HTML and CSS Reference
Exploring Digital Video
Maxine's next task for you is to embed a video clip on her Web page of Fred Astaire
dancing in Royal Wedding . Video has become one of the most popular methods of shar-
ing information on the Web. Before embedding Maxine's video clip, you'll first examine
some of the issues involved in working with digital video on the Web.
Bit Rates and Video Quality
A video is composed of a series of single images or frames that are played in rapid suc-
cession to create the illusion of motion. Many frames are sized to have width-to-height
ratios or aspect ratios of 4:3, though theatrical releases typically have aspect ratios of
1.85:1 or 2.39:1. The two most common frame sizes for the Web are 160 × 120 pixels
and 320 × 240 pixels; frame sizes as large as 640 × 480 pixels or more are becoming
more common as connection speeds increase. Online movie sites such as Netflix, Hulu,
and Amazon Instant Video offer high-definition movies at resolutions of 1280 × 720.
The speed at which one frame in a video is replaced by the next is called the frame
rate , commonly expressed in frames per second (fps). Higher frame rates usually, but not
always, result in a smoother animation. DVDs typically render video at 24 fps; frame
rates of 10 to 15 fps, commonly used on the Web, still can result in videos of good
However, the frame rate is not the only factor that determines the quality of a video
playback. A more important factor is the video bit rate , which is the amount of data
that has to be processed by the video player each second. Video bit rates are defined
in megabits per second (mbps) or kilobits per second (kbps). DVD-quality video has bit
rates of about 4 to 8 mbps, while Blu-ray video requires roughly 25 mbps. Most Web
video has a bit rate of about 800 kbps, which is slightly less than 1 mbps. Movie Web
sites typically require bit rates of 2 to 3 mbps.
It is a mistake to assume that a higher bit rate always results in better video quality.
You also must take into account the bandwidth of the connection to your Web site. The
bandwidth must be large enough to accommodate the amount of information processed
each second to smoothly play the video. If a user attempts to play a video with too high
of a bit rate, the playback will be choppy and uneven as the connection tries in vain to
keep up with the pace of the clip.
File Formats and Codecs
Video size can be greatly reduced through the use of file compression. When a com-
pressed video is replayed, each frame is decompressed as it is rendered by the video
player. The software that compresses and decompresses a media clip is called a codec
(short for co mpression/ dec ompression). Some codecs create smaller video files but at the
expense of choppier playback. Video editing software allows you to choose the codec
for a video clip, but you might need to experiment to determine which codec provides
the best file compression without sacrificing video quality. During the last 15 years,
advances in video technology and increases in computer processor speed have allowed
for codecs that greatly reduce video size, resulting in high-quality video at data rates that
are reasonable even under lower bandwidths.
Each codec is stored within a file called a container file . Figure 7-19 lists four popular
container files along with some of the codecs they support. Note that a single file format
might support several different possible codecs. You should not confuse the video file for-
mat with the codec of the clip it contains; for example, a Flash video file might contain a
video clip that has been compressed using either the H.264 or the VP6 codec.