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Unique switching scheme breaks T1s for video network.

Vermont Interactive Television (VIT) is a partnership of state government, education and private business that has been helping to meet the needs of underserved populations in Vermont since 1988.

Currently, nine sites are linked with two-way audio and compressed digital video, using T1 (1.544 Mb/s) phone lines in a star configuration. Any site can also be linked to the Sprint Meeting Channel system of national and international teleconferencing rooms.

While VIT has a strong emphasis on two-way audio and video communications, a dual-band satellite receiver at the hub is often used to bring in teleconferences from outside Vermont.

VIT has also been linked up to a two-way cable TV system on a pilot-project basis to originate nursing courses from the University of Vermont. Similar future connections are anticipated to deliver programming to and from K-12 institutions.

VIT has been funded primarily by state appropriations, local contributions and user fees. The annual operating budget is $600,000. Under an extension of the Vermont Telecommunications Agreement with New England Telephone, VIT pays 30% less than the FCC tariff for T1 lines.

Although VIT is administered through the Vermont State Colleges System, it is available to all educational institutions, state agencies, non-profit organizations and private businesses. In 1992, over 4,000 people participated in over 2,000 hours of programming.

VIT sites are located at various institutions, including secondary and post-secondary educational facilities, a hospital, a public utility and a state training center. These host institutions provide space, utilities and other services for one dollar per year and a 50% discount on system time.

The typical VIT site consists of a studio/classroom varying in size from 20 x 30-feet to 30 x 30-feet, a control room, a waiting/reception area and an office for the site coordinator.

All but one of the sites have either three or four single-chip cameras which are remotely-controlled by Telemetrics pan/tilt controllers. Cameras are custom-mounted on the stands holding the remote-site monitors to maximize apparent eye contact. 27-inch Sony monitors are used because their flat vertical plane minimizes room light reflections.

An overhead camera is usually used instead of a chalkboard, since this image can be combined with a shot of the presenter. Use of the overhead camera also limits the total amount of motion, which might otherwise be distracting in compressed digital video.

Audio is from directional, voice-activated Shure AMS-series microphones and mixers. This is acceptable for most events, but VIT is exploring other options, including overhead microphones and speakers, and echo cancellation.

VIT began multipoint operations before CLI started marketing its multipoint control unit. This problem was solved through the use of Coastcom T1 multiplexers and a Coastcom DXC-II digital access cross-connect switcher (DACS) controlled by custom software written at Vermont Technical College.

This unique switching scheme allocates one of the 24 DS0s (64 kb/s data channels) in the Tl signal to sequentially poll computers at each of the sites for switching requests. These requests are received through a serial port of a computer at the system hub. There, the information is processed and then the appropriate command is sent through a second serial port to the DACS.

Additional DS0s are also allocated for audio contact with the sites that are not being seen. The switching software ensures that audio from each site is coming through the codec if that site is being seen, and through the multiplexer if it is not.

All sites in the same session are in constant audio contact, and can switch to any other site in the same session whenever they want to. The network can be partitioned into any combination of simultaneous events.

One of the most important features of the switching system is a three-line window at the bottom of the computer screens for real-time E-mail (no message storage). The shift key and the appropriate function key are used to address any number of one-line messages to any or all sites. PgUp and PgDn keys enable toggling to or from full-screen E-mail.

Site coordinators and part-time technicians find E-mail contact to be invaluable for communicating about events without disrupting the events, and for maintaining a sense of coherence within a statewide organization.

VIT originally integrated its own multipoint system out of necessity, and has continued using it because of its functionality (primarily the built-in E-mail) and its cost-effectiveness. In a conventional nine-site system, two MCUs costing a total of about $200,000 would have to be linked together. In contrast, VIT spent about $70,000 for its multipoint capability.

There are some potential problems. If many sites were added, switching speed would be slower, since more sites would have to be polled. Also, because bandwidth is taken from the codec video in order to add voice channels for new sites, it is possible to expand the network to the point where video quality would no longer be acceptable.
COPYRIGHT 1993 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1993 Gale, Cengage Learning. All rights reserved.

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Title Annotation:Vermont Interactive Television
Author:Thompson, Darrell
Publication:Communications News
Date:Oct 1, 1993
Words:814
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