Measuring HD performance with Tektronix' Picture Quality Analyser and other tools

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.

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