Streaming

Overview

Streaming involves sending movies from a server to a client over a network such as the Internet. The server breaks the movie into packets that can be sent over the network. At the receiving end, the packets are reassembled by the client and the movie is played as it comes in.

QuickTime movies can be real-time streamed using:

• RTP (Realtime Transport Protocol)

Streaming is different from Fast-Start download in that the client plays the movie as it comes in over the network, rather than downloading the entire movie and playing it as it is being downloaded. A streaming movie is never actually downloaded; its packets are played as they come in, then discarded.

QuickTime supports RTP streaming of video, audio, text, and MIDI.

RTP is used for real-time streaming. Movie packets are sent in real time, so that a one-minute movie is sent over the network in one minute. The packets are time-stamped, so they can be displayed in time-synchronized order. Because packets are sent in real time, RTP streaming works with live content in addition to pre-recorded movies and can even carry a mixture of the two. RTP can be used for live transmission and multicast, this allows the user to view long movies or continuous transmissions without having to store more than a few seconds of data locally. Using RTP transmission under RTSP control, a user can skip to any point in a movie on a server without downloading the intervening material.

RTP streams can be incorporated in a movie using streaming tracks.

A streaming track is a track in a QuickTime movie that contains the URL of the streaming content. A QuickTime movie that contains streaming tracks can also include non-streaming tracks whose media exist on the client's computer. This allows a live transmission, or data stored on the Internet, to be incorporated into a movie along with material stored on locally or distributed over CD-ROM.

RTP uses UDP protocol, which doesn't attempt to retransmit lost packets. This allows multicasts as well as live streams, both cases where retransmission would not be practical.

Application Considerations

An application receives streaming content by opening a movie and playing it. In general, opening a streaming movie is like opening any QuickTime movie. You can open a streaming movie by opening a movie file that contains streaming tracks, a session description file (.SDP) or a URL.

If you open a streaming movie from a URL or an SDP file, the Movie Toolbox will call the appropriate movie importer. If you open a movie file that contains streaming tracks, stream importers will be called.

Applications that can already play QuickTime movies need to take the following points into account to reliably support real-time streaming movies:

• Open movies with high-level Movie Toolbox calls or a movie importer.
• Do not assume that the track structure of the movie you play reflects the track structure of the original movie; a streaming track can contain sound, video, text, MIDI, or all of these.
• Use a movie controller to play the movie, or make sure to Pre-Preroll in addition to Prerolling a movie to set up any streams before playing the movie.
• Display status messages returned by the movie controller.
• Be prepared to deal with network errors when your application plays a movie.
• Be prepared for movie characteristics to change dynamically; the height, width, duration, and whether or not the movie has audio or video can all change as the movie plays.
• Do not assume that the movie will begin immediately.

Track Structure

Unlike other QuickTime movies, streaming movies consist of two distinct movie filesÑone on the server and the other on the client machineÑoften with different track structures. You sometimes need to distinguish between the server movie and the client movie. The media of each track in the server movie is transmitted as a separate RTP stream. On the client side, multiple RTP streams can be combined into a single streaming track.

If you open a movie from a URL, for example, the movie importer may create a client movie that contains only a streaming ('strm') track. Unlike most other QuickTime track types, a streaming track can contain multiple media streams of different types. A streaming track in a client movie may contain the URL of a server movie with audio, video, text, or MIDI tracks.

The client movie file never contains audio or video media from the server movie; they are displayed and discarded. If a client movie is saved, the saved movie contains information such as the URL of the server movie and the currently displayed movie time. The client movie can also contain local tracks and local media, but the media samples in the server movie remain on the server except when playing.

Preroll and Pre-Preroll

Before any movie is played, QuickTime needs to allocate buffers and open appropriate media handlers. This process is called prerolling the movie. Before a streaming movie can be played, additional steps need to be taken, such as establishing RTP streams between the client and the server. This setup process is called pre-prerolling. Pre-prerolling is performed automatically when a streaming movie is played using a movie controller. If your application uses movie controllers to play movies, you do not need to take any special steps to pre-preroll a streaming movie.

If you are playing movies using the Movie Toolbox (without a Movie Controller), you will need to pre-preroll the movie to set up the network connections for a streaming movie before you can preroll or play the movie. Pre-prerolling does nothing unless a movie contains streaming content, so it's safe to do for all movies.

Pre-prerolling can be done either synchronously or asynchronously, depending on whether you specify a completion procedure. If called asynchronously, control will return almost immediately even if the movie contains streaming content. This allows your application to perform other tasks while awaiting the completion of the pre-prerolling. You will need to periodically to grant time to the Movie Toolbox for the task of pre-prerolling when using it asynchronously. When pre-prerolling is finished, your completion procedure will be called. In the simplest case, the completion procedure will want to preroll and start the movie.

Reacting to Changes in Movie Characteristics

One feature of streamed movies is that their characteristics may change dynamically during playback. For example, when you open a movie from a URL you may not know the actual height and width of the movie, its duration or how many streams it contains. You also might not know if the movie has a sound track, or whether it is an audio-only movie.

QuickTime will assign default values to these characteristics if they are unknown.

QuickTime can notify your application when the movie characteristics become known or are changed and in most cases, your application will want to reflect such changes. Changes in the movie size and other movie characteristics are announced and can be handled in a action filter procedure.

Duration Changes

The duration of a streaming movie or a streaming track may not be initially known. A track or movie whose duration is not known is assigned an indefinite duration: 0x7FFFFFF . If you do not treat this as a special case, a streaming movie will appear to your application as a very long movie. Once the pre-preroll process is complete, QuickTime should know the actual duration of the streams. Duration changes are also handled in the application's action filter procedure.

If the movie contains live content, it may not have a specific duration. In this case, the duration remains indefinite: 0x7FFFFFF . You might want to set a timeout in your code that detects the fact that QuickTime has not adjusted the duration from 0x7FFFFFF after a few seconds of movie play and use such a timeout to notify the viewer that the movie is a live broadcast.

Sound and Video Changes

Whether a streaming movie has sound or is sound only (no video) may not be initially known or may change dynamically. If you have an action filter procedure and set the movie hints, your applications filter procedure will be called when the sound or video characteristics change.

Other Playback Considerations

Streaming movies only play back at a rate of 1. Other playback rates, such as playing backward, are not supported when streaming in real-time.

References

Streaming

Customizing Movie Controllers