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Example set of Picture Quality Analysis Charts TV-BAY Questions... Monitoring image & signal degradation with Mike Hodson, OmniTek OmniTek’s latest waveform analysis software focused on assessing the degradation in video and audio quality and timing suffered by images as they are transmitted or stored. What approach has OmniTek taken to measuring these effects? I n an ideal world, all broadcasters would love to have a piece of equipment that simply looks at a video signal and tells the operator what the quality of the signal is. Unfortunately this is an almost impossible thing to do, for many reasons. What is the meaning of ‘quality’, and how do you measure it? What happens if the video material has deliberately been distorted or de- focussed by the producer, for effect? Will this confuse the measurement system? In practice, the only reliable way to assess the quality of a signal that has been through a lossy transmission path is to compare the received signal with the original transmitted signal, and look at the differences. This is called a ‘full-reference quality measurement’, and it is what the OmniTek PQA system does. With access to both the source and received signals, it is then possible to make a quantitative, repeatable assessment of the amount of degradation that has taken place. Different full-reference comparisons can be made, varying from comparisons of two stored copies (used to assess the quality of a copy stored in a particular way), through comparisons of issued and received transmissions (used to determine the effect of the transmission path on the quality of the signal), to the comparison of the quality of the signal at different points along a transmission path (used to assess how the signal is degrading). What metrics are used? A range of different metrics are used for making Picture Quality assessments. The simplest and most ubiquitous is the Peak Signal to Noise Ratio (PSNR), which is a calculation of the actual differences in the pixel values between transmitted and received signals, summed over the whole screen area and expressed as a ratio in dB. This gives a quick and easy comparison of the two images, but is not very good at determining subtle differences in images that the human eye is more receptive to. The OmniTek PQA also offers an improved algorithm called the Compensated Signal to Noise Ratio (CSNR), where the pixel difference values are modulated by an edge map of the source image (to highlight compression artefacts) and also a luminance characteristic curve (to mask errors where the image is very dark or very bright). This technique gives results which more accurately reflect a viewer’s perception of the image quality. Of course, Picture Quality isn’t the only factor to be considered in relation to transmission and storage of video. There’s also Audio Quality which, in accordance with ITU-R BS.1387-1 and the PEAQ audio model, is assessed in terms of an Objective Difference Grade, a Distortion Index, and loss of Amplitude. Also important are the delays that become applied to the audio and video streams, which when different give rise to Lip-Sync problems. How does OmniTek measure Lip-Sync delay? OmniTek offers two different methods for measuring lip-sync delay. The first technique involves the use of a special video sequence, which consists of a test pattern plus an audio channel ‘blip’. This sequence is played out from a VTR or file server, through a transmission path: when received by any OmniTek measurement system, this displays the lip-sync delay. Calculation of lip- sync using this technique is accurate to one audio sample (typically 21 microseconds). The latest version of OmniTek software now also includes an optional “in- service” lip-sync delay measurement system, which does not require any special video sequence and is at least as accurate if not slightly more accurate than the test sequence technique. As long as the OmniTek equipment is provided with signals from both the transmission and the reception paths, the total loop delay of both video and audio components (and hence the relative A/V lip-sync delay) can be calculated and displayed. Where did OmniTek gain its expertise in Waveform Analysis? OmniTek has been creating high- resolution waveform monitoring systems for over 10 years. Our in- house design team have a strong background in audio and video signal analysis and digital image manipulation, and generating waveforms is a special application of these signal processing techniques. 54 | TV-BAY MAGAZINE: ISSUE 79 JULY 2013 TV-BAY079JUL13.indd 54 09/07/2013 16:51