It can be reduced to 16 bits (5:6:5,
It can be reduced to 16 bits (5:6:5, giving 65,536 colors), 15 bits (5:5:5 giving 32,786 colors), or to 8 bits (giving 256 colors). Note that the designation indicates the bit depths for R:G:B. Eight-bit coding uses a predefined palette of colors (indexed). For very low bit rate channels, the reduction in color resolution can be acceptable, but continuous-tone objects will exhibit a posterization effect. Subsampling The alternative to reducing the bit depth is to use the perceptual redundancy of human visual perception. This allows the color resolution to be reduced relative to the luminance resolution, without apparent loss of picture resolution. This is achieved by subsampling the color information. YUV format The red, green, and blue signals from the camera are matrixed to give a luminance signal (Y) and two color-difference signals (Blue Y) or U, and (Red Y) or V. This is called YUV. It allows compatibility with legacy monochrome systems, and allows the color resolution to be reduced to save on channel bandwidth and reduce program storage requirements. YUV coding is the standard for interconnecting video equipment, whether as the 601 digital format, in IEEE- 1394 compressed digital format, or encoded as a composite analog signal (NTSC and PAL). Raw RGB occasionally is found in graphics equipment, and is the standard for display drivers (VGA). Early composite digital video systems chose to sample analog video at four times the NTSC color subcarrier. This rate of 13.5 MHz (4 3.375) was adopted for digital component sampling standards. Since the human visual system primarily uses the luminance information to resolve scene detail, the color information can be sampled at a lower rate with no apparent loss of detail. This is a form of perceptive compression. The luminance sampling rate is referred to as 4 after the multiplier used. The color sampling rate may be a half (2) or one quarter (1) depending on the performance required. So a YUV sample is referred to by the ratios of the sampling rates of the three different channels 4:2:2 or 4:1:1. YUV formats are divided into two groups: packed and planar. In the packed format, the Y, U, and V components are stored in a single array. The 601 standard uses a single array. With the AVI formats two pixels are stored as a single unsigned integer value in one macro-pixel. In the planar format, the Y, U, and V pixels are encoded as three separate arrays. It should be noted that 4:2:2 sampling represents a two-thirds reduction in bit rate over RGB 24, and 4:1:1 is a reduction of one-half. The number of 166 The Technology of Video and Audio Streaming
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