Appendix 1
The 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.
Another way of looking at the matter is to say that 709 379/283.751 6 = 2 500 lines = 4 frames (8 fields).
The NTSC 4-field sequence
The NTSC system does not employ the line-by-line phase alternation of the PAL system, so the mathematics is simpler.
In NTSC,
- fsc = 3 579 545 Hz
- fv = 29.97 002 617 Hz
and there are 525 lines per frame. Therefore there are 3 579 545/(29.970 026 17 x 525) = 227.5 cycles of sub-carrier per line.
With 525 lines per frame there are 227.5 × 525 = 119 437.5 cycles of subcarrier per frame. Therefore, not until 2 frames (238 875 cycles of subcarrier) have to pass before we get matching (extrapolated back) subcarrier phases at the start of the first line of each frame.
Another way of looking at the matter is to say that 238 875/227.5 lines = 1 050 lines = 2 frames (4 fields).
The relationship between the colour sub-carrier and the video sync pulses is complicated. However, it must be respected during video editing if picture disturbance is to be avoided. For NTSC, SECAM and simple editing in PAL the sequence cycles over four fields. In more complex (invisible) editing in PAL an eight-field sequence must be respected.
These sequences can be accurately followed by using the timecode signal as a guide. This can be done only if there is a known relationship between each timecode word and the position of its associated frame in the sequence. The IEC has established a standard for this relationship for PAL, SECAM and NTSC systems.
The 625/50 field sequence
When a timecode generator is locked to the 8-field sequence then the remainder (R) obtained from the equation R = (S + F)/4, where S is the number of seconds and F the number of frames, will be
- 0 for fields 7 and 8
- 1 for fields 1 and 2
- 2 for fields 3 and 4
- 3 for fields 5 and 6
Let the bits of the longitudinal timecode word be labelled
- bit 0 = A
- bit 16 = B
- bit 1 = C
- bit 8 = D
- bit 17 = E
- bit 24 = F
For correct colour framing of timecode in the 4-field sequence (SECAM and simple PAL editing):
- fields 1 and 2 shall occur when A ⊕ B = 1
- fields 3 and 4 shall occur when A ⊕ B = 0
- where + represents the exclusive-OR function.
Thus the first field of the day at 00h 00m 00s 01f has A=1 B = C = D = E = F = 0, and A + B = 1
- At 09h 11m 03s 07f, A= 1 B = 1 and A⊕B = 0
- At 12h 22m 10s 12f, A = 0 B = 0 and A⊕B = 0
- At 21h 59m 47s lOf, A = 0 B = 1 and A ⊕ B = 1
For correct colour framing in the 8-field sequence, the same conditions as above apply, together with the following relationship:
- fields 1-4 shall occur when (A⊕B)⊕C⊕D⊕E⊕F = 1
- fields 5-8 shall occur when (A+B)⊕C⊕D⊕E⊕F = 0
where + represents the exclusive-OR function.
If the timecode is displayed in decimal numbers, let S and P designate the values of seconds and pictures respectively.
The 4-field condition is met when
(S + P)/4 is odd for fields 1 & 2 and fields 5 & 6
(S + P)/4 is even for fields 3 & 4 and fields 7 & 8
The 8-field sequence is met when in addition to the above the remainder on dividing (S + P) by 4 is:
- 0 for fields 7 & 8
- 1 for fields 1 & 2
- 2 for fields 3 & 4
- 3 for fields 5 & 6.
Editing in SECAM
In the SECAM system the two colour sub-carriers (having different frequencies) are modulated on alternate lines by two signals, designated D′b and D′r. For correct colour framing of timecode, frames in which the second field begins with a line having sub-carrier modulated by D′b, require the sum of the numbers of frames and seconds in the associated timecode address to be odd.
Where the second field of the frame starts with a sub-carrier modulated by D′r then the sum of the numbers of frames and seconds is even.
The relationship can also be defined as follows:
- Let bit 0 = A; bit 16 = B
- Then the timecode generator must be locked to the video so that
- A ⊕ B = 1 for fields where line 24 (or 336) is modulated by D′b
- A⊕ B = 0 for fields where line 24 (or 336) is modulated by D′r
Editing in NTSC
For correct colour framing in the 4-field sequence, even-numbered frames will contain fields 1 and 2, odd-numbered frames will contain fields 3 and 4. Fields 1 and 2 are commonly referred to as together constituting Colour Frame A; fields 3 and 4, Colour Frame B.
To permit correct selection of 2- or 4-field editing the following relationship between timecode and the associated video signal is that LTC bit no. 0 contains:
- 0 for fields 1 & 2
- 1 for fields 3 & 4
Colour framing and VITC
For the VITC word the logic is identical. The bit representation differs because the distribution of data through the 90-bit VITC word differs from the 80-bit LTC word:
- bit 2 = A, bit 22 = B, bit 3 = C, bit 12 = D, bit 23 = E, bit 32 = F.