Appendix 1The colour frame sequence and timecode
The PAL 8-field sequence
In the PAL system the colour sub-carrier frequency is calculated as fsc = (284 - 0.25) fh + 0.5 fv, where fsc is the colour sub-carrier frequency, fh is the line (horizontal) frequency, fv is the field (vertical) frequency.
This gives fsc = (284 - 0·25) X 15 625 + 0.5 X 50
= 4 433 618.75 Hz.
This means that there are 4 433 618.75/15 622 = 283.751 6 cycles of colour sub-carrier per line.
With 625 lines to each frame, there are 283.751 6 x 625 = 177 344.75 cycles of sub-carrier per frame.
Not until 4 frames have passed (177 344.75 × 4 = 709 379 cycles of subcarrier) do we get matching (extrapolated back) sub-carrier phases at the start of the first line of each frame.
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