Ycbcr To Rgb Considerations

YCbCr to RGB Considerations
TM

Application Note

March 1997

AN9717
Author: Keith Jack

Introduction
Many video ICs now generate 4:2:2 YCbCr video data. The YCbCr color space was developed as part of ITU-R BT.601 (formerly CCIR 601) during the development of a world-wide digital component video standard. Some of these video ICs also generate digital RGB video data, using lookup tablesto assist with the YCbCr to RGB conversion. By understanding the YCbCr to RGB conversion process, the lookup tables can be eliminated, resulting in a substantial cost savings. This application note covers some of the considerations for converting the YCbCr data to RGB data without the use of lookup tables. The process basically consists of three steps: 1. 4:2:2 to 4:4:4 YCbCr Conversion 2. 4:4:4YCbCr to RGB Conversion 3. Linear RGB Generation

Converting 4:2:2 to 4:4:4 YCbCr
Prior to converting YCbCr data to R´G´B´ data, the 4:2:2 YCbCr data must be converted to 4:4:4 YCbCr data. For the YCbCr to RGB conversion process, each Y sample must have a corresponding Cb and Cr sample. Figure 1 illustrates the positioning of YCbCr samples for the 4:4:4 format. Each sample has a Y, a Cb, and aCr value. Each sample is typically 8 bits (consumer applications) or 10 bits (professional editing applications) per component. Figure 2 illustrates the positioning of YCbCr samples for the 4:2:2 format. For every two horizontal Y samples, there is one Cb and Cr sample. Each sample is typically 8 bits (consumer applications) or 10 bits (professional editing applications) per component. The 4:2:2to 4:4:4 conversion consists of generating interpolated Cb and Cr data for those Y samples that have no Cb and Cr data. Although complex interpolation filtering is specified in BT.601, most NTSC and PAL video decoder chips (such as the HMP8115) use simple linear interpolation. This is usually acceptable since the bandwidth of the color data is 0.6MHz-1.3MHz, rather than the 3.375MHz bandwidthspecified by BT.601.

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FIGURE 1. 4:4:4 YCbCr SAMPLING STRUCTURE FOR A 625-LINE INTERLACED SYSTEM

FIGURE 2. 4:2:2 YCbCr SAMPLING STRUCTURE FOR A 625-LINE INTERLACED SYSTEM

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R´G´B´ Generation
BT.601 defines Y to have a nominal range of 16-235 (blackwhite); Cb and Cr are defined to have a nominal range of 16240, with 128 corresponding to zero. YCbCr is also defined to have been derived from gamma-corrected RGB (R´G´B´) data. The BT.601 equations are used by many video ICs to convert between digital R´G´B´ data and YCbCrare: Y = (77/256)R´ + (150/256)G´ + (29/256)B´ Cb = -(44/256)R´ - (87/256)G´ + (131/256)B´ + 128 Cr = (131/256)R´ - (110/256)G´ - (21/256)B´ + 128 R´ = Y + 1.371(Cr - 128) G´ = Y - 0.698(Cr - 128) - 0.336(Cb - 128) B´ = Y + 1.732(Cb - 128) When performing YCbCr to R´G´B´ conversion using the above equations, note that the resulting R´G´B´ values have a nominal range of 16-235 (black-white).Occasional excursions into the 0-15 and 236-255 values are possible due to Y and CbCr occasionally going outside the 16-235 and 16-240 ranges, respectively, due to video processing, rounding errors, and noise. However, if the 24-bit R´G´B´ data are to have a range of 0255 (black-white), as is commonly found in PCs, the following equations (used by the HMP8115) should be used to maintain the correct blackand white levels: Y = 0.257R´ + 0.504G´ + 0.098B´ + 16 Cb = -0.148R´ - 0.291G´ + 0.439B´ + 128 Cr = 0.439R´ - 0.368G´ - 0.071B´ + 128 R´ = 1.164(Y - 16) + 1.596(Cr - 128) G´ = 1.164(Y - 16) - 0.813(Cr - 128) 0.392(Cb - 128) B´ = 1.164(Y - 16) + 2.017(Cb - 128) For the YCbCr to R´G´B´ equations, the R´G´B´ values must be saturated at the 0 and 255 levels due to occasional excursions outside the...