Section 34.1. Basic Digital Video Concepts


34.1. 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 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. "Full-screen" video is 640 x 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 320 x 240 pixels. Some producers go as small as 160 x 120 pixels.


Frame rate

The frame rate is measured in number of frames per second (fps) . Standard TV-quality video uses a frame rate of approximately 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).


Quality

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, frame size, 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 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 because 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 about 16K of data per second.




Web Design in a Nutshell
Web Design in a Nutshell: A Desktop Quick Reference (In a Nutshell (OReilly))
ISBN: 0596009879
EAN: 2147483647
Year: 2006
Pages: 325

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