Timing is the process of adjusting video signals so that their synchronizing information (sync and burst) arrive in-step with each other, usually at the input of a switcher, router, patch panel, etc., so that mixing or cutting between the sources can be accomplished seamlessly.
Timing assumes that the signals are synchronous with each other. That is, that they are "referenced" (locked, genlocked, or frame-sync'ed) to the same sync generator. Any time differences which exist between sources must be fixed in amount, not-varying. Correct "timing" reduces these time differences to a negligible amount.
Beginning with the (coarse) vertical adjustment, timing is generally done in four steps with ever increasing precision:
I. Vertical timing (Vertical Phase)
II. Horizontal timing (H-Phase)
III. Color, or burst, timing (including SCH phase)
IV. Picture Centering, or Video H & V Phase
Normally these steps are performed in the order shown, With some equipment however, steps (2) and (3) are reversed.
Vertical timing is the coarse adjustment of coincidence between the synchronizing pulses of video sources. Simply, the video signal being adjusted (the "adjustable source") must be operating on the same field and the same horizontal line as the reference signal (the "reference source".) That is, line-one/field-one of the adjustable source's vertical interval must "line up" with line-one/field-one of the reference signal; line-two/field-one with line-two/field-one and so on for all 525 lines.
A displacement of a half-horizontal line or more may generally be considered to be a vertical-timing error. On older analog equipment horizontal timing errors are usually on the order of a few microseconds. Vertical timing controls usually adjust timing in half-line or whole-line increments.
Vertical timing is checked using an externally-locked waveform monitor (such as a Tektronix 1750 or 1760) or an oscilloscope. [If a dual-channel oscilloscope is being used, it must be set to "chop" (mode) between channels and not to "alternate"(mode) between channels.] Signals are observed for coincidence during the vertical blanking interval. Since vertical timing is a coarse adjustment, many pieces of equipment genlock to the correct field and horizontal line automatically and don't provide half or whole-line adjustments.
[Sidebar: A tip for identifying fields: With color black or other video signals where set-up and burst are present, it is relatively easy to identify field one and two. But when a sync-only signal is being observed (with no video or burst) just remember that field one still begins with six half-line intervals immediately preceding vertical sync broad pulses, and six half-line intervals follow. Field two's broad pulses are preceded by seven half-line intervals and followed by five half-line intervals.]
Horizontal timing is a fine adjustment for coincidence between video sources. Signals are observed during the horizontal blanking interval. The adjustable source is tweaked until the (minus 50%) point on the leading edge of horizontal sync (usually the -20IRE point) is coincident with that same point on the reference signal. This assumes that vertical timing is correct and there are no whole-line or half-line errors.
If a Tektronix 1750 or 1760 unit is used for horizontal timing, set up the unit with external reference, a simultaneous display of Waveform Vector and SCH modes, and a (1H) magnified sweep rate. If an externally-locked oscilloscope is used, the horizontal sweep rate must be set to (50 ns/div.) to provide sufficient resolution. Also if an oscilloscope is used, externally trigger it using negative (4 V) sync, not (1.0 V p-p) color black. This will reduce scope trigger jitter. Adjust horizontal timing controls to align the adjustable source.
Color timing simply refers to aligning or matching the burst vector of the adjustable source to match that of the reference signal. The phase control of an externally-locked 1750 / 1760 or vectorscope, is used to position the reference signal burst vector on a convenient scope graticule point, usually the 180-degree axis, and then the burst phase control of the adjustable video source is tweaked to match that of the reference signal. On some equipment this is done via a "subcarrier phase" control. On some equipment a subcarrier phase control will change the phase of burst and chroma together. On others, equipment burst phase and chroma phase have separate controls. If burst phase moves independently of chroma, another adjustment will most likely be required to correct chroma phase (hue) after the phase of burst has been set.
[Sidebar: Chroma phase is the phase difference between the chroma information and phase of burst. The "phase" (or timing relationship) between burst and horizontal sync is known as subcarrier-to-horizontal (sync) phase, or SC/H phase. If the reference signal is correctly SC/H-phased, any signal correctly timed to it will also be correctly SC/H phased. A discussion of SC/H phase is beyond the limits of this short article, but future on-line material will be made available.]
"Picture centering" refers to the positioning of video relative to sync. That is, video is adjusted, or centered, within the "frame" created by the horizontal and vertical blanking intervals. This adjustment is critical when video is being processed in an animation, or matched-cut, editing environment or where it is desirable to minimize horizontal blanking stretch, but for most day-to-day purposes this is usually ignored. Once a signal has been correctly been vertically, horizontally, and color-timed, picture centered can be performed as the last step.
The purpose of picture centering is to prevent, or minimize, unintentional picture movement on the screen due to digital processing; the time relationship between any given pixel and sync at the output of a piece of equipment, should match what was input to it. Picture centering adjustment is often done in whole-cycle (of subcarrier) (279 ns) increments.
oOo
-Al Stephens
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