52 I want to add an MPEG card to my system. How does it work?
Description
This article is from the comp.sys.ibm.pc.hardware.video Frequently Asked Questions, by Michael Scott with numerous contributions by others. (v1.0).
52 I want to add an MPEG card to my system. How does it work?
The Motion Pictures Experts Group (MPEG) has released a series of standards which describe a lossy digital video compression technique. In some cases, MPEG can reach compression rates of 100:1. It works by removing redundant information and details that most people would generally miss, and in later versions storing only the differences between successive frames.
When an MPEG video clip is viewed on the screen, the video stream must be decoded on-the-fly. If done in software, this operation can be quite demanding of the system CPU. An alternative is to have a dedicated coprocessor do the MPEG decoding, then feed the resulting video stream to the video card. Because this type of coprocessor is dedicated to MPEG decoding, it can be optimized to perform the operation very fast, and can also be used to scale up the size of the resulting video with little or no degradation in performance. Even a relatively small 320x200 video displayed at 30 frames per second requires a bandwidth of 15.4 million bits. This would seriously decrease available bandwidth for other purposes like disk i/o if all of that data was dumped down the peripheral bus (ISA, VLB, PCI, etc). As a result, many video card manufacturers incorporated a feature connector on their VGA cards. This connector gives direct access to video display memory, allowing high frame rate video to be dumped to the monitor. One limitation of this adapter is that it can only provide 8-bit (256 unique) colour.
If you're planning on using your PC as a VCR, you'll be disappointed with an MPEG card playing the cdrom version of your favourite film. The resolution will be inferior to that provided by your television. If you want to get smoother video playback and/or free-up your CPU for other tasks, then the addition of an MPEG decoder card may be worth the cost.
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