Town of Arlington I-Net
Design

After a meeting in mid October with the ACMI Executive Director, Norm McLeod, Greg Hall of Integrisys, and myself, we decided to investigate a new signal path plan. For information on the existing signal path, please see the Research page.

This new approach would bring signals for all three channels, Public, Educational, and Government (PEG) to the studio at 85 Park Avenue. That way, what is going out to subscribers on all three channels could be monitored for signal quality at a single location. In addition, if there is any signal failure from any other location on the I-Net, a substitute signal could be sent to subscribers from the studio until the problem was corrected. In addition, program playback and even studio facilities at 85 Park Ave., could be used for any of the three purposed, Publlc, Educational, or Government without any restrictions posed by the arrangement of the I-Net.

The question we had toanswer was: how do we get signals from the studio at 85 Park Avenue out to the cable head ends and thus out to subscribers? As of November 2006, both RCN and Comcast took their signals from connections in the basement of the high school.

Our answer has been to use a proprietary multiplexing technology that allows up to four video and audio channels (so called 'base band') to be carried by a single fiber. This scheme is depicted in the system overview block diagram.

This means that, of the four fibers now connecting the studio with the RCN head end on Mass. Ave. (from which signals may be connected to other locations via fiber) only one need be used to feed RCN customers and one need be used to feed Comcast customers. One must be used to connect to the existing data processing fiber network, which we are sharing for video and audio, from the high school to the studio (goes to "Video over IP Box" in the "Dallin Studio" in the above diagram). That leaves one more.

This brings us to the existing I-Net technology, a hybrid coaxial/fiber network supported by Comcast, still in place. Originally, there were only to be five locations still served by this network. Then, it was discovered that RCN fiber had been installed at the Veterans Memorial skating rink, so that left four. Of those remaining, the Spy Pond field has no connection that we can determine (see research page for details), so the three that are left are Arlington Catholic High School, the Gibbs Junor High building (housing various tenants, including the Arlington Center for the Arts), and the Arlington Boys & Girls Club. Also, programs originating within the high school itself or on its grounds can be carried by this network, allowing the use of existing coaxial cable already installed in the building and modulators connected at each location.

Signals from the existing ("old") I-Net locations, including locations within the high school itself, such as the School Committee meeting room, enter the high school basement and are combined into a single coaxial cable. This cable may be carrying modulated video and audio, on any one of a number of TV channels. To get these signals to 85 Park Ave., everything being carried on this cable is sent over fiber to the studio, using an optical transciever at each end of the fiber. At the studio, demodulators (one for each channel being transmitted from the remote locations) convert each signal back to baseband video and audio. For example, if the the high school is carrying a football game from the field, on channel 6 and there is an open house at Arlington Center for the Arts in the Gibbs Junior High building in East Arlington at the same time, say modulated on channel 20, then those two channels are carried to the high school basement and come in on coaxial cable. The combined signal (both channels 6 and 20) are sent over fiber to the studio, then converted from fiber back to coax and then each demodulated with a demodulator, one set to channel 6 and one set to channel 20. Studio personnel would monitor the signal from each location and arrange the "ACMI switch" so that each program would go out to subscribers over the correct service, perhaps "Public" for the Arlington Center for the Arts program, and "Educational" for the football game, since it's sponsored by the school system (there being no "Sports" service, per se.).

At some point in the future, if there is significant demand for live programming from these locations, we may propose to install fiber to these locations and abandon the old I-Net completely. At the present, however, based on some limitations of funding, we must work on a way to ensure that they are still served and live cablecasts can be originated from them.

 

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