The Digital Fact Book - Quantel

For the second major element the signals must be low-pass filtered to prevent the passageof information beyond the luminance band limit of 5.75 MHz and the color difference bandlimit of 2.75 MHz, from reaching their respective ADCs. If they did, aliasing artifacts wouldresult and be visible in the picture (more later). For this reason low pass (anti-aliasing)filters sharply cut off any frequencies beyond the band limit. For HD, the principle remainsthe same but the frequencies are all 5.5 times higher, generally, depending on the HDstandard being used.Sampling and digitizationThe low-pass filtered signals of the correct amplitudes are then passed to the ADCs wherethey are sampled and digitized. Normally two ADCs are used, one for the luminance Y,and the other for both color difference signals, R-Y and B-Y. Within the active picture theADCs take a sample of the analog signals (to create pixels) each time they receive a clockpulse (generated from the sync signal). For Y the clock frequency in SD is 13.5 MHz and foreach color difference channel half that – 6.75 MHz – making a total sampling rate of 27 MHz(74.25 MHz, 37.125 MHz and 148.5 MHz respectively for HD). It is vital that the pattern ofsampling is rigidly adhered to, otherwise onward systems, and eventual conversion backto analog, will not know where each sample fits into the picture – hence the need forstandards! Co-sited sampling is used, alternately making samples of Y, R-Y, and B-Y on oneclock pulse and then on the next, Y only (i.e. there are half the color samples comparedwith the luminance). This sampling format used in 601 is generally referred to as 4:2:2 andis designed to minimize chrominance/luminance delay – any timing offset between thecolor and luminance information. Other sampling formats are used in other applications– for example 4:2:0 for MPEG-2 compression used for transmission.The amplitude of each sample is held and precisely measured in the ADC. Its value is thenexpressed and output as a binary number and the analog to digital conversion is complete.Note that the digitized forms of R-Y and B-Y are referred as Cr and Cb.185