In order to be able to switch, mix, and combine different television pictures, it's necessary for them all to be scanned at the same time... starting line number one in a frame within a few billionths (!) of a second of each other. Television pictures have a lot of reasons to be Non-synchronous, though.

Television devices are all driven by Sync Generators. Originally, a television system would have one sync generator, with as many as six cables carrying pulse signals to each device in the plant. These days, each device like a camera, tape machine, or computer graphics machine has an internal sync generator. 

Genlock

The internal sync generators in studio equipment must be capable of Genlock. Genlock allows the sync generator to accept a signal like colour bars or black, and lock all of its timing to it. Sync generators, colour bar generators, and black generators are often found in an "all-in-one" unit. Whatever signal is used to distribute timing (bars or black) is called "reference"

A Sync Generator

This is a simplified diagram of a sync generator, for the curious. The various signals are all tightly related to each other because they are all derived from the same master oscillator. Dividing the master frequency by 4 results in the colour subcarrier signal. The countdown results in an ugly, but accurate square wave which has to be shaped into the beautiful sine wave that we see in the video. Those stages are not shown in this diagram. Dividing the master clock by 910 gives an ugly square wave that is internally shaped into a narrow pulse that serves as horizontal sync. Divide further by 525 (the number of lines in the picture... think about it), and shape into the vertical sync pulse. Actually, vertical sync is at twice the repetition rate (59.94) because of the interlace. Mixing, shaping, and application of logic (if this, then not that) results in the signal that is added to the final picture.... composite sync. Composite blanking is a lot like sync, except it starts before a sync (H or V) pulse, and ends after the pulse. Its purpose is to blank out the picture while a sync pulse is occurring, with a little safety time before and after.

Down to the Nanosecond

Please don't panic, but the video signal starts a new line every 63.5 millionths of a second... this makes a timing error of one millionth of a second equal to a sideways jump about one 64th of the picture width. Worse yet, the color system is sensitive to timing errors of a few billionths of a second! To save you looking it up, electrical signals travel through coaxial cable at two-thirds the speed of light. The long and the short of it is that changing a cable length by just a couple of meters will ruin the horizontal timing of a signal, and about 25 centimeters of cable length will affect the colour.

Because devices are located at different distances around the television plant, and may have almost any cable length, genlock circuits are made with timing adjustments that are wide enough to take cable length into account.

Termination

The electrical fluctuations that are the video signal can have very rapid changes. A piece of video detail might occur in as little as a fraction of a microsecond (millionth of a second). Since it takes time for a signal to reach the far end of a cable, if it is not absorbed completely at the end of its travel, it will exist a slightly later time at each end of the cable, and will reflect back to the start of its journey as a slight double image (ghost). 

For this reason, all video must dead end into a load. In analog video, using the cable that we do, this is a 75 ohm resistor, also known as a termination. As well, un-terminated video will have too high an electrical level. Video may be "looped" through a device, most often a monitor. 

The reverse situation also happens... double termination. Flipping the 75ohm switch on a monitor pass-through is the usual culprit. This results in low video level, from the double load.

Switching

There are two main types of switching devices in a television plant, the Routing switcher, and the Production switcher. Both types allow different video sources to be selected. The Routing switcher is used to select sources for recording, or monitoring. The production switcher is used to select sources that are going to air live, or being recorded as a final program. The Production switcher has additional circuitry that allows fading, wiping, overlay, and various other effects between two or more video signals. 

	This is a diagram of a simple "8x5" routing switcher. Note that any one input can be switched to many outputs. Here, Input #1 is routed to outputs #1 and #3.
	The Production Switcher has more complexity. In this simple example, eight inputs are available on three "busses". Two switch busses feed a simple mixing amplifier that allows fading from one source to another. Another bus, called "preview" simply feeds a monitor.  Note that the output of the mixing amp is available on preview.

Sources are usually timed for proper arrival at the Production Switcher, but in a well-wired plant, they will also be "zero-timed" to the routing switcher, so that timing can be done at any position with routing switcher panel and a waveform/vector. You will have to ask, or do some experimentation to know whether or not your system is zero timed to the router.

Have another look at the Production Switcher.

Horizontal Timing

To set up for Horizontal timing a source, lock the waveform monitor to "external sync". Use the vertical size, position, and horizontal position to place the leading edge of bars's (barzzez?) sync on a reference mark as seen here. Once the Waveform Monitor is set, do not touch it's controls again. Now, select any sources you want to time, and adjust their h-phase (horizontal timing) controls until their leading edge of sync is on the same reference mark. 

Rapidly switching between the source you are timing, and bars is a good way to fine tune the timing match.

Subcarrier Timing

Colour timing is done in a similar fashion, but using a vectorscope. Using bars, expand the display until burst touches the outer circle. Use the vectorscope's phase adjustment to put the burst right at zero on the scale. Don't touch the vectorscope controls after that. Rapid switching between bars and the source should show no jumping of the burst in the up and down direction (rotating). The vector line of the burst may lengthen and shorten a tad (differing burst levels in the real world), but this is not as critical as getting the phase exact.

Timing reminders

• You need to know where to time.... at the output of a tech monitoring position using the router? Preview out of the Production Switcher? Other?

• The waveform monitor and vectorscope must be locked to external sync (called "ext ref" on the vectorscope).

• First adjust the Waveform and Vector displays to colour bars... they are fixed in time in most systems. Once the Waveform and Vector displays are correct on bars, don't touch their timing and phase adjustments.

•  Adjust each source using its h-phase and subcarrier phase controls. To time any unit to the switcher.... observe only it's burst phase and sync pulse timing... not the colour bar dots. This will be more clear after you read about TBC's and Frame Synchronizers.