TV-BAY Issue 79 July 13

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its base band channels. When moving SDI video containing Dolby® audio around a facility the highest risks of failure are likely to be the caused by timing issues that critically affect the detection of the Dolby® header as well as interruptions or corruptions of the data stream. Dolby® E Framing Values Figure 1 - Dolby® audio data burst within HANC area primary programme (Programme 1) and optional ancillary programs that can be delivered at much lower bit rates than Dolby® Digital. These programme channels are compressed (lossy) into multiple independent digital data stream plus up to 8 dependent sub stream that can be transferred between compatible devices and stored on a standard stereo pair of audio tracks. HANC Encoding Unlike PCM audio the Dolby® data burst contains both the encoded audio channels as well as metadata. This Dolby® metadata carries specific information about the encoded surround sound audio including the Dolby® encoding method, the number and type of audio channels and the specific matrix coefficients required to re-assemble the surround sound audio at the receiver. The Dolby® metadata is be delivered to the receiver with the encoded audio channels to ensure that the correct audio levels and the correct channel separation The complexity of Dolby® encoding, its metadata and its transport using HANC SMPTE 337M data packet embedded within the SDI data, means that it is susceptible to video timing, switching issues, decoding and encoding and the insertion of additional broadcast metadata within the broadcast chain. It is therefore important to be able to analyse the metadata at each stage to ensure that all data is transmitted transparently and decoded successfully at its final destinations. The header information within the Dolby® E and Dolby® Digital encoded audio package is used by Dolby® decoders to identify specific encoding method used. In the case of Dolby® E, if this header information is missing then the audio is assumed to be PCM which can effectively cause a full scale noise burst that can damage audio monitoring equipment. Only when the next frame where a valid Dolby® E header appears, will the Dolby® circuitry be able to decode the data correctly. In the case of Dolby® Digital and Dolby® Digital Plus, if this header information is missing, or if the Dolby® programme is interrupted, then the audio is assumed to be PCM which can effectively cause a full scale noise burst that can damage audio monitoring equipment. Only when the next valid Dolby® Digital header appears will the Dolby® circuitry decode the data correctly. Note that this occurs every 3072 audio data samples. Things to look out for In most cases it has to be assumed that the actual Dolby® Surround Sound programme data is correct as it is difficult to interpret without decoding it first to A Dolby® E encoded audio programme is a video frame-based system whose data occupies the area of ancillary data normally occupied by the AES/EBU PCM audio. Unlike PCM audio that can tolerate video switching anywhere within a large range of lines in the vertical interval, Dolby® E has a narrow guard band which contains no audio and in which video switching can take place without loss of critical Dolby® header information. The Dolby® Reference Point (immediately following the Dolby® guard band) is where the encoded Dolby® E audio packet starts. This can be defined as specific video lines and timed approximately 700µs ±80µS from the SMPTE RP156 Reference Point. It is important for the Dolby® E packet to be positioned well away from the video switching line so that Dolby® E packets are not corrupted by downstream switchers. Test equipment such as the Sx hand held range with Dolby® option can measure the timing of the Dolby® E packet relative to the SDI input or the External reference as shown in Figure 1. Corruption of Dolby® Metadata As mentioned earlier, the Dolby® metadata contained with the audio data burst is as important as the encoded audio itself. Equipment such as play-out servers, that store complete Dolby® Digital and Dolby® Digital Plus video/audio programs, must deliver the Dolby® data burst exactly as it was created otherwise it will not be decoded correctly at the receiver. TV-BAY MAGAZINE: ISSUE 79 JULY 2013 | 57 TV-BAY079JUL13.indd 57 09/07/2013 16:51