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Chapter 9. Media Streaming > Synchronizing Data with Video - Pg. 331

Media Streaming 331 Synchronous data represents the regular delivery of bits at a constant average bitrate. For example, a synchronous stream may deliver a stream of bits such that each second 192 Kbits are transmitted, received, decoded, and rendered. Like synchronized data, synchronous MPEG-2 program elements require the use of a PCR and PTSs. However, unlike Synchronized data, Synchronous data streams carry a small fixed length DAU of 2 bytes, thereby significantly simplifying their implementation which is driven only by the PCR and bitrate. Synchronous streams are useful in scenarios when the data displayed on the screen complements the video but is not directly related to it. As an example, synchronous data streams may be useful for stock ticker, weather information, and headline news banners. Synchronizing Data with Video The key technical challenge when attempting to synchronize data with video is the lack of data transmission and decoding time guarantees needed to achieve accurate frame synchronization of the data. For example, a problem occurs when complex data objects with long decoding times are combined with time-line (PCR) discontinuities. In this circumstance, there can be ambiguity in the meaning of the PTS value in the encapsulation. The ATSC A93 Synchronized/Asynchronous Trigger Standard was designed to address this situa- tion, as well as other complications [A93]. It allows an arbitrary complex DAU to be activated by an arbitrary simplified receiver to achieve tight synchronization in the context of a discontinuous time line. The reminder of this section describes ATSC A93; at the time of writing this book, I am not aware of a parallel DVB or ISO standard. A key property of the ATSC trigger design (see Figure 9.23), which enables synchronizing data with video at frame-level accuracy, is its ability to decouple the asynchronous DAU delivery (and de- coding) from their (synchronized or asynchronous) activation. Synchronized triggers carry a PTS that indicates the point along the content time-line at which the DAU is to be presented. The trigger and the DAU referenced by a trigger are emitted received and decoded before the time it is to be presented. Both the DAU and trigger can be repeatedly transmitted to enable acquisition in the presence of random tuning, namely viewers may tune to a trigger carrying channel at random time points. Figure 9.23. Overall trigger structure.