Chapter 25. Video on the Web
audio, video clips were linked to web pages in the Web's
earliest days. Delivering video via the Web is especially problematic
because video files require huge amounts of data to describe the
video and audio components, making for extremely large files. Few
people will sit and wait an hour for a couple of minutes of video
Many of the same technologies that have improved the experience of
receiving audio over the Web have been applied to video as well. As
with audio, you have the option of simply linking a video to your web
page for download and playback, or you can choose from a number of
streaming solutions. "Streaming" means the file begins
playing almost immediately after the request is made and continues
playing as the data is transferred; however, the file is never
downloaded to the user's machine. For a more complete
description of streaming versus nonstreaming media, see Chapter 24, "Audio on the Web".
Many of the principles for developing and delivering video content
for the Web are the same as those for audio (in fact, some of the
file formats are the same as well). This chapter introduces you to
basic video technology and concepts, including introductions to the
video file formats QuickTime, RealMedia, Windows Media, AVI, and
MPEG. If you are interested in learning how to produce video files
for the Web, the books listed at the end of this chapter are a good
25.1. Basic Digital Video Concepts
The following is a list of aspects of digital video that can be
manipulated with standard video-editing software. It is important to
be familiar with these terms so you can create video optimized for
- Movie length
It's a simple
principle -- limiting the length of your video clip limits its
file size. Videos longer than a minute or two may cause prohibitively
long download times. If you must serve longer videos, consider one of
the streaming video solutions.
- Frame size
Obviously, the size of the frame has an
impact on the size of the file. "Full-screen" video is
640 480 pixels. The amount of data required to deliver an
image of that size would be prohibitive for most web applications.
The most common frame size for web video is 160 120 pixels.
Some producers go as small as 120 90 pixels. It is not
recommended that you use a frame size larger than 320 240
with current technology. Actual size limits depend mostly on CPU
power and bandwidth of the user's Internet link.
- Frame rate
rate is measured in number of frames per second (fps). Standard
TV-quality video uses a frame rate of 30 frames per second to create
the effect of smooth movement. For the Web, a frame rate of 15 or
even 10 fps is more appropriate and still capable of producing fairly
smooth video playback. For "talking head" and other
low-motion subjects, even lower frame rates may be satisfactory.
Commercial Internet broadcasts are routinely done as low as 0.5,
0.25, or even 0.05 frames per second (resulting in a slideshow effect
rather than moving video).
Many video-editing applications allow you to set the overall quality
of the video image. The degree to which the compression algorithms
crunch and discard data is determined by the target quality setting.
A setting of Low or Medium results in fairly high compression and is
appropriate for web delivery. Frame rate and quality are often traded
off in different degrees in relation to each other, depending on the
application, to reduce bandwidth requirements.
- Color bit depth
The size of the video is affected by the
number of pixel colors in each frame. Reducing the number of colors
from 24- to 8-bit color will drastically reduce the file size of your
video, just as it does for still images. Of course, you also
sacrifice image quality.
- Data rate (bit rate)
This is the rate at which data must be
transferred in order for the video to play smoothly without
interruption. The data rate (also called "bit rate") for
a movie is measured in kilobytes per second (K/sec or Kbps). It can
be calculated by dividing the size of the file (in K) by the length
of the movie (in seconds). So, for example, a highly compressed movie
that is 1900K (1.9 MB) and 40 seconds long has a data rate of
For streaming media in particular, a file's data rate is more
important than its total size. This is due to the fact that the total
bandwidth available for delivery may be severely limited,
particularly over a dial-up connection. For example, even an ISDN
line at 128 Kbps offers a capacity to deliver only 16K of data per
|24.8. For Further Reading||25.2. Compression|
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