In media streaming, how can we better understand the terms Digitizing and Compressing?

If you use an analog camcorder, you'll need a capture card to digitize the video. A video capture card is an expansion card that works in conjunction with, or replaces, the existing graphics adapter inside your computer. If you used a DV (Digital Video) camcorder with a FireWire jack, you'll be able to transfer the video directly onto your hard drive with a FireWire capture card.

With analog video, the first step is compressing the video into an AVI, MOV, or MPEG format to store on your hard drive. Compress the video at the lowest available ratio that allows the clip to fit on your hard drive. Don't worry about the file size at this point, because you will have to compress the video again into a streamable format.

Once your video is captured, edited and rendered into a file onto your hard drive, you need to select one of the streaming software packages to further compress the file into a streamable format. The software you choose needs to be supported by the streaming server you select. Find out which software packages your prospective server supports before purchasing software or hosting services (See sidebars of streaming software and streaming servers).

When you compress the MPEG, MOV, or AVI into the streamable format you have to choose exactly how you want it compressed. The first decision to make is how much of the video quality you want to sacrifice to make the video play better over the Internet to phone-modem-connected users. The three factors that make up the quality of a video are frame rate, color depth and resolution.

Frame rate is the number of still images that make up one second of a moving video image. At 30 frames per second (fps), images seem to move fluidly and naturally. Video digitized at a frame rate of less than 15fps becomes noticeably jumpy. Since current phone and modem technology limits the frame rate to 10fps, you might as well set 10fps as the target in your compression scheme. If a video clip contains a lot of action, then limit it to 7fps.

An alternative to using 7fps streamed video is the Web slideshow, which limits the frame rate to one frame every five seconds. This allows for one higher resolution image to be displayed with an audio track, instead of 35 low resolution frames. (See the May 1998 issue for more about Web slideshows.)

The second quality variable is color depth, the number of bits of data the computer assigns to each pixel of the frame. When there are more bits of data assigned to color each pixel, there are more colors that can be emulated on the screen. Most video is either 8-bit 256 color, 16-bit 64,000 color, or 24-bit 16.8 million color. 256-color video is very grainy and unsuitable for viewing. 24-bit color is optimal, but because it greatly increases the size of the streaming file, you'll want to settle for 16-bit color.

The third factor in determining the visual quality of the picture is resolution, measured in number of pixels. The more pixels there are in your picture, the higher the resolution of the video. For example, if your video is 640x480, you have 640 pixels across each of the 480 vertical lines of pixels. Streamed video ranges in resolution from postage-stamp size (49x49 pixels) to 160x120 pixels, the highest resolution you can stream to users connected through a telephone line, all the way up to 640x480 and beyond, which is considered full-screen video.