File Size Determines Format; Format Determines File Size
Typically, the better the quality of the image you want, the larger the file size will be. A few key factors that affect the file size are the clarity of the image, the physical size of the display area, the number of frames per second, and the duration of the video clip. The Internet is growing in popularity for use with video, but many people still do not have access to a large enough amount of bandwidth to receive high-quality video. Therefore, when distributing video over the Internet, most people choose to process their video content with a smaller physical display area (number of pixels wide by number of pixels highsomething like 160x120 is common), reduce the frame rate (possibly as low as just a few frames per second, where broadcast video runs at 29.97 frames per second), and keep the duration of the clips down to a few seconds to a few minutes. CD-ROMs allow you to distribute your content in a manner that's a step above the Internet. The speed at which the data is read from a CD-ROM is far greater than that coming over the Internet. Therefore, we can now increase the number of frames per second (typically 15 frames per second works well), the physical display areas can be much larger (320x240 pixels or greater), and the clips can be much longer in length.
In general, the image quality of the codecs used for video content that will be created for CD-ROM distribution will be much clearer (needing less compression) because you are now talking about having 700MB of space to work with. Try streaming a video file over the Internet that is several hundred megabytes large. As technologies improved, MPEG codecs with unique compression schemes became available that allowed even longer lengths of video content with higher quality to be created. Another benefit is that you can usually bundle the appropriate video drivers or media players directly on the CD-ROM. This way, your end user can easily view the content without having to struggle to find the appropriate players. Some players need to be installed on the end user's system, whereas others allow the file to be displayed correctly while running directly from the CD-ROM.
So what happened when people wanted to see video files run at full screen, running at full frame rates, longer durations of the video, with the best possible image quality? That's where MPEG-2 and DVD technologies came into play. MPEG-2 compression produced the highest quality video reproduction, but of course required a larger format than CD-ROM technology for distribution. DVD platforms increased the available file space from 700MB on a CD-ROM to 4.7GB on a single-sided DVD. If you needed even more space than that, that's where you get into double-sided, double-layer DVDs, giving you more than 15GB of space on a disc that looks like and is the same physical size of a CD-ROM. I just can't wait until I can get DVD-quality imagery on my cell phone. OK, so I'm pushing it a bit. See my article on the inner workings of audio and video compression to get a more in-depth look at the variables that affect the final quality of video media.