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14/11/2016

T&M. It's Not All About 'Video and CRI Issues'.

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There is a tendency these days to associate Test and Measurement with 'Video and Bit streams' and perhaps to forget about other system infrastructure.

So what do you do when you suspect an audio or video system problem? Well of course by applying 'first principles', you should confirm that the Source IS actually reaching the Destination, and IS what you expect it to be – but what if it is corrupted?

Any Testing device worth its salt will be able to supply a suitable substitute signal to eliminate the original source, and see if this localises the fault. After all, you can't fix a problem unless you know what it is! Assuming the problem is unchanged, reconnect the original source and move the Tester to the receive end of the path and then confirm whether the problem is still present, or not. This fundamental 'section by section' technique is always applied, whether the routing be in the digital or analogue domain. So, assuming that the problem has been localised to one particular section of cable, what equipment might we use to identify and locate a video cable problem, for example?

Well, the popular Murraypro Test Chest is an excellent place to start as it provides a number of powerful investigative tools in one compact unit, which are particularly quick and easy to use. Naturally it offers users a variety of A+V Generation and Analysis capability from 3G SDI, through tri-level and mixed sync on a wide range of standards, with AES audio and heritage composite formats too.

An extremely powerful investigative utility, and one which is totally unique to Test Chest is Time Domain Reflectometry. TDR testing is a diagnostic technique which is particularly suited for installation investigations and just like Radar, uses a very short pulse of energy to probe into the 'unknown'. The TDR utility then monitors the network for reflections which are displayed ripples on a scope trace.

When a healthy cable, correctly terminated in its characteristic impedance (normally 75 ohms) is tested this way there should be NO reflected energy to display, since any reflection is an indication of an imperfect network. Reflected pulse energy is clearly displayed on the TDR display as one or more 'echoes' from which three vital pieces of information are readily divined. The echo polarity, either positive or negative, indicates whether the reflected energy emanates from an anomaly of higher or lower impedance respectively than 75 ohms, whilst the echo amplitude is an indication of the severity of the mismatch. TDR readily displays all impedance anomalies ranging between the extremes of open circuit through to short circuit. Conveniently, and exactly as with a Radar reflection, the distance to the anomaly is indicated directly in metres on a calibrated scale. One very impressed Test Chest User told us of the TDR Utility: "I never knew I needed it – and then I used it three times in one day measuring coiled cable lengths!"

What if the problem is audio related, and in the analogue domain too, how could this be investigated? Once again Test Chest springs to the rescue, and when using its included Audio-Pod XLR interface, is able to directly diagnose balanced audio cable problems. Test Chest is able to show when a cable is 100% serviceable; or whether it is suffering from a phase reversal on pins 2&3, or it might be open circuit on one leg, short circuiting a leg to ground, or whether it is totally open or short circuit? The presence of any of these faults is quite sufficient to wreck programme audio, but Test Chest's diagnostic power will be available immediately the suspect cable is connected.

To see a demonstration, learn about Test Chest's magic Which-Wire? automatic source identifier, or for more details generally, visit www.murraypro.com.

This article is also available to read at BFV online as part of this issue's Test & Measurement feature here, page 46.

(JP/LM)
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