3.4 Summary

The commonly used video compression and decompression methods are H.261, H.263 and MPEG. Each one has its own advantages and shortcomings.

The ITU recommendation H.261 support CIF format with the resolution of 352 * 288 pixels and QCIF format with the resolution of 176 * 144 pixels. It was targeted for the transmission of limited motion pictures such as teleconferencing. [33]

H.263 supports SQCIF at 128 * 96 pixels and QCIF at 176 * 144 pixels. As an option, it also supports CIF, 4CIF 8CIF and 16CIF that are 2, 4, 8 and 16 times the resolution of QCIF. It is an improved version of H.261 compression algorithm and it significantly increases the compression efficiency. With H.263 it is possible to achieve the same quality as H.261 with 30-50% of the bit usage. Most of this is due to the half pixel prediction and negotiable options in H.263. [18]

MPEG is derived from the H.261 video standard and from the ITU and JPEG image formats. [65] MPEG-1 was originally optimised to work at video resolutions of 352 * 240 pixels at 30 fps (NTSC) or 352 * 288 pixels at 25 fps (PAL). The bit-rate is optimised for applications of around 1.5 Mb/sec. It has a high compression ratio (up to 150:1) and therefore needs less bandwidth for the data and requires less space to store it. MPEG-2 is used at 720 * 480 resolution video at 30 fps (NTSC) and 720 * 576 at 25 fps (PAL), at bit-rates up to 15 Mbps. It also supports the HDTV resolution of 1920 * 1080 pixels at 30 fps at a bit-rate of up to 80 Mbps.

MPEG is a suitable codec for high-resolution, full-motional pictures, to be transmitted over wide bandwidth network such as LANs. H.261 is a suitable codec for video through the mid or lower bandwidth networks, such as ISDN, where the motion picture and its resolution is not crucial. Finally H.263 is suitable for codecs for communicating over lower bandwidth networks such as POTS with little demand for clear and full motion pictures.