Color space conversion for HDTV on computer displays -

Color space conversion for HDTV on computer displays

The adoption of high definition (HDTV) by the broadcast industry is rising worldwide as networks rapidly increase HD programming. Video game console makers have joined the high-definition video movement in recent years. This has created heightened consumer demand for high-definition television sets, since traditional television sets do not have the capabilities for HD formats. In order for viewers to realize HD video content, display resolution, horizontal line rate, and vertical refresh rate, among other requirements, must be met by the TV or display set. Computer display monitors meet these requirements, but require color space conversion to adapt from TV formats to RGB.

Video Formats
Conventional televisions were made for displaying standard NTSC video, which has a resolution of 640×480 at 29.97 frames per second with each frame consisting of two interlaced fields in the United States. (In other parts of the world such as Europe, PAL video has a resolution of 720×576 at 25 frames per seconds and is interlaced as well.)

In moving forward with higher fidelity video, EDTV, or enhanced-definition television, was introduced, improving the picture quality by scanning lines progressively rather than in an interlaced fashion. EDTV formats are typically referred to as 480p or 576p, signifying progressive-scan versions of NTSC and PAL, respectively. This advancement requires a display set capable of handling progressive scan video; something that NTSC and PAL TVs are unable to do.

Taking a step further in video format evolution, HDTV came about to improve the viewing experience by increasing resolution and scanning progressively. Currently, HD consists of three main formats: 720p, 1080i and 1080p. 720p has a resolution of 1280×720 lines that are scanned progressively. 1080i has a resolution of 1920×1080 lines that are interlaced, and 1080p is its progressive scan counterpart.

HD on PC Monitor Displays
Although the world of computer video and graphics may seem distant from television, consider that most computer video formats are non-interlaced, or progressive. Moreover, PC graphic resolutions such as VGA, SXGA or UXGA are in line with, if not exceeding, the resolutions of the TV formats mentioned above. Accordingly, the display sets or monitors that are made for PC video are designed to handle very high resolutions and frame rates. As such, these inherent attributes of PC monitors make them excellent for displaying ED and HD video.

In other words, computer monitors are in fact, high-definition display sets. The only caveat is that ED and HD video employ the YPB PR standard, while computer-based graphics employ the RGBHV standard. Bridging these two standards requires color space conversion, as well as complicated tri-level sync stripping and separation.

Color Spaces
Color is what is perceived by the human brain when the photoreceptors in the eyes are excited with a visual sensation. These colors can be interpreted or defined by specifications or models known as color spaces. Color is described using different color spaces, with each space associated to different applications based on system requirements and needs.

The YPB PR color space is a scaled version of the YUV color space. Color information is determined by two separate chrominance (color) signals, Pb and Pr, which are a function of a third signal, Y, the luminance (brightness) signal. The RGB color space is defined as three components – red, green and blue. It stems from the concept that the human eye is most sensitive to red, green and blue, and all other colors are perceived as a combination of the three. The RGB color space is excellent for ensuring that colors are correctly mapped from input to output without color degradation. Computer graphics have adopted the RGB color space due to these facts, as well as its simplicity. On the other hand, television has adopted the more complex YUV color space in order to economize bandwidth for transmission and broadcast of its video signals.

The components of the YPB PR and RGB color spaces are related to each other by a matrix algebraic relationship, with the coefficients of the matrices varying from format to format, i.e. SDTV, HDTV, etc. These relationship equations are as follows:


For HDTV (720p, 1080i, 1080p):

Figure 1. YPB PR and RGB Color Bar Relationship

Next Page: Sync and YPbPr to RGBHV Conversion Syncs
Aside from color spaces differing between computer graphics and television video, synchronization schemes also must be addressed. With computer graphics, the horizontal and vertical syncs are separate from one another, as individual components, H and V. Hence, the standard referred to as, RGBHV, where all five signal components are transmitted over a cable with a 15-pin d-sub connector. With ED/HDTV’s YPB PR standard, the horizontal and vertical syncs are compositely combined and embedded on the luminance, Y, signal. Since the horizontal and vertical syncs are combined, equalization period and serration pulse schemes are implemented for vertical blanking. Also, syncs are embedded as bi-level syncs for ED and tri-level syncs for HD.

To strip off the composite sync portion of the luminance signal and to separate it into horizontal and vertical components is no trivial task. Dealing with tri-level syncs for HD formats and embedded copyright protection syncs are certainly an added challenge, relative to old bi-level sync separation techniques. As sync separation in itself is beyond the scope of this discussion, take blocks such as comparators for threshold detection, logic gates, sync level-slicing and integrators as just a few of the elements involved in the complex operation.

YPB PR to RGBHV Conversion
Although there are quite a few ways to convert YPB PR to RGBHV, key considerations for implementation are cost, color space decoding precision, sync processing performance and, in some applications, minimal impact to system architecture. Three available options are a discrete design, integration into a costly and complex digital video processor, or a monolithic IC solution that captures the desired function concisely. The former two choices compromise color decoding quality and system cost effectiveness. Carrying out the conversion discretely is cumbersome, requiring a number of devices and board space, as shown in Figure 2. Depending on the desired color accuracy, the discrete design can pose an additional challenge for engineers due to the level of precision in components, and the higher the precision, the higher the component costs. On the other hand, highly integrated digital video processors that may include the YPB PR – to-RGBHV conversion are bundled with an excess of functions, features and programmable registers which make them very cost inefficient and difficult to use and design in.

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Figure 2. Example discrete design for YPB PR to RGBHV conversion

A single IC solution, such as the LMH1251 from National Semiconductor shown in Figure 3, increases quality, performance and ease of operation, while reducing system design complexity, costs and board space. With high-precision resistors internal to the chip, it can provide down to less than 1.5 percent of amplitude and 0.75° of phase color accuracy. The LMH1251 can also detect the incoming video format on its own and automatically apply the appropriate set of color space conversion equations for either ED or HD. The integrated sync processor further simplifies the total conversion process, as tri-level syncs, vertical serrations and copyright protection syncs, among other issues, are all resolved.

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Figure 3. YPB PR to RGBHV conversion simplified by the LMH1251

Another added feature of the chip is a 2:1 high-speed video MUX, well-suited for direct drop-in into the RGBHV signal path in PC monitor designs. Implementing the configuration shown in Figure 4 allows the monitor user to switch between ED/HD video and PC graphics video instantly. For the monitor designer, this simple solution minimizes the re-work of the existing system architecture without the need for lower performing, digitally integrated solutions that require extensive firmware changes.

Featuring this type of conversion capability at the analog front end of PC monitors can be an excellent way to add total system value, as supporting YPB PR component inputs ultimately results in an “HD Ready” display set. Moreover, this addition aligns well with the ongoing trend of integrating multi-media functionality to desktop PCs and displays, and could quite possibly provide a second wind for CRT monitors.

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Figure 4. Example PC monitor application

About the Author
Cliff Win is a Senior Applications Engineer for National Semiconductor's Amplifier Product Line. Previously, he has worked extensively with analog and mixed signal video display ICs in National's CRT and Digital Television group. Cliff holds a Bachelor of Science degree in Electrical Engineering from the University of California at Davis. He can be reached at .

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