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[ The PC Guide | Systems and Components Reference Guide | Video Cards | Full-Motion Video ]

Video Performance Issues and the MPEG Format

The idea of playing a movie or TV program on the PC is a relatively new one, and these applications make demands on the performance of a PC system that few others do. The main problem is the sheer volume of data that is involved.

Let's suppose that we wanted to display a 2-hour movie in high-quality, high-resolution graphics. To keep things from getting out of hand, let's choose just 640x480 resolution in high color (16 bits per pixel) instead of going "all out" to say, 1024x768 in true color. Full-motion video typically means 30 frames per second. To store the data required for a full-motion, two-hour movie using these parameters, would require approximately 133 GB. (That's not a typo--133 billion bytes, and that doesn't include sound.) Also, the data to support this video stream would need to be pumped to the video card at the rate of about 150 million bits per second. In ten years these numbers won't seem like that big of a deal but of course, right now they sure are.

There is a solution to this dilemma: data compression. Due to the nature of the data, video can be easily compressed by a large factor. There are several reasons for this:

  • Every image that is displayed has large areas of redundancy that can be represented by a smaller amount of data using an encoding algorithm. For example, if the bottom half of the screen in a particular frame is black, that can be represented using a small number of bytes; we don't need to have 150,000 2-byte pixels all filled with zeroes.
  • The human eye is not sensitive to certain details in a video, which can be removed without appreciable loss to the perceived signal.
  • If you take any two consecutive frames, the changes from one to the next are usually rather small. For example, if you have a scene centered on someone's face, the background images are probably going to remain static for hundreds or thousands of consecutive frames. There are special algorithms that can describe a frame only by the changes that it represents from the one before, which dramatically cuts down both on the amount of storage required and the time to transmit each frame of data.

The most popular compressed video format is MPEG, which stands for the Motion Pictures Expert Group, the body that defined the standard. MPEG can typically compress as high as 100 to 1, bringing our 2-hour movie down to a much more manageable 1.33 GB. To use MPEG the video must be encoded using an MPEG encoder, and then viewed using an MPEG decoder. Not surprisingly, MPEG has its own problems. The chief one is that the process of encoding and decoding takes a lot of processing power, which is why dedicated MPEG encoding and decoding hardware is used for high-performance applications.

Next: Video Decoding Hardware and Software

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