DTV stations looking at alternate sites and testing new technology

While much higher in elevation, Squaw is farther from the plains that Denver sits on than Lookout Mountain.

The majority of Denver, Colorado's NTSC transmitters are on Lookout Mountain just west of the city. The stations would like their DTV facilities there also. The local mountain community has been vocal enough about their reservation to new RF facilities in the area to make Jefferson county officials hesitant to issue the proper permits for the DTV CPs to be acted upon.

The FCC has also responded by revoking most of the CPs. Even though a number of stations have banded together to promote a consolidation of facilities onto a single tower, the opposition has not abated. The thought of locating to another mountaintop has increased. One possible site is Squaw Mountain, where one of Denver’s two PBS stations, KBDI (NTSC 12/DTV 38), is located. While Squaw Mountain is almost 4000 feet higher, it is also 13 miles further away from Denver than the front-range Lookout site. This presents greater likelihood of coverage shadows, especially for communities nuzzled against the mountains.

While not prevalent east of the Mississippi, many western states, with mountain topologies, use hundreds of translators to fill in the many coverage shadows created by ridgelines.

A little north of Denver, Fort Collins, has similar topology to Denver, in that the front range of the Rockies rises sharply from the prairies.

Colorado is one of 10 states that have more than 300 translators in use. A significant portion of the state receives Denver television via translators. The FCC, and the industry in general, has not yet come to grips as to what will replace these translators when NTSC goes dark.

A number of proposals have been suggested to solve DTV coverage holes, and in October one was put to the test in part because of the uncertainty surrounding Denver DTV’s build out. Current NTSC coverage of the Denver area from Lookout has been spotty in areas, namely the northwest suburb of Boulder, which lies in a coverage shadow created by Lookout Mountain. A test of an on-channel booster to fill the Boulder shadow was conducted by off-air reception of the KBDI-DT signal radiating from Squaw and then retransmitting it using a Larcan DTT 500 watt transmitter from a site overlooking Boulder.

Even though the Denver test was a simple on-channel booster, the results from the 184 sites tested were considered “encouraging,” even with the timing differences between the main and booster transmitters which appear as multipath to a receiver.

A more elaborate approach, known as a Single Frequency Network (SFN)has been proposed. Each active transmitter in a SFN system would present another multi-path looking source to a receiver increasing tap energy at the receiver.

Squaw Mountain RF site

The SFN system requires the main and all booster transmitters to be fed the same ATSC MPEG data, and that each perform it’s own error correction encoding, pilot insertion and modulation. This eliminates many of the propagation offsets between the main rig and booster units.

New receiver technology offered by Linx Electronics might help make the SFN technology commonplace. Advances in processing technology allows Linx to offer “Casper” technology (Casper because the technology is considered “ghost-friendly"), which can take tap energy with a wide time difference from the DTV pilot and shift it into time with the pilot. Thus, more of the energy arriving from multiple paths and sources, even if displaced in time, are used to recover the signal.

Other tests are underway to help determine the widespread feasibility of this technology.

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