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MONITORING Test & Measurement Ask the Expert with Alan Wheable FISTC, MITOL What should an Eye diagram look like? There are basically two different approaches to capturing the Eye Diagram from an SDI signal. These are real-time capture (employed by Real Time oscilloscopes) and signal subsampling (employed by Sampling oscilloscopes such as the Omnitek Ultra 4k Tool Box). All broadcast T&M equipment will use one of these approaches to create the Eye Diagram using a variety of proprietary techniques. Real Time oscilloscopes build the Eye using the complete waveform from a single trigger point and captured from the signal at a frequency at least 2.5 times higher than the signal. Sampling oscilloscopes build the Eye pattern from individual pixels sub- sampled at a frequency lower than the signal with position of each sample offset from the previous. These two approaches produce a subtly different pseudo-live display which the T&M equipment has processed and fi ltered to give the most accurate results. The Eye pattern can be shown with or without jitter superimposed. With jitter the resultant Eye can be thicker, it can have movement and it can close. With 12G-SDI the permitted jitter for some frequencies is 8UI and even at 0.2UI the Eye will close making it diffi cult to measure accurately. So for most purposes the Eye should be jitter-free so that the signal shape can be seen and the critical parameters of amplitude, rise time and fall time can be measured. A common misbelief with Eye Diagrams is that one display method is somehow better than another. In practice the method is irrelevant as the displayed Eye Diagram is nearly always the same and this is the important thing when making measurements. How can I gauge the quality of an SDI signal and why is it so important for 12G-SDI? The quality of the SDI signal is important as this determines the maximum cable length the signal can be sent and can be defi ned by the waveform shape and the jitter level. 52 | KITPLUS - THE TV-BAY MAGAZINE: ISSUE 107 NOVEMBER 2015 The waveform shape is different for SD, HD, 3G, 6G and 12G SDI signals. For SD it is almost a square wave but for 12G it is more like a sinewave due to the attenuation of the high harmonic frequencies of the fundamental clock frequency. Regardless of the clock rate, the most important waveform aspects are the amplitude, rise and fall times. Overshoot and undershoot are indicative of phase differences in the harmonic frequencies (normally due to return loss and signal driver quality) but as long as they are within specifi ed limits they should not affect the overall cable length. Note that current generation, high slew rate driver chips for 12G-SDI can transmit over 60m with Belden 1694 A cable. Even with the perfect waveform shape, the level of jitter will affect the cable length because the jitter effect is accentuated with lower signal level as the signal rise / fall times are longer. It is essential to remove jitter from the Eye Diagram and monitor it separately. Omnitek’s Ultra 4k Tool Box employs this method for this reason. It is essential that Eye Pattern shape and Jitter measurement are within specifi ed limits at the sending equipment’s output, over a 1m cable and using 100% bars as this is the best way to ensure that the signal can be delivered over reasonable cable length when actual live video is used. How do I know that my equipment is immune to ‘Potholes’? ‘Potholes’ (or TRS-Induced Pathological Conditions) are long sequences of ‘ones’ and ‘zeros’ that can occur in the SDI data steam when the 8 data streams of the 4k / UHD image are multiplexed onto a single 6G-SDI or 12G-SDI cable. The effect of ‘potholes’ is jitter, and potentially data errors, in the receiving equipment as it loses and re-locks after each occurrence. These ‘potholes’ potentially occur twice in every 512 lines when the TRS preamble sequences from the 8 different streams are multiplexed. For 6G-SDI the concatenated TRS preambles result in 8 consecutive