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MPEG

MPEG (Motion Picture Experts Group) standards are several and still in final development stages. MPEG standards provide very high compression levels and excellent presentation quality. MPEG is a joint technical committee of the International Standards Organization (ISO) and the IEC (International Electrotechnical Commission). MPEG offers the critical advantage of asymmetrical compression and decompression. Additionally, the standard is supported by IBM, Apple, AT&T, and a host of other manufacturers and carriers. While the compression of the video signal is time-consuming and expensive, the decompression process is rapid and involves relatively inexpensive equipment. MPEG compression is as high as 200:1 for low-motion video of VHS-quality; broadcast quality can be achieved at 6 Mbps. Audio is supported at rates from 32 Kbps to 384 Kbps for up to 2 stereo channels.

MPEG 1
standardized in 1992, provides VHS (videotape) quality at 1.544 Mbps and is compatible with single-speed CD-ROM technology. In fact, it was designed to put movies on compact disc. MPEG 1 integrates synchronous and isochronous audio with video, and allows the random access required by interactive multimedia applications. Intended for limited-bandwidth transmission, it provides acceptable quality and output compatible with standard televisions. Current applications include video kiosks, video-on-demand, and training and education. Compression of about 100:1 is supported by MPEG-1.
MPEG 2
(1994) is the proposed standard for digital video at 4 to 100 Mbps over transmission facilities capable of such support (fiber optics, hybrid fiber/coax and satellite). While MPEG-2 requires much more bandwidth than MPEG-1, it provides much better resolution and image quality at much greater speed. While there remain technical issues associated with the transmission of MPEG 2 video over ATM, those issues are being addressed. MPEG 2 already has found application in Direct Broadcast Satellite (DBS) services, also known as Direct Satellite Systems (DSS). Such services employ Ku-band satellites and VSAT dishes in competition with CATV, running MPEG at rates of about 3 and 7.5 Mbps [14-11]. In a convergence scenario (Chapter 15), MPEG-2 is the standard of choice, supporting compression rates of about 200:1. MPEG 3, designed for HDTV application, was folded into MPEG-2 in 1992. [14-6].
MPEG-4
is a low bit-rate version intended for application in videophones and other small-screen devices. It is still under development.

Video Equipment

Video equipment includes transmit (camera) and receive (display) equipment that operate in concert with and through various intermediate devices in order to format the signal properly and otherwise treat it for effective transmission over a network. The intermediate devices include codecs, inverse multiplexers, servers and control units. A simple videoconferencing arrangement is presented in Figure 14.1.


Figure 14.1  Videoconferencing network employing cameras, codecs, and monitors.

Codecs
accomplish the process of digitizing, or coding, the analog signal on the transmit side and decoding it on the receive end. The codecs also accomplish the process of data compression and decompression, according to the specifics of the compression algorithm used. Additionally, codecs may include encryption features for security purposes. Codecs can be very inexpensive for PC-based systems, although, as in all of life, you get only what you pay for. Realtime MPEG 1 and MPEG 2 encoders range between $15,000 and $75,000; MPEG1/2 decoders can cost as little as $300.
Inverse Multiplexers (Inverse MUXs)
are used in commercial videoconferencing systems where dedicated bandwidth is not available for relatively bandwidth-intensive communications. An inverse MUX splits the video signal into two or more component parts that are transmitted over separate circuits or, perhaps, separate channels of multiple DS1 circuits. The Inverse MUX on the receiving end reassembles and resynchronizes the complete video signal for proper presentation.
Servers
are extremely high-capacity storage devices, containing many GBs (GigaBytes) of memory. Servers store video and audio data for delivery to clients on demand. While some video servers have found their way into business applications in a LAN environment, they are expensive and, therefore, unusual. Server technology primarily is aimed at a convergence scenario, in which video-on-demand will be delivered over ATM-based, fiber optic networks.
Multi-point Control Units (MCUs)
are digital switching and bridging devices which support multipoint videoconferencing, with up to 28 parties (sites) supported. MCUs must be compatible with the compression standards employed with the codecs. H.231 describes ITU-T MCU standards and T.120 describes generic conference control functions. MCUs are costly, ranging from $40,000 to over $200,000, depending on configuration and capability. MCUs may be found in the carrier network in support of a carrier videoconferencing service, or on the end user premise in support of a videoconferencing network based on leased lines.
Videophones
originated with the AT&T Picturephone, which was demonstrated at the New York World’s Fair in 1964. Never intended for legitimate application, the Picturephone was extremely bandwidth-intensive, requiring about 90 MHz, and weighed about 26 pounds [14-6]. During the past few years, AT&T, BT and others have developed videophones that sell for less than $1,000. As the cost is high—each party must have a videophone of the same manufacture—and the picture quality is poor (2 fps), videophones have been a market failure at the time of this writing.
But hold the phone, literally! Matsushita, the parent company of Panasonic, has developed and demonstrated the first version of a wireless videophone for use in Japan’s cordless phone system. The phone uses ITU-T standard video compression techniques to squeeze video down to a 32-Kbps channel for display on a 2.5-inch color LCD display. A 1/3-inch camera also is built into the device for video transmission at 3-7 frames per second [14-12].


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