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

Media Data Atom TypesFinding an Original Media Track From a Hint TrackType'tmax''pmax''dmax''payt'Value4 bytes4 bytes4 bytesVariableDescriptionThe largest relative transmission time, in milliseconds.The largest packet, in bytes; includes 12-byte RTP header.The largest packet duration, in milliseconds.The payload type, which includes payload number (32-bits) followed byrtpmap payload string (Pascal string).Note Any of the atoms shown in Table 4-17 (page 254) may or may not be present. These atoms arenot guaranteed.Finding an Original Media Track From a Hint TrackLike any other QuickTime track, hint tracks can contain track reference atoms. Exactly one of these must be oftrack reference type 'hint', and its internal list must contain at least one track ID, which is the track ID of theoriginal media track. Like other track reference atoms, there may be empty references in this list, indicated bya track ID of 0. For hint tracks that refer to more than one track, the index number (starting at 1, and includingany 0 entries) is used in the media track reference index field in some of the packet data table entry modes.For example, if you have MPEG-1 video at track ID 11 and MPEG-1 layer 2 audio at track ID 12, and you arecreating a RTP hint track that encapsulates these in an MPEG-2 transport, you need to refer to both tracks. Youcan also assume that there are some empty entries and other track references in your hint track atom referenceatom’s list. So it might look like this: 11, 0, 0, 14, 0, 12, 0. When you are assembling packets from audio andvideo tracks 11 and 12, you use their list indexes (1 and 6) in the media track ref index field.If you have only one media track listed in your hint track reference, you may simply use a 0 in the media trackref index field.RTP Hint TracksRTP hint tracks contain information that allows a streaming server to create RTP streams from a QuickTimemovie, without requiring the server to know anything about the media type, compression, or payload format.In RTP, each media stream, such as an audio or video track, is sent as a separate RTP stream. Consequently,each media track in the movie has an associated RTP hint track containing the data necessary to packetize itfor RTP transport, and each hint track contains a track reference back to its associated media track.2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.255