2GHz re-banding creates urgent need for patience, COFDM training

According to consulting engineer George Maier, using COFDM can serve up blank screens that indicate a signal characteristic that’s counter-intuitive
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While the industry waits for the official implementation of 2GHz re-banding to begin, some are already expressing concern that the transition from analog point-to-point microwave to COFDM line-of-sight transmission will be fraught with uncertainty and some pain as ENG crews learn to live in a digital transmission environment.

On one level, there’s the natural limitation of vendors to re-equip a national fleet of ENG vehicles. News directors will have to understand that re-equipping existing analog-transmission-based ENG trucks with new COFDM equipment will take time, said Tom Jennings of Wolf Coach.

For trucks that are four-years-old or newer, the actual changeover could require only a few hours. However, older trucks that need new wiring, NYCOIL and racks could be out of commission for up to three days, he said.

On another level, to transmit ENG footage digitally from the field will require training at the station. Unlike analog transmission, where an operator can see a snowy picture and tune the system, COFDM is subject to the cliff effect where there’s either a signal, or there’s not. And that’s only one of the problems.

According to consulting engineer George Maier of Orion Broadcast Solutions, using COFDM can serve up blank screens that indicate a signal characteristic that’s counter-intuitive: not a lack of signal, but too much. The problem centers on digital AGC that can allow a digital signal to get so strong that it creates a condition in which the demodulator is overloaded and won’t decode the signal, he said.

In most of the current antennas, LNAs are full on, full off or have only one step of gain reduction, Maier said. However, help is on the way with the newest breed of LNAs that have continuous AGC loops from the receiver so they can back off as required.

Signal overload will be a common problem because of the way many broadcasters are likely to lock in their signal path, Maier said. Many will come up transmitting analog, panning their antenna until they’ve established a good signal path and then switch their transmission mode to digital, causing the signal overload.

Interference is likely to be another common COFDM transmission problem for ENG applications, Maier said. An ENG crew that’s first at a news event may establish a clean transmission path back to their receiver. However, as other ENG crews show up and begin establishing their digital signal path pointed at “the same big tower,” what was once a strong digital signal will simply go away due to interference.

Maier pointed out that these are not insurmountable obstacles and that manufacturers are already addressing these issues with the latest generation of equipment. However, such a large-scale implementation of a new transmission system is bound to create difficulties – many of which can be overcome with the right user training, he said.

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