Currently there are two main players in the field of desktop video production and playback for PC compatible computers, Apple's QuickTime and Microsoft's Video for Windows. These two video systems are similar in that they are both software-based. No extra hardware is required for full functionality. Both bypass standard video driver software in order to increase throughput speed and enhance playback performance. Despite these similarities there are also many differences between these two engines. Let's roll up our sleeves and take a look under the hood, and see where features and functionality diverge.
Frame-based vs. time-based
First of all, Video for Windows (VFW) is frame-based. Every VFW movie plays back at an unchanging user-specified frame rate. This is similar to film and videotape playback. Movie film, for example, plays back at a constant rate of 30 frames per second (fps). Once it starts, the projector never slows down or speeds up -- at least the theater management hopes it doesn't. A constant playback rate offers many advantages, including ease of editing and synchronization of multiple tracks. However, when applied to digital video files it also has some drawbacks.
Consider, for example, a movie presentation that consists of two segments or clips. Clip #1 is a shot of a history professor lecturing to a group of students from a podium. Clip #2 is a static shot of a time-line graph showing the important events leading to the rise and subsequent fall of the Roman Empire over 700 years. Now, let's say that we want clip #1 (the live shot of the moving professor) to be displayed for 20 seconds. For the movements of the professor to look natural and fluid, we will need to select a playback rate of at least 15 fps. This means that the professor's image will use 150 frames of digital video (10 sec. x 15 fps). The unchanging image of the time-line graph would use 300 frames of digital video (20 sec. x 15 fps). Processing digital video takes a lot of computing power, and storing it takes a lot of memory and disk space. Wouldn't it be great if every time we had a static image such as the time-line graph, we could tell the video software to wait instead of reading and writing the same image over and over? It would save a lot of disc space and processing power, in this case over 60 per cent!
In effect, this is what the QuickTime system does. The playback rate of QuickTime video is variable or time-based. This means that only images that change need to be stored and that a single image can be displayed for a long period of time. Smaller file sizes for longer videos is one advantage of time-based video systems. But not all video clips will benefit from time-based playback, and using varying playback rates in a time-based system can make editing and synchronization a tricky business.
Platform adaptability
Another benefit of the QuickTime system is that QuickTime movies can be played on Macintosh and PC compatible machines without conversion. AVI (Audio Video Interleave, the file format for Video for Windows) movies must be converted to be viewed on a Macintosh. However, this is a relatively simple procedure if you have the right software.
The future
QuickTime has been around longer than VFW and this maturity has contributed to its reputation for robustness and support for such advanced features as multiple audio and video tracks. VFW, on the other hand, benefits from Microsoft's marketing clout and will undoubtedly become the engine of choice for many PC compatible software developers.
