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Desktop Video AGOCG Report

CCITT Standards

In late 1990, the CCITT adopted a world-wide specification for video compression called H.261 or Px64 that made it possible for different systems to interoperate. Unfortunately, many suppliers feel that the specification is poor compared with their own proprietary algorithms.

H.261 defines a scheme for sending video at speeds of from 64 Kbps to 2 Mbps. At the low end of the spectrum H.261 fits into an ISDN channel; at the high end, it needs wider bandwidth dedicated lines. The standard defines a video window of 352 x 288 pixels, known as CIF (Common Intermedia Format). It also supports QCIF (Quarter CIF), a smaller window of 176 x 144 pixels. Related specifications cover still frame graphics, call set up protocols and other issues.

PictureTel, Compression Labs and other manufacturers of large scale conference room video systems offer H.261 as an option, but traditionally rely on their own proprietary codes. In the desktop market it seems that not everybody is happy with the H.261 standard since it requires very expensive hardware. This has led to new proprietary CODECs being developed including MediaVision's MotiVE (Motion Video Engine) and Captain Crunch.

A company called Knex in Fremont, California is developing a radical new compression scheme that can send 320 x 240 pixel colour images at 15 fps over telephone systems with a transmission delay of less than 200 milliseconds. However, this has yet to be realised.

Systems which use video rely on a CODEC to compress the image. When compressing, there is a trade-off between the size, quality and time taken to produce the image. The optimum CODEC used to compress the data will depend on exactly what is required to be displayed. A high level recording with much changing detail will need a more complex compression algorithm than an animation with minimal changes. In the case of Microsoft's Video for Windows, the default compression method is Video 1 which guarantees a specified data rate for 8, 16 and 24-bit colour sequences, as mentioned above.

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