Quicktime File Format (2012-08-14).pdf

Media Data Atom TypesHint Sample Data FormatMedia tracks that do not have an associated RTP hint track cannot be streamed over RTP and should be ignoredby RTP streaming servers.It is possible for a media track to have more than one associated hint track. The hint track contains informationsuch as the packet size and time scale in the hint track’s sample description. This minimizes the runtime serverload, but in order to support multiple packet sizes it is necessary to have multiple RTP hint tracks for eachmedia track, each with different a packet size. A similar mechanism could be used to provide hint tracks formultiple protocols in the future.It is also possible for a single hint track to refer to more than one media stream. For example, audio and videoMPEG elementary streams could be multiplexed into a single systems stream RTP payload format, and a singlehint track would contain the necessary information to combine both elementary streams into a single seriesof RTP packets.This is the exception rather than the rule, however. In general, multiplexing is achieved by using IP’s port-levelmultiplexing, not by interleaving the data from multiple streams into a single RTP session.The hint track is related to each base media track by a track reference declaration. The sample description forRTP declares the maximum packet size that this hint track will generate. Partial session description (SDP)information is stored in the track’s user data atom.Hint Sample Data FormatThe sample description atom ('stsd') contains information about the hint track samples. It specifies the dataformat (note that currently only RTP data format is defined) and the data reference to use (if more than one isdefined) to locate the hint track sample data. It also contains some general information about this hint track,such as the hint track version number, the maximum packet size allowed by this hint track, and the RTP timescale. It may contain additional information, such as the random offsets to add to the RTP time stamp andsequence number.The sample description atom can contain a table of sample descriptions to accommodate media that areencoded in multiple formats, but a hint track can be expected to have a single sample description at this time.The sample description for hint tracks is defined in Table 4-18 (page 256).Table 4-18Hint track sample descriptionFieldSizeData formatBytes442012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.256