Here is a representation of a video frame both for NTSC and PAL under the 656 recommendation. This shows that the horizontal blanking region is demarcated by the H bit, so EAV or End of Active Video is when H equals 1. The transition to H equals 0 is the SAV, the Start of Active Video. Likewise when V changes from 1 to 0 it denotes a new region, blanking versus active video. Then the field, Field 1 and Field 2 are also denoted by the change between 0 and 1 on the F line. These control codes are sent in the data stream, along with the data. If you were to take a snap shot of the data stream you would see a few leading bytes, (we’re looking at an 8-bit case of 8-bit video components). “FF 00 00” -- that’s the preamble associated with a control code. It lets the end device know that a control code is coming.