gnals overview For 8-bit systems, the values of 0x00 and 0xFF are re- a common number of samples for the active line period served for timing information. For 10-bit systems, the values simplifies the design of multistandard equipment and of 0x000-0x003 and 0x3FC-0x3FF are reserved for timing standards conversion. With a sample rate of 6.75 MHz information, to maintain compatibility with 8-bit systems. for Cb and Cr (4:2:2 sampling), each active line period The YCbCr or R'G'B' levels to generate 75% color contains 360 Cr samples and 360 Cb samples. bars are discussed in Chapter 7.2. Digital R'G'B' signals With analog systems, problems may arise with are defined to have the same nominal levels as Y to repeated processing, causing an extension of the blanking provide processing margin and simplify the digital matrix intervals and softening of the blanking edges. Using 720 conversions between R'G'B' and YCbCr. digital samples for the active line period accommodates the range of analog blanking tolerances of both the 480i and 576i systems. Therefore, repeated processing may be SDTV sample rate selection done without affecting the digital blanking interval. Blanking to define the analog picture width need only be Line-locked sampling of analog R'G'B' or YUV video done once, preferably at the display or upon conversion signals is done. This technique produces a static orthog- to analog video. onal sampling grid in which samples on the current scan Initially, BT.601 supported only 480i and 576i systems line fall directly beneath those on previous scan lines and with a 4:3 aspect ratio (720 X 480i and 720 X 576i active fields, as shown Figures 7.2-2 through 7.2-11. resolutions). Support for a 16:9 aspect ratio was then Another important feature is that the sampling is added (960 X 480i and 960 X 576i active resolutions) locked in phase SO that one sample is coincident with the using an 18 MHz sample rate. 50% amplitude point of the falling edge of analog hori- zontal sync (0x0). This ensures that different sources EDTV s