Is Low Power DTV A Good Idea?

[Gregg Lengling]

By Ed Williams

The FCC seemed to think so in its recent MO&O (MM Docket 00-39, adopted November 8, 2001), in which TV stations will be allowed to construct DTV facilities at power levels well under those allocated, yet retain the allocated interference protection for at least a year or longer. The FCC believes allowing lower power will increase the DTV buildout rate while allowing it to maintain its deadlines for DTV construction and analog termination. While the concept is good in principle, unwary stations should not be lulled into a sense of euphoria. At some point, the full DTV facility will have to be built or surrender a significant portion of its protected DTV service area.

A station could purchase a low-cost 150 W (average) DTV transmitter with an 8-VSB modulator and standard definition (SD) encoder (with simple PSIP) and an FCC-compliant bandpass filter for installation at the current transmitter site. It could feed the DTV encoder from a DA output from the current analog STL. The DTV facility will be simulcasting at 100 percent, easily meeting FCC requirements to the letter. With a small, 24-gain, transmit antenna side-mounted on the tower and 500 feet of 3-inch flex line, 150 W results in about 2 kW ERP which, at 500 feet HAAT, will produce a city grade (48 dBuV/m) signal extending about 25 miles based on the f(50,90) propagation curves.

Being on the air sooner could mean earlier cable TV system carriage, being received at a local TV dealership for display, and providing a signal (albeit weak) for reception in the home. Low power DTV also means lower AC power bills. Finally, with a low power DTV facility duplicating the NTSC programming, there is no need for an expensive HDTV, or any, infrastructure for DTV. Sounds like a win-win situation and it may be for some stations.

The Disadvantages Of Low Power DTV

If there is a DTV station in the same market with 200 kW or 1 MW, then 2 kW looks pretty weak in comparison. How weak? The difference between 2 kW and 200 kW is 20 dB (100 times) and it is another 7 dB to 1 MW, or a total of 27 dB difference. A viewer able to receive that 1 MW DTV signal with an indoor antenna may discover the 2 kW signal to be practically non-existent in comparison. A better solution would be to run at 25 percent or 50 percent of allocated power rather than 1 percent. Imagine running an NTSC facility at 1 percent of normal power levels.

While low power DTV may enable a station to achieve the required FCC 48 dBuV/m (for UHF) DTV "City Grade" contour covering the community of license, this signal level is nearly the equivalent to Grade B NTSC and requires an outdoor antenna for reception. An indoor antenna requires at least a 20-dB (100 times) stronger signal at a given location than an outdoor antenna.

However, it depends upon where the audience is located with respect to the transmission facility. Two kW at 500 feet delivers a "city grade" signal about 25 miles from the tower. Viewers well within that distance (depending upon terrain, of course) using outdoor antennas and cable TV headends will pick up such signals reliably.

A transmit antenna with higher gain or mounted higher on the tower may increase signal level beyond 10 miles, but the higher cost and wind load on the tower may not be acceptable for a temporary installation. This gain will be offset by substantially lowering the signal level within the first 10 miles that in turn will make indoor antenna reception even more difficult. With the DTV video encoder located at the transmitter site, there may be little or no incentive to improve the quality of the digital signal since it is simply fed with an analog source and the power consumption of a low power transmitter is low enough to be tolerable for long periods of time. Transmitting SDTV video that started as analog will look no better than a DVD and may be worse if the station's plant is not fully digital.

Low power DTV puts the station in a "demo mode" situation rather than appearing to take DTV operation seriously. This will disappoint viewers who may turn to other stations or delivery services for digital or HDTV programming. Bottom line: Install low power to meet the FCC deadline if there is no other alternative, but be aware of the consequences and don't count on it to satisfy viewer's expectations of digital television broadcasting. Stay tuned.