Color bars are the most ubiquitous of all test signals. When not following a TV station sign-off, they are most often perceived as a preprogram place holder -- something like a blank electronic film leader. But, as technical pros know, color bars provide both serious at-a-glance confirmation of signal path completion and confidence of video signal acceptability.

Merely observing bars on a TV monitor, however, is a subjective assessment. The monitor will display a signal problem, but it reveals little about the source, nature or degree of the signal impairment. To extract detailed information about video signal fitness requires careful examination of one or more test signals on a waveform monitor and/or vectorscope.

Color bars are only one of many test signals. Test signals are precisely defined electronic signals that serve as performance measurement benchmarks. The purpose of each signal is to make one or more types of video distortion easy to see and quantify.

Testing 1, 2, 3

The principles behind video signal testing are quite simple. The output of a test signal generator is applied to the input of a piece of video equipment or distribution system. The test signal, after passing through the components, is displayed on a waveform monitor and vectorscope. Any significant change in the signal becomes fairly conspicuous, and the type of change gives many clues about the possible source of the problem.

Color bars are handy for setting up such equipment as time base correctors, or TBCs. However, measuring the performance, such as the bandwidth of a TBC, requires more specific testing. Periodic measurement of overall system performance helps spot minor problems before they grow into noticeable picture problems. The result is consistently higher picture quality.

What test signals ensure video signal quality? Color bars are most obvious. They monitor or measure several amplitude, timing and color parameters. However, no one test signal defines all of the amplitude and timing relationships of the NTSC signal. Signal path impairments and system performance are often best detected with several types of test signals. You'll find the most common of these signals in the Summary of Video Test Signals chart.

The types of signals suited for an application depend mainly on two factors: the environment and the video format.

Standard values

Most studio, production and post facilities use a variety of equipment and systems. These require a broad corresponding selection of signals for setup, maintenance and calibration.

Calibrating a picture monitor requires SMPTE color bars and a crosshatch (convergence) signal. Color bars are used to set chroma, hue and brightness adjustments. The convergence signal helps align the red, green and blue beams. The multiburst signal is used to check the picture monitor's horizontal resolution.

Observing technical values within a system on a waveform monitor or vectorscope requires a generator with test signals such as pulse-and-bar, modulated staircase, multipulse or multiburst. However, evaluation with each test signal is time consuming.

Combination signals, such as NTC 7 Composite, FCC Composite and NTC 7 Combination, contain two or more test signals as elements on each video line. Each element of a combination signal is basically a narrow version of regular signals, allowing two or more to fit side by side on a single line of video.

Many generators also produce matrix test signals that combine two or more different signals in a single field of video. Different matrix signals are available for transmission and system testing and picture monitor setup applications. A matrix typically consists of 40 or more consecutive video lines of one signal followed by the same number of lines of the next signal, throughout the active video lines of a field.

Like the combination signals, matrix signals speed the process of equipment and system evaluation and adjustment. With several signal types, you can observe multiple problems (or the absence of multiple problems) without changing the test signal.

Continuous technical monitoring of a source is enhanced with Vertical Interval Test Signal (VITS) insertion. By including one or two test signals on unused lines in the vertical interval, you can make objective judgements about the extent of the signal degradation instantaneously, at any point in the program. Most TV stations and networks use VITS to monitor and evaluate technical parameters without interrupting programming.

Once you've decided which signals and formats you need, there are still more considerations for choosing a test signal generator. Space is always important; usually the smaller the generator's package size, the better, provided the unit supplies the signals you need. Depending on the complexity of the facility, other video gear in the system may need to be genlocked. Some test signal generators eliminate the need for an extra distribution amplifier (DA) by providing multiple black burst outputs.

System performance checks require only a few other signals. The NTC 7 Composite and Combination signals or a system test matrix signal can provide all components necessary for quick checks of system linearity, insertion gain, frequency response, differential phase and gain, chrominance-to-luminance gain and delay, and other short- and long-time distortions.

More than test

Other signals and functions provided by test signal generators can simplify several routine production tasks. A safe-title/ safe-action-area signal helps operators position and size the critical parts of a scene so they don't appear somewhere off the edge of the presentation screen. Blacking tapes is a task that some generators can reduce to the push of two buttons. (You need only press Record on the VTR and select a countdown sequence on the generator.)

Evaluating the performance of 2-wire (Y/C) or 3-wire analog component systems requires special signals in the appropriate component form, in addition to the composite signals. Bowtie is an essential signal for 3-wire component systems designed specifically for precise amplitude and timing adjustments. An economical alternative to multiple test signal generators is a multiformat generator with signals in NTSC, Y/B-Y/R-Y and Y/C formats.

Cameras usually don't require external test signals, but in the studio they must be genlocked to the system. In smaller facilities or Electronic Field Production (EFP) applications, a generator with multiple black burst outputs might eliminate the need for a separate master sync generator or DA. A black burst input to a production switcher is commonly used as the source of black for a fade-to-black.

Distribution distortions

Distribution paths can be subject to myriad distortions. A distribution path may be as simple as a camera output looped through a monitor to a switcher and terminated, or as complicated as a 1,000-machine duplication system. That's why multipurpose combination signals, such as the FCC and NTC 7 Composite and the NTC 7 Combination, are so widely used as VITS in transmission testing.

Multiburst (which is part of the NTC 7 Combination signal), multipulse and (sinX)/X are all used extensively for frequency-response testing of various distribution systems. Multipulse and (sinX)/X indicate group delay as well, but (sinX)/X requires a spectrum analyzer or automated video measurement set for display. Big ticket items, such as spectrum analyzers or automated measurement systems, often aren't a liability, because many other standard transmission and distribution tests require these instruments. To optimize performance, particularly in this era of higher-resolution formats, good test equipment is a basic requirement, not an option.

Maintaining performance

Maintenance areas have some special requirements. Flexibility is the key, as the proliferation of interconnect and recording formats continues.

It's not uncommon to find composite NTSC gear in use with component analog or Y/C gear. Although all of this gear has NTSC inputs and outputs, equipment in each video format requires test signals in the same format to fully exercise its circuitry. In addition to the luminance and color difference signals primarily used with component analog video equipment, the time-compressed versions (CTCM or CTDM, the actual recording formats used) must also be available.

Serial digital video is finding favor as an interconnect format within many video facilities. With it comes the need for yet another test signal format and, of course, new testing issues. Serial digital requires the same analog test signals -- you simply need to convert them to the new digital format and add a number of very complex digital goodies to the vertical interval.

Operational equipment in need of routine maintenance or troubleshooting can't always go to the shop. So, with the proper signals and formats, a small, lightweight package can make it a lot easier to bring the tools to the problem.

The economics of testing

One thing is certain today -- viewers expect high video quality, all the time.

Gone are the days when a single, snowy TV channel was revered as a miracle of modern technology and occasional problems from technical difficulties were tolerated. Today, home viewers fix technical difficulties on their own without leaving their armchairs by switching channels with remotes. Viewers of corporate and other nonbroadcast video have similar expectations and reactions, except instead of tuning out with a remote control, they tune out their brains.

Reliable, high-quality, uninterrupted video -- the type clients pay for -- requires extensive testing and preventative maintenance to catch and fix declining system performance before visible picture impairment occurs. Once quality starts to slip, so will the attention of your viewers, as well as future business from your clients.

Producing video without adequate test and measurement gear is as risky as driving a car at night without headlights. Today's digital-based test signal generators provide many high-precision signals in small, economical packages. Waveform monitors and vectorscopes come in a variety of affordable packages, and sometimes both functions are combined for even greater economy. With compact, easy- to-use gear, the serious video producer can't afford not to use test equipment every day.

Summary of Video Test Signals