Previous Table of Contents Next


Video Standards

According to an anonymous, but very wise, man: “The nice thing about standards is that there are so many of them!” Video compression standards are no exception—they are numerous and incompatible. Early standards were developed for specific purposes, such as motion picture production (MPEG) and photographic editing (JPEG). Only recently has the ITU-T become involved in the development of international standards towards B-ISDN. In the videoconferencing world, numerous proprietary standards have been developed that require that the network involve equipment from only a single vendor.

As of late, a number of true standards (as opposed to proprietary standards) have developed. These video compression standards include Px64, JPEG, and MPEG and its variations. Virtually all manufacturers have embraced these standards, incorporating one or more of them into their systems alongside a proprietary compression technique. Generally involving Px64, this approach provides at least a minimum level of communication between systems of disparate origin—at least between fairly substantial systems. The world of desktop videoconferencing is much more parochial. Proprietary standards predominate, with the yield all too often being incompatibility [14-9]. As is always the case in networking, the lowest common denominator rules; lacking a common denominator, anarchy rules.

Px64

Px64is an ITU-T standard designed to support videoconferencing and various levels of bandwidth in increments of 64 Kbps up to a maximum of 2.048 Mbps (E1). (Note: p = 1-30 channels of 64 Kbps, with the maximum of 30 bearer channels being supported by E1.) Px64 specifies various frame rates and levels of resolution. Most manufacturers of videoconferencing equipment support Px64 as a lowest common denominator, although they each prefer their own proprietary standards in promotion of their own equipment and unique feature sets. Px64 also is known as H.261, referring to the specific ITU-T video coding standard, as well as the video formats. Those formats include CIF (Common Intermediate Format), which is optional, and QCIF (Quarter-CIF), which is mandatory in compliant codecs. H.261 supports 352 pixels per frame, 288 lines per frame and 30 frames per second, although lower frame rates also are supported.

Actually, H.261 is an element of the ITU-T H.320 umbrella standard for videoconferencing that addresses narrowband visual telecommunications systems and terminal equipment. Related ITU-T standards include H.221, which defines a frame structure for support of audiovisual teleservices in 64-Kbps channels; and H.222, which defines the frame structure for such services in an ISDN environment. While these umbrella standards truly are international in nature, the United States and European camps unfortunately (and not surprisingly) are divided over certain implementation aspects of the ITU-T standards. Specifically, the differences deal with the manner in which audio and still-frame graphics are handled. For example, the Europeans have adopted JPEG for graphics (ITU-T T.81), which is compatible with most PC-based graphics software. That approach is totally incompatible with the H.261 standard implemented in the United States, although it does provide a bridge to true multimedia applications [14-10].

JPEG

JPEG (Joint Photographic Experts Group) was developed jointly by the ISO and ITU-T. JPEG is a compression standard that is used for editing of still images, as well as color facsimile, desktop publishing, graphic arts, and medical imaging. A symmetrical compression technique, JPEG is equally expensive, processor-intensive, and time-consuming in terms of both compression and decompression. Motion-JPEG is used in the editing of digital video. JPEG is not appropriate for video transmission, as the compression rate is in the range of only 20:1-30:1. JPEG transmission in support of videoconferencing requires bandwidth in the range of 10 to 240 Mbps.


Previous Table of Contents Next