Progressive download
A progressive download (German: progressive download ) is the transfer of digital media files from a server to a client . Typically, that is HTTP - protocol used when the transfer is initiated from a computer. The user can start playing before the media file has fully downloaded. The key difference between streaming and progressive downloading media files is how the digital media file is received and stored by the end user's device .
A media player capable of progressive download playback relies on intact metadata in the file's header and a local buffer of the digital media file downloaded from the web server. The medium is played from the point in time at which a certain amount of data is available on the local playback device. The specific size of this buffer has been embedded by the manufacturer of the file content in the encoder settings of the file. It is supplemented by additional buffer settings of the media player.
history
Originally, the JPEG digital media file type was the first visual medium to build up a progressive image representation while the digital medium was still being downloaded , and was therefore referred to as a progressive download. The distinction between the technical behavior of Progressive Download versus the common use of the term Progressive Download to describe this behavior has not been documented and there are very many questions about the origin of the term compared to that of the technical implementation. In 1997, Apple used the term Fast Start in the reference of its QuickTime media player to describe what is commonly referred to as progressive download playback of encoded digital media content .
The rapid start of playback was the result of a shift in metadata from the end of the digital media file to its beginning. This shift in metadata gave the media player all the information it needed to begin playback while the file was being downloaded. Before this change, the metadata was stored at the end of the digital media file and the entire file had to be downloaded in order to read the metadata and let the media player start playing.
Progressive download versus streaming media
The perception for the end user is similar to that of streaming media . When a digital media file is embedded in a web page, it is downloaded to the device memory by the end user. The digital media file is typically either saved in the temporary directory of the web browser involved or redirected to a directory that is specified in the settings of the media player for playback. The playback of the digital media file will jerk or even stop if the data rate for playback exceeds the data rate for downloading. If enough data has been downloaded again, the file will continue to play.
Do the washing up
First, a media file will play as quickly as possible from its beginning. The user may now wish to jump to a point in time in the playback that has not yet been downloaded. This capability is called spooling and allows any portion of the media file to be downloaded and played. This is also known as pseudo streaming.
When winding in Flash video , a list of winding points is required in the metadata of the media file. These points are offsets in the video (both in bytes and in seconds) at which a new section begins. A web server or a media server processing a download must support such spooling points in the requests for data to be downloaded.
For other types of media files like MP4 or MKV , the web server needs to be able to handle special offset parameters. The names of the offset parameters differ for the different servers and must therefore be specified in the settings of the media player.
Some servers support spooling with additional modules, as shown in the table below. The names of the winding parameters are written in italics .
| server | Flash spools | Spool MP4 |
|---|---|---|
| Apache HTTP Server |
mod_h264 start mod_flvx start |
mod_h264 starttime |
| lighttpd | begin | mod_h264 starttime |
| Nginx | HttpFlvModule start | HttpMp4Module start |
| Nimble Streamer | begin | begin |
Web links
- Streaming vs Progressive Download. Archived from the original on August 18, 2010 ; accessed on August 18, 2010 (English, not accessible).
- Dave Nelson: Windows Media Server or Web Server? Microsoft, May 15, 2008, accessed February 9, 2014 .
- Flash Video: Progressive Download. Media College.com, accessed February 9, 2014 .
Individual evidence
- ↑ QuickTime Streaming Server 4.1. (PDF) (No longer available online.) Archived from the original on July 6, 2011 ; accessed on September 21, 2010 : "Two methods are commonly used to deliver media over the web for real-time viewing: progressive download (Fast Start) and real-time streaming." Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
