JOHN LUFF /
11.01.2007 12:00 PM
Test & measurement tools
In this DTV age, monitoring is more important than ever.

Today's technology spans generations of products used in systems with interconnections that vary from coax carrying analog video to high-speed networks on fiber-optic cable carrying multiple gigabits per second of file transfers. This presents challenges to those responsible for designing and maintaining facilities that use complex and often misunderstood technology tools. Fortunately, as computer power has grown, it has facilitated human interfaces that make technology more approachable. This allows complex systems to be understood at fundamental levels, or viewed with simple pass- and fail-oriented tools.

Video testing

Today's broadcast facility is almost guaranteed to contain both analog and digital signals. In fact, it likely supports analog SD as well as digital HD signals. We often forget that reference signals today are still analog color black, with NTSC or PAL characteristics. Most modern facilities use SMPTE 259M (270Mb, component digital signals), or SMPTE 292M (1.485Gb component HD digital signals) for interconnection. During this extended change from analog to digital, and from SD to HD facilities, we need to embrace tools that help us bridge multiple worlds.

Waveform monitors have been a part of TV facilities since the first monochrome television went on-air in the late 1930s. For decades, they helped us adjust and understand signal degradation. The first units were actually special-purpose oscilloscopes tailored to the amplitude of the TV signal. The featured special display times that showed frame and line detail in analog TV signals. When color replaced B&W in the 1960s, new, partially solid-state waveform monitors were developed. Today, that lineage continues with powerful tools that can display the line rate waveform of active video as well as vector displays, gamut displays, and picture and sound information.

Today, CRT instruments have been increasingly replaced by digital, often color, displays that allow sophisticated views of audio, video, picture, vector display, signal jitter and data analysis. Though we often think traditional CRT waveform monitors and vectorscopes are disappearing, they aren't. The flexibility of instruments that are based on digital technology, however, brings many new features, along with the replacement of CRT technology, for which it is increasingly hard to find replacement parts. Like TV sets and professional monitors, the industry is moving to more modern technology and the benefits that color displays afford.

Conventional instruments with internal displays offer flexible HD and SD inputs, and decoding of ancillary data carrying audio and other information. Rasterizers can perform the same digitization and analysis functions but output to external displays. These rasterizers have become common in modern systems because they offer much larger display areas and can input to monitor wall processors over VGA and DVI connections.

Since the 1980s, there has beena steady growth in products that automate parts of a facility's monitoring needs. Fully automatic systems have replaced those from the 80s. Manufacturers now build systems that monitor signals and send alarms when signals are out of a tolerance window.

Such automated monitoring is also done by companies not normally thought of as test and measurement specialists. These manufacturers build monitoring systems that can be embedded in a facility. This powerful technique allows remote full-time sampling of common parameters, such as audio and video levels. It also enables comprehensive alarms for conditions that might be detrimental to operations, such as signal presence and freeze detection. When aggregated in a system using SNMP monitoring or proprietary user interfaces, entire networks of widely dispersed facilities can be monitored from a central site. For example, a cable network might monitor its reception by headend facilities where advertising revenue is particularly important. Broadcasters can build complex centralized monitoring for stations centrally programmed, or just keep track of the quality of their stations' transmitted signal.

Often, these remote monitoring systems are combined with displays that are integral to multiplexed monitor walls. If a facility control system is integrated with monitoring and display, a holistic and powerful combination is formed. Alarms can trigger actions that might correct for problems sensed. For example, a frame synchronizer that has failed can automatically be replaced by a spare. Used in a thoughtfully constructed system, these remote monitoring capabilities extend the reach of operators and maintenance personnel.

Portable instruments are particularly valuable when building facilities and working on remote location productions. Some portable monitors come with generators included for SD and HD signals.

Increasingly, it is important to have optical test equipment in a facilities when long links between devices are installed on fiber. Power level, signal presence and optical time domain reflectometer (OTDR) capabilities are important. Some devices can detect a signal in a fiber without opening it or disturbing the signal being passed. Test instruments like power meters, when coupled with a calibrated source, can permit accurate testing of optical quality for installed cabling.

Audio testing

Audio levels and phase are common tests. With the introduction of multichannel sound in DTV, it is important to know the levels of the signal as well as what aural image it represents. Many manufacturers of video test equipment have also included audio test options for embedded and discrete audio signals (analog and digital). In addition to the usual cast of TV test equipment manufacturers, many audio console manufacturers have designed audio-only monitors that show phase and level for two- to eight-channel monitoring.

Subjective loudness can be monitored and compared to the dialnorm value contained in the metadata of a Dolby E encoded stream. There are also handheld, portable devices available that test AES, and Dolby AC3 and Dolby E compressed streams carried on AES paths.

A serious issue for all broadcasters is lip sync. Until recently, there were few methods of testing for lip sync, though methods for correcting it were easily available. Now several methodologies exist, both in-service and out-of-service. Out-of-service systems and backhaul links can be qualified for A/V delay to less than one frame of video. Other systems work in-service by comparing audio with the faces of people on-camera and using a sophisticated algorithm that figures out A/V delay.

Compressed bit stream analysis

It no longer suffices to test baseband quality because compression systems introduce nonlinear distortions that can be quite disturbing to viewers. As a result, two important classes of test instruments are aimed at verifying the quality of the bit stream delivered over DTV, or any compressed link or system. The most prevalent is bit stream verification and compliance testing. Systems analyze the syntax in the MPEG-2 transport stream and display statistics about the content of the stream, including syntax errors that can lead to problems in the decoder or home receiver.

Beyond the syntax, however, there are issues related to the quality of the content itself. Subject viewing is not always a good way to qualify a link because if the content stresses the system only occasionally, it may be difficult to catch errors that won't be accepted by the home environment. Fortunately, several manufacturers offer products that correlate double-blind studies of video artifacts and their impact on viewer perception of quality to numeric scores used as a measurement of picture impairments. Devices that test in an open loop, or in-service, can monitor quality on a daily basis. Systems that test in a closed loop, or out-of-service, may be better suited to evaluate equipment under consideration for purchase, or in design laboratories.

Conclusion

Test and measurement equipment is moving toward more sophisticated analysis of trends and hands-off monitoring. As systems move inexorably toward less baseband and more compressed content, it will be important for most facilities to invest in tools that can both monitor quality and adherence to standards for compression systems and transmission equipment. Beauty is in the eye of the beholder, and with compressed video, that is only a statistically significant viewer, not an objective measuring device like waveform monitors of old.


John Luff is a broadcast technology consultant.



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