Section One: The Digital Video Process
Step
Two: Sending Digital Video to the Desktop
Once a video is
created, it is stored and then transported to the desktop for playback.
Digital video created on a computer can be stored on the computer,
opened and played back, just as a document is opened in a word processing
program for reading, editing and printing.
A server must
generally be employed to store and share a video over a network —
whether a campus or building LAN or the Internet. Digital service
includes real-time broadcast, non-streamed downloading or streaming
to the desktop. Video service may be multicast ("one to many") where
one video stream is served to many viewing clients or unicast ("one
to one") where one video stream is served to one viewing client.
Real-time broadcasting
converts analog video to digital on the fly. Analog video is received
by the video server directly from a broadcast feed or a video camera,
encoded in real time, and then served as a multicast video stream
to many clients. Real-time
broadcasting also includes the real-time delivery of files already
in a digital format, such as a digital camera or satellite transmission.
Video files are
meaningful only when forward progression, providing continuity of
information, is maintained. A cartoon coyote cannot be running off
the cliff in one frame and standing on the edge of the cliff, looking
down, in the next frame, if the video file is to make sense to the
viewer. The coyote also cannot be running swiftly to the cliff in
one frame and moving slowly and jerkily in the next. Video data must
be played in the correct order, with little or no packet loss, and
with smooth, continuous timing; or else essential information will
be missing. To insure that video files are usable at the desktop,
the frames must be received in order and timed for playback. Digital
video can be received at the desktop for playback in two ways: non-streaming
or streaming video.
Non-streaming
video downloads an entire video file and lacks the timing functionality
for smooth packet streaming. (Some clients, such as QuickTime 3, will
begin to play the video before the file has completely downloaded
— this is referred to as "preloading" — and thus may simulate
streaming.) A video server is not required to store and serve non-streaming
digital video. Any server can store and serve non-streaming video,
or the non-streamed file may be stored on the microcomputer hard drive
or a CD-ROM and played back.
Digital video
functionality, for opening the non-streaming file and playing it back
on the client machine, is provided by the client software. Downloaded
video is an option when the latency (elapsed time) required for the
download process, which can range from several minutes to more than
an hour, is not an issue.
Non-streaming
video is also employed when a video server is not available to provide
streaming. Non-streaming video files may also be provided when the
maximum number of concurrent video streams supported by a video server
has been exceeded. Most video server vendors do not support download
of non-streaming videos. If an institution wants to offer nonstreamed
video files for download, to insure high availability for the files,
an FTP server or other download site must be separately provided.
Streaming video
begins playback on the client as soon as enough of the video has loaded
to begin and sustain playback at a continuous rate. Cache is established
from random access memory (RAM) on the client desktop and is used
to receive the file, insure that frames are in the correct order,
establish timing, refresh compressed frames and check for dropped
packets. The video file continues to download into the client cache
even as the beginning of the video is being viewed. Video streaming
relies on technology at the video server and at the client, such as
caching and control bits, to receive and assemble a video in which
all data bits play smoothly, in progressive frame order.
Video streamed
via the Web must be transported within the IP architecture. Streamed
video has low tolerance for the enforced reliability of TCP, which
would keep an application waiting for the retransmission of dropped
packets. UDP (User Datagram Protocol) is frequently used in place
of TCP as a transport protocol for real time applications, such as
digital video.
UDP uses the Internet
Protocol (IP) to transport a data unit ("datagram"). UDP supports
digital video because it does not divide the data stream into packets
for reassembly at the client end. However, UDP also does not order
the datagrams into the correct sequence. Applications using UDP must
insure, at the receiving end, that the complete message has arrived,
in the correct sequence order.
