Kitplus - The TV-Bay Magazine

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words of 3FFh and 16 consecutive words of 000h. When this is serialised, it will cause 80 ‘ones’ followed by 160 ‘zeros’ at the scrambler input. In the worst case scenario, after the NRZI coding, this could produce a run of 179 ‘zeros’ or ‘ones’ in the resultant data stream. To prevent this, a technique known as ‘syncbit insertion’ is employed to stop these ‘pathological’ sequences occurring. This requires the modifi cation of the 10-bit parallel interface data stream to replaces the two least signifi cant bits of repeated 3FF or 000 words, on all but the fi rst set of words, with values of 10b for 000h words and 01b for 3FFh words. Not all equipment employs ‘syncbit insertion’ so the use of 6G-SDI and 12G-SDI pathological test patterns available on the Omnitek Ultra 4k Tool Box can ensure that receiving equipment is immune to ‘potholes’. Which is the best Test Pattern to use with 4k / UHD equipment? Due to the construction of 4k / UHD data streams, traditional line test patterns such as colour bars and multi-burst do not always show up underlying issues. With quad link 3G / square division implementations there is a risk that cables (links) can be swapped or delayed with respect to each other which will only show up if an appropriate moving test pattern or pattern sequence is used. Test equipment that only shows one or two frequency bands simultaneously does not give a realistic jitter measurement. Ideally all jitter frequency bands need to be monitored simultaneously. Tools such as Jitter Meters, Waveform, Histogram and Spectrum available on the Omnitek Ultra 4k Tool Box can give an in- depth view of the jitter landscape of the SDI signal. What should I be monitoring in Ancillary Data? With the increasingly sophisticated use of metadata to defi ne how the video image is displayed, and audio is heard, it is essential that all Ancillary Data is correct to ensure that the viewer experience is as intended. Firstly the structure of all data needs to be checked using the CRC word built-in to each data packet. Secondly the continuous presence of the data packets needs to be monitored. Thirdly the content of all data packet need to be checked to ensure that they have been correctly formatted and to do this the packets need to be decoded. In the case of the SMPTE 352 packet (which defi nes the video format, image size, frame rate, colour space and link construction) this can be done by simply checking that the signal can be correctly decoded by the receiving equipment. For other data packets, appropriate test equipment such as the Omnitek Ultra 4k Tool Box is required to fi nd and interpret the 50 plus different Ancillary Data packets that may be present on the SDI signal. Ideally the test sequence should contain elements that will identify square division and 2 sample interleave artefacts as well as possible temporal issues if the individual link signals (ie quad link 3G or dual link 6G) are compressed or delayed with respect to each other. What do good Jitter meters tell me? As the clock frequency of the SDI data streams increases, the effect of jitter becomes potentially more damaging to the data. Timing and Alignment jitter at different frequencies can combine to exceed the permitted jitter level and cause data loss if the receiving equipment cannot tolerate it. Jitter meters show the Timing and Alignment jitter level at different frequencies and match the SMPTE RP 184 Jitter Tolerance Template which defi nes the permitted jitter level within each frequency band for each SDI interface type. KITPLUS - THE TV-BAY MAGAZINE: ISSUE 107 NOVEMBER 2015 | 53