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Figure 2 - PHABRIX® SxA with Dolby® Analysis Option Equipment that decodes and re- encodes the video/audio data stream must re-assemble the data stream exactly to ensure that it can be decoded by the receiver as intended. allows the integrity of the Dolby® audio data to be checked by the receiving equipment. It can be difficult to inspect or interpret a Dolby® data stream as it is passes through a broadcast chain. It can be more useful to inject a known Dolby® program with known metadata values and then check at each stage in the broadcast chain that the injected program and metadata is the same. Equipment such as the PHABRIX® Sx hand held and PHABRIX® Rx rack mount systems allow the generation of Dolby® E, Dolby® Digital and Dolby® Digital Plus metadata and test tones embedded within an SDI data stream so that closed loop testing of any equipment can be performed. Dolby® CRC and SMPTE Pa/ Pb Sync Word Spacing There are a number of critical measurements that can be used to ensure that the Dolby® data burst is correct: The Dolby® data burst provides a CRC word for each data burst. This With Dolby® Digital and Dolby® Digital Plus, the audio data burst is a constant length each frame and is bounded by the SMPTE 377M-2000 Pa and Pb sync words. If this spacing is incorrect, or changes during a programme, it can indicate that the data burst is not generated correctly or that it is corrupt. It may not be possible to detect random Pa/Pb spacing changes or CRC errors during a program transmission so it is may be necessary to log these errors for later analysis. Dolby® Programme Combinations Dolby® programs can consist of mono channels, stereo pairs, 5.1 surround sound and 7.1 surround sound and even 13.1 surround sound as Main, Dependent or Independent programme streams. There are a large number of possible programme combinations that are permitted by Dolby® E, Dolby® Digital and Dolby® Digital Plus and therefore equipment manufacturers and installers need to test all of these combinations to ensure that they are compliant. Products such as the PHABRIX® Sx hand held and Rx rack mount equipment allow the automated testing of each of the possible combinations to ensure compliance. Audio Buzz When Dolby® encoded audio is not processed or decoded correctly it can produce a distinctive audio buzz. If the audio programme is interrupted the decoder may repeat the same audio segment repeatedly unit the programme is restored. Conclusion At first sight Dolby® audio encoding looks like a black art but in reality it is only a black art if you don’t have an understanding of the principles that have been adopted and the equipment to analyse it. As I said in my introduction that if you can hear the audio you can tell if it is alright. With Dolby® E, Dolby® Digital and Dolby® Digital Plus it’s a bit like the ‘Matrix’ where you have to concentrate on the moving letters and symbol to see the picture. Or in this case see the audio[1]. Digital audio is here to stay and will steadily become more sophisticated as more and more programme content is delivered over increasingly narrow channels but with the expectation of increasingly higher quality. So to deliver this expectation we need to ‘tool-up’, both our understanding of digital audio as well the equipment required to do so. [1] If you view the audio data stream on a monitor as ‘pulse cross’, it looks a bit like the cascading symbol view of the ‘Matrix’ from the film of the same name. [2] Dolby® E, Dolby® Digital and Dolby® Digital Plus are trademarks of Dolby Laboratories™ Figure 3 – PHABRIX® Sx Dolby® Generator Option Alan Wheable FISTC, MITOL Senior Technical Author PHABRIX® 58 | TV-BAY MAGAZINE: ISSUE 79 JULY 2013 TV-BAY079JUL13.indd 58 09/07/2013 16:51