摘录


Streaming of audio and video is a confusing subject. This page is aimed at providing some of the basic concepts.

Streaming means sending data, usually audio or video, in a way that allows it to start being processed before it's completely received. Video clips on Web pages are a familiar example.

Progressive streaming, aka progressive downloading, means receiving an ordinary file and starting to process it before it's completely downloaded. It requires no special protocols, but it requires a format that can be processed based on partial content. This has been around for a long time; interleaved images, where the odd-numbered pixel rows are received and displayed before any of the even ones, are a familiar example. They're displayed at half resolution before the remaining rows fill in the full resolution.

Progressive streaming doesn't have the flexibility of true streaming, since the data rate can't be adjusted on the fly and the transmission can't be separated into multiple streams. If it delivers a whole file quickly and the user listens to or watches just the beginning, it wastes bandwidth. The user is given the whole file and can copy it without any effort.

"True" streaming uses a streaming protocol to control the transfer. The packets received don't add up to a file. Don't mistake streaming for copy protection, though; unless there's server-to-application encryption, it's not hard to reconstruct a file from the data.

True streaming may be adaptive. This means that the rate of transfer will automatically change in response to the transfer conditions. If the receiver isn't able to keep up with a higher data rate, the sender will drop to a lower data rate and quality. This may be done by changes within the stream, or by switching the client to a different stream, possibly from another server. Streamingmedia.com has a discussion of adaptive streaming.

Streaming can be broadly divided into on-demand and real-time categories. With on-demand streaming, the client requests a recording or movie and receives it; normally no one else will receive the same recording at the same time. With real-time streaming, the sender determines what to send, and the receiver plays it back as it's sent, with a slight and consistent delay.

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提问于 23 一月, 15:33

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