XvColour or xvYCC denotes a Sony based on the Kodak PhotoCD YCC color space (YCbCr ) developed, expanded color space for the transmission of digital video signals. The corresponding color standard is IEC61966 -2- 4th
Implementation and effects of the extended color space
YCC as a basis
The color gamut of the YCC color space is the same as that of the sRGB color space, as well as the white point D65 and the gamma value 2.2. During sRGB colors than three gamma corrected channels R ', G ' and B ' to be coded, the YCC color space in the color three channels Y, Cb and Cr are encoded ( hence the name). The channel Y encodes the luminance ( brightness ), the two values Cb and Cr encode two chroma channels ( chroma ). Cb is the difference between blue and yellow, Cr represents the difference between green and red dar. through a transformation matrix can be converted to sRGB YCC signals signals simple. The luminance Y is sufficient for the used 8-bit code from 16 to 235, the chroma values of each range from 16 to 240 ( out of the theoretically possible value of 0-255). The reason for limiting the range of values in such a way was a desire to avoid clipping signal peaks at the upper and lower end of the chroma and luminance channels, which may result from overshoot in the transmission of analog signals. As a result, the area reduction is that luminance values between 0 and 16, ie dark gray colors as black and light gray colors 236-255 are shown as white. In the chroma channels, range restriction has the effect that the colors green and red and yellow and blue to achieve the highest saturation value already at values of 16 or 240. This of course means a strong reduction of the 8-bit theoretically representable chroma and brightness values .
Innovations of the xvYCC color space
In digital transmission of signals, which are increasingly widespread in the field of television technology, no more area must be reserved for the overshoot. The extended color space xvYCC therefore encodes luminance and chroma channels with the full range of values from 0 to 255 and includes all body colors and optimal colors, ie all physically realizable colors that can be caused by reflection or transmission. Since the encoding of xvYCC in principle no different from YCC, the backward compatibility of the standards is guaranteed, provided that the recording standard used supports a full 8 -bit range of values ( eg this is the case with MPEG -2). In xvYCC encoded signals in the sRGB color space and negative values can result. This results in colors that are outside the representable in RGB range, so colors, the playback devices can not produce. Front of the display so the conversion to the color can be represented with the device to be made. The ability to theoretically produce with xvYCC all displayable colors with the result that 8 -bit coded signals in playback devices prove with extended color space to be disadvantageous. Even though it is a larger color space available, but the colors can be resolved only in 255 color gradations. The effect is that no smooth color transitions with large device color spaces are possible and the image quality suffers greatly despite satterer colors. The only possibility is to encode the xvYCC signals with higher bit widths. We recommend a minimum width of 10 bits to up to 16 bits. But even such high bit widths can still be - cause perceptible gradations, such Experience with extended color spaces such as ProPhoto, WideGamutRGB and others - added small. The use of a higher bit width for data transmission has a naturally higher required transmission bandwidth which can be partially compensated by more efficient compression algorithms for sequence.
For the presentation of xvYCC -encoded signals following requirements are necessary:
- A xvYCC -compatible signal source. These are so far only HD video camcorder and BluRay players (especially for AVCREC DVDs).
- A digital connection between source and display. Single medium is in this case so far HDMI version 1.3.
- A display device that can display xvYCC -encoded image data. This condition shall comply with flat screens now many manufacturers.
Whether DVDs, Blu -ray or satellite television in the future xvYCC -encoded, remains to be seen and depends on the behavior of old play equipment at loading with appropriately coded signals from ( backward compatibility ).
- Video technology
- Color system