DTV Conversion: Take a 24-Unit Condo Complex Digital
This is part of IEEE Spectrum's Special Report: The Day Analog TV Dies
It is clear that many people are going to experience considerable difficulty in obtaining satisfactory reception due to their distance from television transmitters, local reception problems, inadequate antennas, and less-than-optimum signal distribution systems. However, there are solutions available for some of these problems.
I recently upgraded a signal distribution system in a 6-building, 24-unit condominium development with the assistance of Steve Zahn, an engineer. This development is located on a bluff overlooking the waters of Lake Michigan in Wisconsin’s Door County, about 120 kilometers from television transmitters in Green Bay. It is surrounded by dense forests; to the west are the waters of Green Bay, and a series of tall bluffs and islands extend to the south in the same direction as the television signals.
The original antenna and distribution system were constructed about 20 years ago. At that time, there were only two buildings in the development. Antennas on top of a 9-meter tower provided signals through a splitter to both buildings. Over the years, as additional buildings were constructed, the distribution system was extended from the second building, ultimately forming a daisy chain from the antennas that was about 335 meters long. Four single-channel amplifiers boosted signal levels along the system.
About four years ago, the original antennas were replaced with large new multielement VHF and UHF antennas. It was never necessary to use a rotor with the antennas since at a distance of 120 km from Green Bay, all stations were essentially in the same direction.
For analog reception, the performance of this system was fair to good. Most of the analog stations in Green Bay used VHF frequencies and could usually be received fairly well. The few UHF stations produced fuzzy pictures. Reception of all channels was highly dependent on atmospheric conditions.
For our initial evaluation of the system for digital television, we used an analog television and a DTV converter box. To our surprise, a low-cost television with a built-in digital tuner did not work as well as the DTV converter box. At the output of the antenna preamp, we received strong digital signals and excellent pictures from all major networks. However, in units farther away from the antennas, only one station produced an adequate digital signal.
In order to improve reception, we reviewed the entire signal distribution system. We found that many of the couplers in the individual units had less than optimum tap values—that is, they caused significant losses between the in and out ports. We moved or replaced many coupler boxes, often using higher tap values to reduce attenuation along the trunk line, and placing boxes with the highest tap values immediately after the amplifiers, where signal levels are highest. In addition, the existing amplifiers with a flat gain response were replaced with four multiband distribution amplifiers (model ZTA-35 by Blonder-Tongue) that allowed us to adjust the gain independently for low-band VHF frequencies, high-band VHF frequencies, and UHF frequencies.
Since the attenuation of UHF signals along the distribution system is much greater than for VHF signals, we adjusted the amplifier gains to tilt the overall system gain toward higher frequencies. We optimized the final gain settings by using two television sets, one at the output of a given amplifier and one at the input to the next amplifier. In this manner, we worked our way along the system to produce the best possible results.
After making these changes, we obtained medium to strong digital signals and excellent pictures from all major networks throughout the condo development, even in units up to 335 meters from antennas that are 120 km from stations in Green Bay. We have yet to fully evaluate the impact of changes in atmospheric conditions on reception, but initial results are promising.
The cost of the four new distribution amplifiers and new coupler boxes was about $500. This does not include installation labor or our engineering time. This project demonstrates that serious problems in digital conversion exist but that they can be overcome in rather extreme rural locations with a properly designed and implemented system.
Unfortunately, many people are likely to find it difficult to obtain the needed assistance. Without adequate antennas and distribution systems, today’s fuzzy analog pictures will be tomorrow’s blank screens.
To Probe Further
For more articles and special features, go to IEEE Spectrum's Special Report: The Day Analog TV Dies.