Digital TV switch raises questions for some schools

Although the transition to digital television (DTV) broadcasting across the United States is still a few months away, the impending switch has prompted many school technologists to consider "what-if" scenarios. One of their chief concerns is whether the coaxial radio-frequency (RF) cable networks that currently carry analog TV content in schools will be able to handle digital content adequately.

In the accompanying article, Tim Beekman of SAFARI Montage writes that some school systems could experience a degradation of their DTV signal as the signal travels over their coaxial cable network.

"While some schools will have 1GHz RF networks that can support digital TV, the traditional model of RF cabling in most schools uses a trunk-based configuration with a series of taps where cables drop off for each display device; thus, one classroom accesses the content stream before it reaches the second classroom, which accesses the content stream before it reaches the third classroom, and so on," Beekman writes. "Not only is there loss of power at every tap, making it difficult for the signal to be accessible at more than a few classrooms down the line, but the system was likely not designed to carry digital signals in the first place. The coaxial cable and network used in most schools have a bandwidth of 750 MHz or less, while 1 GHz or better is required for DTV."

Others offer additional perspectives on the situation facing schools.

"Digital television broadcast channels operate in the same channel band…as their analog counterparts; therefore, any well-designed system carrying analog broadcast RF services will carry digital broadcast RF services," said Bill Hayes, the director of engineering and technology for Iowa Public Television. "In addition, the digital television signal fits into the same channel space (6 megahertz per channel) as an analog channel. In most cases, the RF infrastructure will support the digital service."

Hayes continued: "The challenge is that the actual information carried on the individual channels has changed. In order to receive service, the end user …must either have a digital television or a digital converter box to translate the digital content to analog so that an existing analog receiver or television can display it."

Jim Hirsch, associate superintendent for technology at the Plano Independent School District in Texas, agreed with Hayes that concerns about the degradation of DTV signals over an RF network are largely unfounded.

"There are taps that are designed to carry the signal farther–this is part of any initial design," Hirsch said.  At each tap, schools will incur some signal loss–but it will be the same loss for analog as it is for digital, he said, so a well-designed system should be sufficient.
"As long as school districts are getting the signal from the cable company, [they] should be fine," Hirsch said. 

The science of splitting the signals into their proper channels for high definition or standard digital or analog service comes from a school’s cable TV provider, he said.

"The real question is, what is your cable company going to do with the current analog frequencies below 750 MHz?" Hirsch said. "Our thoughts are that they are going to rebroadcast the signals on those frequencies like they are doing right now, so everyone will be fine–although we don’t have anything in writing."

However, if a cable company starts broadcasting channels that are above that 750 MHz range, Hirsch said the nodes and taps can come into play, because they have different "maximum" levels.

"Our taps can go to 1 GHz, but our nodes can only get to 860 MHz," he said. "That is a different discussion, and we don’t think we will be talking about that for another two to three years."

The signal strength in the trunk is determined by the output of the amps and how much loss is introduced into the line by the various pieces of equipment that are attached to it, added Bob Moore, executive director of IT services for the Blue Valley Union School District in Overland Park, Kan. 

"Taps are selected for their specific loss values, as the signal is naturally attenuated by cable loss as it reaches the end of the trunk. A trunk/tap system that works fine for analog should work fine for digital as well, if the output power and losses are taken into account, just as they should be with an analog system," Moore said.

He added: "At higher frequencies, [signal] losses are greater–but this is true for any cable, whether it is carrying analog or digital signals."

Assuming that DTV channels are carried within the 750 MHz spectrum, Moore said, a 750 MHz analog amp will not be outdated when digital signals come into play.

"The only possible problem that one might foresee with transmitting a digital signal is that the digital pulses themselves can degrade over long distances, to the point where it is difficult for the decoding equipment to tell between 1s and 0s," he said. "In problem cases, an additional amp or re-clocking device might be needed somewhere along the line."