Video Sampling

Digital video is a sampled form of analog video. The most common sampling schemes in use today are:

                  Pixel Clock   Horiz    Horiz    Vert
                   Rate         Total    Active
NTSC square pixel  12.27 MHz    780      640      525
NTSC CCIR-601      13.5  MHz    858      720      525
NTSC 4FSc          14.32 MHz    910      768      525
PAL  square pixel  14.75 MHz    944      768      625
PAL  CCIR-601      13.5  MHz    864      720      625
PAL  4FSc          17.72 MHz   1135      948      625

For the CCIR-601 standards, the sampling is based on a static orthogonal sampling grid. The luminance component (Y) is sampled at 13.5 MHz, while the two color difference signals, Cr and Cb are sampled at half that, or 6.75 MHz. The Cr and Cb samples are colocated with alternate Y samples, and they are taken at the same position on each line, such that one sample is coincident with the 50% point of the falling edge of analog sync. The samples are coded to either 8 or 10 bits per component.

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Rick Davis <rld@sgi.com>