Products that take the mystery out of MPEG
Moving Pictures Experts Group (MPEG) is the International Standards Organization’s (ISO’s) committee that developed the MPEG-1 and MPEG-2 video-compression methods. The high-quality resolution and significant compression ratios of the MPEG-2 format make it the chosen video compression method for DVD media and satellite broadcasts. Recording (i.e., encoding) and decoding MPEG-2 streams is the resource-intensive task of dedicated hardware. For this month's concluding review of multimedia technology, I tested Sigma Designs’ REALmagic Hollywood Plus MPEG-2 PCI decoder card and REALmagic NetStream 2 MPEG-2 PCI decoder card. I also reviewed FutureTel’s PrimeView NS Model 325 MPEG-2 encoder card. Before I dive into these reviews, I’ll give you a brief overview of the MPEG format.
The MPEG format. The MPEG committee created the MPEG standard to provide a format for compressed digital video on a standard compact disc, which has a maximum data transfer rate of only 1.416Mbps. The MPEG-1 standard offers a resolution of 352 × 240 pixels at 30 frames per second for the NTSC standard, and 352 × 288 pixels at 25 frames per second for the PAL standard. Several software applications, such as Microsoft’s Media Player, decode MPEG-1 content, and any MPEG-2 device can decode MPEG-1 content, because MPEG-1 is a subset of MPEG-2.
The MPEG committee developed MPEG-2 to satisfy the needs of the broadcast television industry’s cable and satellite networks, which aren’t limited to the data throughput of a compact disc. A single-speed DVD runs about seven or eight times faster than a compact disc; thus, the MPEG committee developed MPEG-2 and eliminated the speed barrier to MPEG content on the DVD medium. The MPEG-2 format offers a resolution of 704 × 480 pixels in 24-bit (i.e., 16.8 million) color at 30 frames per second for the NTSC standard, and 704 × 576 pixels at 25 frames per second for the PAL standard.
MPEG compression works by encoding only the information that changes between successive video frames, rather than encoding all the information in each frame. For example, in a video of a beach scene that shows sand in the foreground, blue sky in the background, and a boat moving across the horizon, the sand and sky don’t change—only the position of the boat with respect to the frame’s edges changes. In each successive frame, MPEG uses a cached background from a previous frame and records only the changed portions of the frame (the boat in this example) to compile a complete image.
An MPEG stream contains two layers. The compression layer includes compressed audio and video streams, and the system layer includes timing and synchronization information for audio and video streams.
Motherboard manufacturers are considering embedding MPEG-2 decoding application-specific integrated circuits (ASICs) in their future products. This development isn’t surprising if you consider the effort involved in decoding an MPEG-2 stream at 30 frames per second. However, MPEG technology is readily available today, as this month’s product reviews show.