Basic digital video concepts Web Designing

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 web delivery.

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. "Fullscreen" 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
The frame rate is measured in number of frames per second (fps). Standard TVquality 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 videoplayback. For "talking head" and other low-motion subjects, even lowerframe rates may be satisfactory. Commercial Internet broadcasts areroutinely 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 bandwidthrequirements.

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 47.5K/sec.

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 second.

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