Image Processing Reference
content before commencing the playback. The player observes the packets arriving from
the server and is able to calculate the amount of buffering required. The QuickTime plug-
in, for example, will wait until it has buffered sufficient material so that when it com-
mences playback, it has a reasonable chance of continuing through the movie without
pausing to buffer more content. This is very necessary with low-bit-rate networks or high-
bit-rate content. If the two are carefully adjusted to match, then optimum conditions are
created for minimum buffering.
The user may request that the player jump forwards in the buffer to a point beyond
what has been delivered. The RTSP streaming interface will go to that new location and
begin buffering from that point. However, progressive downloading with HTTP doesn't
support that kind of access. It is strictly linear.
Fast start is a QuickTime feature that allows a downloaded file to start playing before it is
fully downloaded. This feature is called progressive downloading in other video players.
When a browser requests a movie file using the HTTP protocol, the download does not
have to be completed before playback begins. Previously the download and playback
processes could only take place sequentially because the cached file could not be opened
for playback while it was still being written. As long as the playback speed is equal to or
less than the download speed for the content, fast start is a neat solution and plays the
video in real time. See the discussion on buffering for what is required when download
speeds are slower than playback speeds.
Another advantage of progressive HTTP-based streaming is that an ordinary
Web server can serve the files. This saves you having to install and deploy an RTP or RTSP
The main disadvantage compared with streaming is that you cannot seek ahead and
only the content that has already downloaded is playable.
Smart players will negotiate with the streaming server to establish the available bit rate for
stream delivery. This is called content negotiation. The player requests a low bit rate
(lower quality) stream if network faults or high traffic levels compromise the networking
bandwidth. Sometimes this feature is called Sure Stream or Smart Stream.
All of the advanced streaming techniques have evolved out of a need to deal with band-
width issues. There is simply not enough bandwidth to deliver video and audio without
carrying out some very complex compression and delivery management.
Since the beginning, innovations in the reduction of bandwidth required to
deliver video and audio at quality worth watching have been matched by the telecoms'
research into more efficient modulation techniques over a wiring system that was origi-