This holds true in any facility from the largest production house or network studio, to corporate studios, and to the smallest freelance production shop. The difference is, the larger facilities have a technical staff devoted to keeping equipment performance and picture quality in peak form. Smaller commercial, business, and industrial facilities have smaller staffs. Sometimes a staff of one!

Whether you're a video veteran or the newest intern, it behooves you to know something more about video quality than what you see on a picture monitor. The goal of this booklet is to help you acquire additional technical knowledge, and be more comfortable and capable with video testing.

You must, however, exercise caution and common sense. Be sure to heed all warnings printed on equipment covers. And never remove any equipment casings or panels unless you are qualified for internal servicing of that equipment. The information in this booklet is intended only for use in external monitoring and adjustment of user accessible controls on video equipment.

A television picture is conveyed by an electrical signal (Figure 1-1). This video signal is carried from one place to another by cables (coax) or by radio-frequency (RF) waves. Along the way, it must pass through various pieces of equipment such as video tape machines, switchers, character generators, special effects generators, and transmitters. Any of this equipment can change or distort the signal in undesirable ways.

 

Figure 1-1. A television picture is created by an electrical video signal. In this case, the picture of color bars is created by a color bars test signal.

Since picture quality is largely determined by signal quality, it's important to detect and correct any signal distortions. The signal has to be right before the picture can be right.

Many video facilities rely heavily on picture monitors for quality checks at various stages of production and distribution. A picture monitor, after all, displays the final product, the picture. It's a quick and easy means of ensuring that there are no obvious picture impairments.

But picture monitors do not tell the whole story. In fact, relying solely on picture monitors for video quality checks can be an open invitation to disaster.

First of all, not every picture impairment is obvious on a picture monitor. Minor problems are easily overlooked. Some cannot be seen at all. For example, a marginal video signal can still produce a seemingly "good" picture on a forgiving monitor. This can produce a false sense of security as signal degradations accumulate through various production stages. The end result can be a nasty surprise that can lead to costly remakes and missed deadlines.

To avoid such surprises, you need to look at more than pictures. You need to look at the video signals that convey the picture information through the various devices and interconnecting cables of a video system. Specialized instruments have been developed to process and display these signals for analysis.

A waveform monitor is an instrument used to measure luminance or picture brightness as well as a high frequency color signal called chrominance. An instrument called a vectorscope is required for quality control of video chrominance, especially in more complex systems.

When properly used, these tools allow you to spot signal problems before they become picture problems. You can make certain that the signals are optimal, rather than marginal. Plus, with regular video system testing, you can catch minor degradations and performance trends that indicate a need for system maintenance. This allows you to avoid costly failures, both in equipment and in program production or distribution.

Test Methodology

There are two somewhat differing ideas used in testing video. Our approach is to test characteristics of a specific signal to ensure that it meets certain artistic or technical requirements. Or, we may test the characteristics of individual pieces of equipment or several pieces in the video signal path to determine that signal distortions introduced by this equipment are acceptable -- or, at least, minimized.

An example of the first case might be monitoring the output of a video source (camera, character generator, etc.) to ensure it is not producing signals that exceed black or peak white signal limits. As an example of the second case, we may wish to ensure the overall gain of a record/playback system is correct, i.e., a recording made with a standard amplitude video signal at the machine's input will produce standard amplitude video at its output during playback.

In the first case (specific signal) we'll need test equipment that enables observation of the signal and knowledge of what the appropriate limits or characteristics are. The second case (equipment or system testing) is more general and is assumed in the following discussion.

The usual method of evaluating video equipment is with a well-defined, highly stable test signal having known characteristics, such as a color bars signal.

Video testing is based on this simple principle of applying a known test signal to the video system or equipment input and observing the signal at the output. Any distortion or impairment caused by the system is observed and measured on the output signal. If there are distortions, the equipment is adjusted to eliminate or minimize them. The point is, if the system can pass the test signal from input to output with little or no distortion, it can cleanly pass picture signals as well.

The signals necessary for such testing are obtained from a test signal generator. This instrument produces a set of precise video signals with carefully defined and controlled characteristics. Each signal is ideal for verifying one or more specific attributes of the video system under test. For all practical purposes, these test signals are "perfect" signals.

Other instruments such as waveform monitors, vectorscopes, combinations of waveform and vector monitors, or specialized video measurement sets are used to evaluate the test signal at the output of the path under test. As an example, Figure 1-2 shows a waveform monitor display of a color bars signal. This display is also called a waveform -- it is actually a graph of the changing voltage of the signal (plotted vertically) and time (plotted horizontally). Calibrated scales on the waveform monitor's screen allow the various amplitude (voltage) and time parameters of the waveform to be measured. Other test signals and their related instrumentation and displays are discussed in the following sections.

 

Figure 1-2. Waveform display of a color bars signal.

Test signal generators and signal evaluation instruments are available in a wide variety of models. These can range from simple production-oriented instruments to highly sophisticated engineering instruments. Waveform monitors, vectorscopes, video test sets, and other specialized equipment to display and/or evaluate the signal are also available in a wide variety of configurations. The following sections will acquaint you with some of these tools and with methods of using them to enhance your effectiveness in maintaining video quality.

Connecting and Terminating Instruments

The Tektronix instruments discussed in this booklet, and most others, have rear panel BNC connectors. For many video tests, you only need to use one connector on each instrument. This is the connector marked TEST SIGNAL on the signal generator and the connector marked CH A INPUT on the waveform monitor or vectorscope.

Notice there are actually two "loop-through" CH A connectors on both the waveform monitor and vectorscope (Figure 1-1). If you feed the signal in one side of these inputs and out the other, the signal will pass through the instrument unaffected. This type of loop-through input lets you connect the same signal to more than one instrument.

For example, after running the test signal from the signal generator through the system under test and from there to a waveform monitor, you can then loop the signal through the waveform monitor into a vectorscope. Using the same method, you can also loop through the vectorscope to a picture monitor. This connection method allows you to look at the same test signal on all three instrument displays (Figure 1-3). The order in which the instruments are connected doesn't matter -- if the connecting cables are short.

 

Figure 1-3. Loop-through inputs on instruments allow the same video source to be viewed simultaneously in waveform, vector and picture forms.

Coaxial cable does have signal loss -- the signal's amplitude decreases as it progresses down the cable. For runs of a few feet, this decrease in amplitude is typically 1% or less and is usually ignored. For longer cable lengths, or when precision measurements are being made, the loss must be taken into account or corrected with a compensating distribution amplifier. Not only is coax lossy, the loss is a function of signal frequency -- higher video frequencies are attenuated more than the low frequencies.

Also, not all cables are well shielded and signals may cross- talk from outside the cable into the video path inside. In other words, the interconnecting cables themselves may be introducing signal distortions.

While small, flexible cables are convenient to handle, they are not without technical cost. Consideration should be given to using larger, double-shielded cables in long or critical runs. The techniques in this booklet can be used to evaluate the distortions introduced by various lengths and/or quality of cable.

While on the subject of cables and interconnections, always remember to properly terminate each signal path. If a signal path is left "open" at the end -- such as a high impedance loop-through with nothing connected on one side -- several problems can result.

Most obvious will be a change in amplitude of the signal on that path. With an open termination the amplitude will be higher than expected (usually about twice amplitude, but actually depending on the signal source impedance). With a double termination, such as will result if an internally terminated loop-through and an external terminator are used on the same path, the amplitude will be decreased. A double termination will often result in a two-thirds amplitude signal, again depending on source impedances.

Amplitude is not the only effect of misterminations. Don't be tempted to make up for improper termination by adjusting the signal amplitude. Misterminations also introduce problems with frequency response (amplitude becomes a function of signal frequency) and with differing response depending on location along the signal path. Use the correct terminating resistor on the end of each coaxial path.

There are three ways a signal path can be properly terminated:

• Some instruments have a single-connector input with a built-in terminator. You can connect such an instrument "as is" at the end of a signal path, but you can't loop the signal through it.
  
  
• Some instruments have a two-connector input with a built-in terminator and a switch for selecting loop-through or terminating connection. You can connect such an instrument anywhere in the signal path, but you must set the switch correctly for the intended use (either Hi-Z for loop-through or 75 ohm for end-of-line termination).
  
  
• Some instruments, such as the 1720 and 1730, have two-connector inputs with no built-in terminators. You can connect such an instrument anywhere in the signal path. But, if you connect it at the end of the signal path, you must attach a 75 ohm terminator to the unused connector.