Copyright © 1997-06-01 Charles Poynton
In 525/59.94 NTSC video, the total number of subcarrier cycles per line is 455/2. This is an odd multiple of one-half. Since there is an odd number of lines per frame, subcarrier can fall in one of two relationships with the start of a frame. Colorframes denoted A and B are distinguished by the phase of subcarrier at 0H at the start of the frame: Frame A at 0°, and Frame B at 180°. This relationship is also referred to as a four-field sequence of fields 1, 2, 3, and 4 (or I, II, III, and I V), corresponding to Aodd, Aeven, Bodd, Beven .
In Figure 1 above, superimposed on line 1, I show a positive-going cycle of subcarrier. Since there is an odd number of halfcycles of subcarrier on that line, line 2 starts with a negative-going cycle. The alternating sequence continues until the end of the field. (I number the figure as if all 263 lines of NTSC are present in the first field, though you could consider this example to show a raster having just 9 lines.)
What is shown here as the top halfline of the second field is actually the continuation of the bottom line of the first field. Since that line starts with a positive-going cycle, the first line of the second field - line 264- starts with a negative-going cycle. The alternating sequence continues to the last (bottom) line of the second field, which is positive-going.
Continuing the sequence to the start of the second frame, the first line of the first field must be opposite to the previous line, that is, it must be negative-going. Every line in colorframe B has the opposite subcarrier phase to the corresponding line of colorframe A. The subcarrier phase sequence through the four fields is 0°, 180°, 180°, and 0°.
For further detail, see Charles Poynton, A Technical Introduction to Digital Video (New York: John Wiley & Sons, 1996).
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