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

Media Data Atom TypesHint MediaThe sample format depends on the dataFormat field of the QTSSampleDescription. The dataFormatfield can be any value you specify. The currently defined values are 'rtsp' and 'sdp '.If 'rtsp', the sample can be just an rtsp URL. It can also be any value that you can put in a .rtsp file, asdefined athttp://streaming.apple.com/qtstreaming/documentation/userdocs/rtsptags.htmIf 'sdp ', then the sample is an SDP file. This would be used to receive a multicast broadcast.Hint MediaThe QuickTime file format supports streaming of media data over a network as well as local playback. Theprocess of sending protocol data units is time-based, just like the display of time-based data, and is thereforesuitably described by a time-based format. A QuickTime file or movie that supports streaming includesinformation about the data units to stream. This information is included in additional tracks of the movie calledhint tracks.Hint tracks contain instructions for a streaming server which assist in the formation of packets. These instructionsmay contain immediate data for the server to send (for example, header information) or reference segmentsof the media data. These instructions are encoded in the QuickTime file in the same way that editing orpresentation information is encoded in a QuickTime file for local playback.Instead of editing or presentation information, information is provided which allows a server to packetize themedia data in a manner suitable for streaming, using a specific network transport.The same media data is used in a QuickTime file which contains hints, whether it is for local playback, orstreaming over a number of different transport types. Separate hint tracks for different transport types maybe included within the same file and the media will play over all such transport types without making anyadditional copies of the media itself. In addition, existing media can be easily made streamable by the additionof appropriate hint tracks for specific transports. The media data itself need not be recast or reformatted inany way.Typically, hinting is performed by media packetizer components. QuickTime selects an appropriate mediapacketizer for each track and routes each packetizer's output through an Apple-provided packet builder tocreate a hint track. One hint track is created for each streamable track in the movie.Hint tracks are quite small compared with audio or video tracks. A movie that contains hint tracks can be playedfrom a local disk or streamed over HTTP, similar to any other QuickTime movie. Hint tracks are only used whenstreaming a movie over a real-time media streaming protocol, such as RTP.Support for streaming in the QuickTime file format is based upon the following considerations:2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.251