MPEG: Pay For Play

Bruce Levy

If multimedia was the consumer hot button last year, this year's clarion call is MPEG. Simply put, MPEG is a method of compressing and decompressing graphics and sound data. Video clips occupy large amounts of disk space, and MPEG compression reduces the amount of storage space needed while speeding file transfers from storage to display. While there are efficiencies in the storage and transferal of MPEG files, compression imposes a performance penalty of its own: decompression.

MPEG decompression can occur in software alone (which is sometimes called native signal processing ), or it can use special hardwa re, such as digital signal processors (DSPs), to decompress files faster. Either solution adds to the cost of a computer. Software MPEG decompression is the less expensive method, because it doesn't cost much for a manufacturer to add an MPEG software driver to a motherboard or a display card. B ecause most systems sold today have fast Pentium processors, an efficient software MPEG driver would seem to offer the best economy, but you must decide for yourself if you want to pay for extra hardware acceleration.

While software can completely decode any given part of the MPEG data, MPEG is defined in such a way that decompression can be partial. That is, while an MPEG implementation is decoding a stream of data, the playback system (if it's falling behind) can omit some of the frames per second. Also, the playback system can partially decode some frames, generating output with a granular, lower-resolution appearance or stripe artifacts.

The most important and noticea ble marker of decoder performance is the video-frame rate on playback. The nominal rate of MPEG playback is dependent on the data stream being decoded, but 30 frames per second is a standard frame rate. This rate provides the appearance of full-motion video. A significant number of dropped frames results in a noticeable drop in visual quality. The system might present the correct number of frames but only partially decompress them, which results in degraded quality. Poor decompression performance can also lower the quality of the audio track that accompanies the video.

To ease your choice between hardware and software MPEG, we measured frame loss during playback. We observed that video playbacks can be maintained at 30 fps with reasonable audio reproduction on 100-MHz Pentium systems with 16 MB of RAM and the best hardware decoders.

On the other hand, even with today's high-powered systems, pure software MPEG solutions are unable to maintain full frame rates with demanding data streams. While th e output frame rate of software decoders varies, depending on several factors (e.g., whether the MPEG display is full-screen or windowed, or with or without audio), we measured output rates from 15 to 24 fps.

On the same software decoder systems, the audio bandwidth reproduction is less than 12 kHz, below that of the better hardware decoders. Any given hardware decoder might, however, have noticeable limits on the quality of reproduction , such as vertical banding or granularity of output on high-resolution video systems.

While current hardware MPEG decompression is imperfect, overall it's better than software-based video playback. Current software MPEG decoders are just on the edge, with performance that's perhaps adequate for short video clips in reference works, such as a video encyclopedia. A hardware decoder is required for VCR-quality MPEG on today's systems.

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Bigger Not Always Better

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With the software-based MPEG engine, image quality greatly degrades as it expands from a postage-stamp-size window to a larger window.