproduce a 4096x4096 image at 24 frames per second or a 1920x1080 one at 120 fps. At the Main Level, a 1920x1080 image could have as many as 60 fames per second where a 2048x2048 one would be limited to a maximum of 30 fps. As a part of MPEG-4, the Studio Profile could use all the scene composition and interactive tools that are included in the lower profiles. But high-end production already has a large number of sophisticated tools for image composition and manipulation, and it's not clear how or if similar components of the MPEG-4 toolkit will be applied to the Studio Profile. One side benefit of a Studio Profile in the MPEG-4 standard is that basic elements such as colorimetry, macroblock alignments and other parameters will be maintained all the way up and down the chain. That should help maintain quality as the material passes from the highest levels of production all the way down to those Dick Tracy wrist receivers.
The Other MPEGs MPEG 7 and 21 are, thankfully, not new compression standards, but rather attempts to manage motion imaging and multimedia technology. MPEG-7 is described as a Multimedia Content Description Interface (MCDI). It's an attempt to provide a standard means of describing multimedia content. Its quest is to build a standard set of descriptors, description schemes and a standardized language that can be used to describe multimedia information. Unlike today's text-based approaches, such a language might let you search for scenes by the colors and textures they contain or the action that occurs in them. You could play a few notes on a keyboard or enter a sample of a singer's voice and get back a list of similar musical pieces and performances. If the MPEG-7 committee is successful, search engines will have at least a fighting chance of finding the needles we want in the haystack of audio visual material we're creating. A completed standard is expected in September 2000. MPEG-21 is the Group's attempt to get a handle on the overall topic of content delivery. By defining a Multimedia Framework from the viewpoint of the consumer, they hope to understand how various components relate to each other and where gaps in the infrastructure might benefit from new standards.The subjects being investigated overlap and interact. There are network issues like speed, reliability, delay, cost performance and so on. Content quality issues include things such as authenticity (is it what it pretends to be?) and timeliness (can you have it when you want it?), as well as technical and artistic attributes. Ease of use, payment models, search techniques and storage options are all part of the study, as are the areas of consumer rights and privacy. What rights do consumers have to use, copy and pass on content to others? Can they understand those rights? How will consumers protect personal data and can they negotiate privacy with content providers? A technical report on the MPEG-21 framework is scheduled for mid-2000. The Missing MPEGs Since we've discussed MPEG 1, 2, 4, 7 and 21, you might wonder what happened to 3, 5, 6 and the rest of the numbers. MPEG-3 was going to be the standard for HDTV. But early on, it became obvious that MPEG-2 would be capable of handling high definition and MPEG-3 was scrapped. When it came time to pick a number for some new work to follow MPEG-4, there was much speculation about what it would be. (Numbering discussions in standards work are like debates about table shape in diplomacy. They give you something to do while you're trying to get a handle on the serious business.) With one, two and four already in the works, the MPEG folks were on their way to a nice binary sequence. Should

the next one be eight, or should it just be five? In the end, they threw logic to the winds and called it seven. Don't even ask where 21 came from (the century perhaps?). Some Final Thoughts Use clean sources. Compression systems work best with clean source material. Noisy signals, film grain, poorly decoded composite video--all give poor results. Preprocessing that reduces noise, shapes the video bandwidth and corrects other problems can improve compression results, but the best bet is a clean source to begin with. Noisy and degraded images can require a premium of 20 to 50 percent more bits. Milder is better. Video compression has always been with us. (Interlace is a compression technique. 4:2:2 color sampling is a compression technique.) It will always be with us. Nonetheless, you should choose the mildest compression you can afford in any application, particularly in post production where video will go through multiple processing generations. Compression schemes using low bit rates and extensive inter-frame processing are best suited to final program distribution. More is better. Despite the fact that there is only a tenuous relationship between data rate and picture quality, more bits are usually better. Lab results suggest that if you acquire material at a low rate such as 25 Mbps and you'll be posting it on a nonlinear system using the same type of compression, the multigeneration performance will be much better if your posting data rate is higher, say 50 Mbps, than if you stay at the 25 Mbps rate. Avoid compression cascades. When compressed video is decoded, small errors in the form of unwanted high frequencies are introduced where no high frequencies were present in the original. If that video is re-encoded without processing (level changes, zooming, rotation, repositioning) and with the same compression scheme, the coding will usually mask these errors and the effect will be minimal. But if the video is processed or reencoded with a different compression scheme, those high frequencies end up in new locations and the coding system will treat them as new information. The result is an additional loss in quality roughly equal to that experienced when the video was first compressed. Re-coding quality can be significantly improved by passing original coding parameters (motion vectors, quantization tables, frame sequences, etc.) between the decoder and subsequent encoder. Cascades between different transforms (i.e. from DCT based compression to Wavelets and vice versa) seem to be more destructive than cascades using the same transform. Since Murphy's Law is always in effect, these losses never seem to cancel each other, but add rapidly as post production generations accumulate.