Computer graphics processing and selective visual display system – Plural display systems – Diverse systems
Reexamination Certificate
1999-01-28
2004-11-16
Wu, Xiao (Department: 2774)
Computer graphics processing and selective visual display system
Plural display systems
Diverse systems
C348S476000
Reexamination Certificate
active
06819305
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to video signals and, more particularly, to a method and apparatus for detecting whether a video output is connected to a monitor or other device.
BACKGROUND OF THE INVENTION
The expanding use of computers for generation and control of entertainment content is giving rise to a need for a means to effectively display computer video output on a standard television. In the home, the television is usually centrally and comfortably located and typically has a larger display than that of a computer monitor, making it attractive for the display of content such as computer video games, Internet web sites and the like. In a business setting, the larger display area of a television is inviting for the display of content such as computer-generated presentations, slides and the like. The video signal format and display generated by a computer, however, differs significantly from the display format and scheme of a television. Moreover, computer monitors are constantly being improved to permit higher resolutions and display of more image information content. Computer users have grown accustomed to these higher resolutions, and wish to duplicate this experience on their televisions. Televisions, however, typically have lower resolutions and utilize a video signal format that complies with stringent video broadcast standards developed nearly fifty years ago. These differing video signal formats and display characteristics have made attainment of an acceptable means for displaying a computer video signal on a television very elusive.
In order to effectively display a computer video signal on a television screen, the computer video signal must be transformed or converted into a television-compatible video signal. This is a complex process involving many issues. While the present invention deals with a particular aspect of this process, stretching or shrinking (“scaling”) horizontal lines of computer video data to fit within the horizontal display area of a television, it is useful to review the fundamental steps of a conventional conversion process. These steps include color space conversion; scan rate conversion; horizontal and vertical scaling, and encoding of the composite waveform in accordance with the desired television signal format.
Color space conversion is performed because computers and televisions generally use different “color spaces” for representation and storage of video color data. A color space is a mathematical representation of a set of colors. Computers typically use the RGB (Red, Green, Blue) color space, while televisions use color spaces based on luminance and chrominance values (the YUV and YCbCr color spaces, for example).
The RGB color space is a digital format widely used in computer graphics and imaging. Red, green and blue are the primary additive colors; components of these primary colors can be combined to form any desired color. The RGB color space is the most prevalent choice for computer graphics frame buffers (the memory used to hold images for display) because computer monitors use red, green and blue phosphors to create the desired color. Consequently, using the RGB color space simplifies the architecture and design of the system.
The RGB color space, however, is not always the best choice when dealing with display of “real world” images. The human eye does not always perceive color as a simple addition of red, blue and green, but rather, sometimes as color difference signals. Moreover, processing an image in the RGB color space is not very efficient. In order to modify the intensity or color of a given pixel, for example, all three RGB values must be read from the frame buffer, the intensity or color calculated and the modifications performed, and the new RGB values calculated and written back to the frame buffer. For these and other reasons, broadcast and television standards generally use color spaces employing luminance and chrominance video signals. Luminance, or luma, refers to the black-and-white information in the video signal and chrominance, or chroma, refers to the color information in the video signal. The YUV, YIQ and YCbCr color spaces are luminance and chrominance based.
The YUV color space is the basic analog format used under the NTSC (National Television Standards Committee) composite color video standard, which is used in North America and other parts of the world, as well as under the PAL (Phase Alternation Line) and SECAM (Sequential Couleur Avec Mémoire) standards, which are used in Europe and elsewhere. The YUV space is comprised of luma (Y) and chroma (U and V) components. The luma component is made up of portions of all three RGB color signals, and the chroma components are color difference signals developed by subtracting the luma component from the blue signal (U) and from the red signal (V). A set of basic equations is used to convert between the RGB and YUV color spaces:
Y
=
0.299
R
′
+
0.587
G
′
+
0.114
B
′
;
U
=
-
0.147
R
′
-
0.289
G
′
+
0.436
B
′
=
0.492
(
B
′
-
Y
)
;
and
V
=
0.615
R
′
-
0.515
G
′
-
0.100
B
′
=
0.877
(
R
′
-
Y
)
.
The prime (′) symbols in the above equations indicate that the RGB values are gamma-corrected. Gamma correction is necessary to compensate for the nonlinear characteristics of displays using phosphors, such as cathode ray tubes (CRTs). In CRT displays, a small change in voltage when the voltage level is low produces a particular change in the output display brightness level, but this same small change in voltage at a higher voltage level will not produce the same magnitude of change in the brightness output level. This effect, or the difference between what should have been measured and what was measured, is known as gamma. Gamma correction adjusts the intensity output of the CRT so that it is roughly linear.
The YUV format is also advantageous in that a black and white display can be driven with just the Y component. For digital RGB values with a range of zero to 255, Y has a range of zero to 255, U a range of zero to ±112 and V a range of zero to ±157.
The YIQ and YCbCr color spaces are derived from the YUV color space and are optionally used by the NTSC composite color video standard. In the YIQ color space, the “I” stands for “in-phase” and the “Q” for “quadrature”, which is the modulation method used to transmit the color information. I and Q are modulated (one modulator is driven by the subcarrier at sine phase; the other modulator is driven by the subcarrier at cosine phase) and added together to form the composite chrominance signal. For digital RGB values with a range of zero to 255, Y has a range of zero to 255, I has a range of zero to ±152, and Q has a range of zero to ±134. The YCbCr color space is a digital component format developed as part of Recommendation ITU-R BT.601 during the development of a worldwide digital component video standard. It is essentially a scaled and offset version of the YUV color space. Y is defined to have a nominal range of 16 to 235; and Cb and Cr are defined to have a range of 16 to 240, with 128 equal to zero.
Another fundamental step in conversion of computer video signals to television video signals is scan rate conversion. Both computers and televisions utilize CRTs having electron guns that produce an electron beam. The beam is attracted to phosphors on the face of the CRT, activating the phosphors and causing them to emit red, green or blue light. The electron beam begins at the top left of the CRT and scans from left to right across the screen, illuminating pixels (which are comprised of the activated phosphors) in the process. Hence, the electron beam is effectively drawing horizontal lines of video, one pixel at a time.
The horizontal scan rate is the number of horizontal lines drawn per second by the electron beam. The horizontal scan rate of a computer monitor is often twice as fast as that of a television monitor. The horizontal
Conexant Systems Inc.
Nguyen Kevin M.
Procopio Cory Hargreaves & Savitch LLP
Wu Xiao
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