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6.2.4.2. |
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6.2.4.2. |
AVI is a technology developed by Microsoft that enables video images to be displayed on 386 or 486 PCs without additional hardware. This "software-only" technology was released in 1992 as an extension of Windows 3.1 called Microsoft Video-for-Windows (VfW). A synchronised sound track may be included in a Video-for-Windows file, and this is delivered through whatever sound board is installed in the PC.
The Video-for-Windows developer's kit contains a suite of AVI devices, compression codecs and editing utilities that enable users to capture video from standard analogue sources using any one of a variety of low-cost capture boards. The developer's kit also contains several hundred sample video clips. Among the kit's utilities is one that will convert QuicktTime video into Video-for-Windows (the reverse translation is not supported), and there is also a freely distributable runtime that will add the extensions required to deliver VfW files to any Windows installation.
The principal codecs in VfW are MS Video 1, Indeo (see below) and Cinepak, but others will be added as they are developed. The quality of AVI clips varies according to the codec used and compression ratio selected (see Table 6.4). At its best, VfW is at least comparable if not superior to QuickTime for the Macintosh, but it suffers from the same problems of data transfer rate (ie the trade-off between frame rate, image size and computer power). On a 386SX it is possible to deliver acceptable quality movie at about one eighth screen, whereas a fast 486 can deliver quarter screen at 30 frames per second.
Pentium machines will probably be able to deliver Video-for-Windows at half or three-quarter screen, but in the meantime acceleration based on Intel's DVI technology (1.2.2.C) promises a simple solution. An alternative is an AVI hardware accelerator such as PC PrimeTime by Cogent Electronics Inc, which can accelerate any AVI file to full screen and costs $499. AVI acceleration is also provided by Matrox's MGA Impression cards.
We have found that using low cost capture boards the quality of video is highly dependent on the quality of the source. For example a Creative Labs' VideoBlaster SE can produce acceptable results from videodisc, but very disappointing results from VHS. However with more expensive boards (the FAST Electronics Screen Machine II is excellent), high quality results can be obtained even from VHS.
It is probable that Video-for-Windows will become a de facto standard for the delivery of video on the Windows platform, as is QuickTime on the Macintosh. There is some advantage in capturing as QuickTime initially because there is much more sophisticated video editing software available on the Macintosh (eg Adobe Premiere), although with the recent release of Adobe Premiere for Windows the gap is probably narrowing.
Indeo is Intel's Real Time Video (RTV) recording and playback technology developed for incorporation within Video-for-Windows as an alternative compression system to Microsoft's Video 1. A number of other less well publicised codecs are also included in Video-for-Windows (eg Cinepack), and there are plans to include Indeo within both QuickTime and IBM's OS/2 multimedia extensions.
Indeo is in many respects superior to Video 1 because it is "scalable" - ie capable of optimising its performance to the available hardware: if this is high-end technology, then it will deliver good quality movie at 30 frames per second. But if it is low-end, the delivery will be correspondingly adjusted to the best the machine can achieve. In practice, since most machines are "low-end" in this respect, quality (though "optimised") tends to be poor using Indeo, even when compared with Video 1, which is not scalable. Video 1, however, will not be significantly better on high-end machines. For acceptable software-only performance even in small windows, Indeo requires a powerful PC, minimally a 33 MHz 486, preferably a 50 MHz machine.
Indeo's playback performance has recently been improved by the release of Indeo 3.1, which can now deliver up to 15 frames per second at full screen on fast 486 machines. With the help of an i750 device, such as an ActionMedia II DVI display board, this performance can be "accelerated" to full screen, true-colour movie at up to 30 frames per second in even low-end machines, though Indeo quality still falls far short of DVI-PLV.