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Concept
Progressive download
Résumé
A progressive download is the transfer of digital media files from a server to a client, typically using the HTTP protocol when initiated from a computer. The consumer may begin playback of the media before the download is complete. The key difference between streaming media and progressive download is in how the digital media data is received and stored by the end user device that is accessing the digital media.
A media player that is capable of progressive download playback relies on meta data located in the header of the file to be intact and a local buffer of the digital media file as it is downloaded from a web server. At the point in which a specified amount of data becomes available to the local playback device, the media will begin to play. This specified amount of buffer is embedded into the file by the producer of the content in the encoder settings and is reinforced by additional buffer settings imposed by the media player.
Initially the digital media file type known as JPEG was the first visual media to render a progressive visual display as the digital media was downloaded and actually referred to as a progressive download. The distinction between the technical behavior of progressive download as opposed to the common or commercial use of the term progressive download to describe that behavior was not documented and there is a good deal of question regarding the origin of the term versus the origin of the technical implementation. Apple in reference to their QuickTime media player employed the term Fast Start in 1997, to describe what was commercially referred to as progressive download playback of encoded digital media content.
The end user experience is similar to streaming media, however the file is downloaded to a physical drive on the end user's device; the file is typically stored in the temporary directory of the associated web browser if the medium was embedded into a web page or is diverted to a storage directory that is set in the preferences of the media player used for playback.
Source officielle
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