Nov 12 2009
IPhone HTTP Video Streaming Explained
One of the more overlooked features of the new iPhone 3.0 is support for a new open standard for live video streaming over HTTP, which promises to open up standards-based video broadcasting to a wide audience while giving mobile users an optimized picture as they roam between WiFi and mobile networks.
For the last decade, Apple has been selling QuickTime Streaming Server, which uses an RTSP (Real-Time Streaming Protocol) server to stream live or rebroadcast video feeds to viewers. Apple uses this technology to stream some of its own live events. However, despite offering royalty free streaming and also delivering it as an open source project, QuickTime's RTSP streaming server hasn't gained the traction it was once expected to achieve. A large part of this is due to the fact that RTSP traffic is blocked by many firewalls, making it difficult to deliver streams reliably. The audio and video conferencing used by iChat also relies on RTSP, causing some users frustrating problems for the same reason. Getting RTSP video streaming to work on the iPhone would be even more difficult, as it routinely moves between mobile and WiFi networks.
Enter HTTP Live Streaming. The basic mechanics involve using software on the server to break an MPEG-2 transport stream into small chunks saved as separate files, and an extension to the .m3u playlist specification (.m3u8) to tell the client where to get the files that make up the complete stream. The media player client merely downloads and plays the small chunks in the order specified in the playlist, and in the case of a live stream, periodically refreshes the playlist to see if there have been any new chunks added to the stream.
What's next? The obvious followup is to add support for HTTP Live Streaming in Apple TV, allowing for HD streams direct from broadcasters, facilitating the ability to only pay for channels you want to watch, skipping around the local cable monopoly while gaining access to content they don't carry. The same content would also be accessible on the iPhone, a desktop PC, or any other device with the capacity to play modern video codecs. And that's why Apple is not supporting Mozilla's efforts to use the obsolete Ogg Theora on the web, which lacks silicon support for hardware acceleration on mobiles and appliances.
Adaptive Streaming allows programmers to offer a higher quality viewing experience to a broader audience. Being able to offer HD streams all the way down to low bandwidth streams, without pauses or interruptions of the video, makes for a significantly improved consumer experience.
For examples of iPhone HTTP Streaming segments, see Ankoder - Online Video Encoding. Viewing live streams requires iPhone 3.0 or Snow Leopard QuickTime X.
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