Digital Signal Processing Reference
In-Depth Information
Movie downloads can be delivered efficiently through the
internet: companies like Netflix are moving away from DVDs by
mail, towards internet downloads of compressed movie content.
One advantage is that the movie can be downloaded in non-real-
time, mitigating the issues of streaming video over IP. This is known
as download and play. Progressive download and play is some-
where in the middle: the video is broken up into segments, and
played as soon as one segment is finished downloading. As long as
the next segment can finish downloading prior to the completion of
the previous segments play time, there is no interruption.
There are many video applications besides movies. Download
and play of short YouTube video clips (often low resolution),
marketing webcasts, investor and analyst briefing or private
corporate broadcast of executive's speeches are all applications
that can be well suited to video over IP transport.
18.7 Video Streaming
Video streaming requires real-time performance of the IP
network to deliver the compressed video content. Usually RTP is
used over the IP network.
The video source can be a video streaming server, a computer
or a webcam. The video server typically has a large storage
capability to house the large amounts of video to be broadcast on
demand. Alternatively, the video may be live (or almost live),
where it is being recorded by a camera, formatted, compressed
and then sent out for viewing across the internet (perhaps
a webcam). The formatting of the video can be of multiple forms,
different video players being supported with one or more of these
formats.
The software to do the video formatting and playing is avail-
able from several companies, and each has its proprietary
methods. Some of the familiar names are: Windows Media Player
by Microsoft; QuickTime by Apple; RealPlayer
by RealNetworks
and Adobe.
Newer video-streaming standards such as HTTP Live
Streaming from Apple have been developed to support video
streaming to iPhones and other smart mobile devices. This
standard uses HTTP (Hypertext Transfer Protocol) IP technology
as opposed to RTSP, which can allow it to bypass many firewalls
in IP networks. Microsoft offers Smooth Streaming, which also
dispenses with RTSP in favor of HTTP IP technology.
HTTP is not designed for video streaming, but it has been found
to be very efficient. It was originally designed for file transfer, and
not to maintain a persistent connection. More recently, a keep-
