Test and measurement is all about producing and managing quality. And like the automotive industry, we have to accept the fact that quality is not a process of fixing something at the end of production, but has to be built in right from start and at every step in the chain.

While the quality of analogue and full-bandwidth digital video can be characterised indirectly by measuring the distortions of static test signals, compressed television systems pose a far more difficult challenge. Picture quality in a compressed system can change dynamically based on a combination of data rate, picture complexity, and the encoding algorithm employed. The static nature of test signals does not provide a true characterisation of picture quality. A test scene with natural content and motion can be used, with human viewers reporting the results, but this method of evaluating the capabilities of a compressed video system is woefully inefficient. However, the Tektronix Picture Quality Analyser PQA500 provides a fast, practical, repeatable and objective measurement alternative to subjective evaluation of picture quality.

The PQA500
The PQA500 takes two video files as inputs: a pristine reference video sequence and a resulting compressed, impaired, or processed version of the reference. First it makes a spatial and temporal alignment between the two sequences without the need for a calibration stripe embedded within the video sequence. Then it analyses the quality of the test video based on human vision system and attention models and outputs quality measurements that are highly correlated with subjective assessments. The results include overall quality summary metrics, frame-by-frame measurement metrics, and an impairment map for each frame. The PQA500 also provides traditional picture quality measures such as PSNR (peak signal to noise ratio) as an industry benchmark and an impairment diagnosis tool kit for measuring typical video impairments and detecting artifacts.

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Figure 1: PQA 500 picture quality analysis unit from Tektronix

Each reference video sequence and test clip can have different resolutions and frame rates. The PQA500 can provide picture quality measurement between HD and SD, SD and CIF or any other combination. This capability supports a variety of repurposing applications such as format conversion, DVD authoring, IP broadcasting, and semiconductor design. The PQA500 can also support measurement clips with unlimited sequence duration, allowing a full-length movie to be quantified for picture quality through various conversion processes.

SMPTE 372M
For acquiring the video material in the best possible quality, 1080p50 or 60 have evolved to be the formats of choice. With requirements in post-production to do RGB 4:4:4, higher-speed interfaces were needed to handle the additional data. Currently, most facilities rely on dual link for this application. However, the SMPTE 372M dual link standard supports a variety of modes and colour spaces when carrying video and audio data. A-channel analysis, inter-channel timing measurement and combined waveform and vector for applications in colour grading are essential to provide first class results. The Tektronix WFM/WVR7120 modular waveform monitor product family provides comprehensive analysis for this, and the accuracy needed at 1.5 Gbit/s to make reliable physical layer measurements.

Next: Lip-sync
Lip-sync
One of the most annoying quality impairments in television occurs when audio and video are not in synchronisation. Known in the industry as the lip-sync issue, it appears when audio and video processing times vary and the alignment of both is done incorrectly.

Several attempts in the industry were undertaken to deploy in-service testing and correction methods, but most encoding schemes actually crop off the ancillary data space, where traditional in-service test signals were situated. The rise of multi-channel audio along with surround sound audio in compressed formats made this task even more challenging.

The out-of-service test with a defined audio and video routine is still the method of choice. Whereas most signals use test patterns relying on the operator to listen and visually find the correct timing between audio and video, Tektronix offers an award-winning test routine now not only available in high-end analysers but also in the new WFM7120.

A correlated audio and video test signal is fed into the system and the Waveform Monitor provides an exact measurement of the timing difference between audio and video. The test signal is available free of charge to all owners of Tektronix TG700 test signal generators and will support SD and HD digital formats.

Cross layer monitoring
The key element to commercial success for HDTV is getting the best possible picture quality to the viewer. H.264/MPEG-4 Part 10 is taking a lead position in satellite and cable applications. In the IP domain, VC-1 and H.264 are dominant.

In order capitalise on all potential benefits of these video codecs, it is imperative to properly debug the distribution channel, whether it be satellite, cable, terrestrial or IP. In mobile TV, lower resolution formats like CIF or QCIF are used, but the challenges are similar to making an IP path work. When looking at traditional systems like satellite, cable or terrestrial, it is mandatory to look at the RF domain simultaneously to the video domain.

This approach, labelled cross layer monitoring, is a precondition for rapid fault isolation and root cause identification in case of a transmission problem. To provide economies of scale, Tektronix has introduced RF Channel Polling to allow intelligent cycling across the channels while still prioritising the channels of high interest or high value. In the IP domain, it is now possible to combine fast monitoring across many sessions, instantly indicating crucial signal failure together with in-depth diagnostics for properly troubleshooting faults.

The MTM400 MPEG, RF and IP monitor (see next page) ensures that quality-of-service requirements and service level agreements are tracked and met.

Next: Elementary stream analysis
Elementary stream analysis
For improving H.264, Tektronix has also developed a Test Suite for elementary stream analysis to improve coding efficiency. It is comprised of an H.264 Multiplexer enabling the creation of test transport streams using the Tektronix Vclips library of test streams, a buffer analyser to verify set top box design and stream compliance and the transports stream analyser to ensure carrying H.264 coded streams.

This Test Suite can be complemented with testing hardware like the MTS430 to allow for realtime measurements. The MTS430 has also been upgraded to host a variety of RF and IP interfaces for improved connectivity and cross layer testing.

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Figure 2: The MTM 400 MPEG transport stream monitor from Tektronix.

Supporting HDTV end-to-end requires a variety of test tools and expertise in different domains, such as uncompressed multi-format video and audio, compressed video for playout and file-based video for storage and workflow.

Tektronix, with a strong commitment to providing testing excellence and accuracy to its customers, offers a wide portfolio of products and solutions that were built on the heritage of serving the TV industry since its inception more than five decades ago.

About the author
Winfried Schultz is Tektronix's Video Segment Marketing Manager for Europe, Middle East and Africa (EMEA). Winfried joined Tektronix in 1997, implementing network surveillance systems for telecom operators across EMEA. Winfried holds a degree in Electrical Engineering from the Fachhochschule Hamburg, Germany. He can be reached at measurement.germany@tektronix.com.