Sampling frequency is best descibed as how the video is divided up in luminance and chromanace ratio. So SD PAL video runs at 270Mbs which the total frequency that this source runs at is 270 MHz. This will help out in understanding how the sampling frequency is seen. An example is 4:2:2, with the first number representing luminance/green Y/G channel, and the last two representing the two colour channels red and blue / R and B / Cr and Cb / R-Y and B-Y. For every 4 luminace packets there will be two each of blue and red information.
We will look at these as individual sampling packets of 8. So 270/8 = 33.75 Mz per packet. with 4 luminace packets and two each of chromanance then the signal information will be divided up like this, 135MHz:67.5MHz:67MHz. this is used with all video signals and is to describe the luminance and colour information ratio.
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