Published:  TV Technology magazine

Issue: June 5, 1997



Harsh Realities of the DTV Allotment Table 

Written by - Joe Fedele





        The new FCC allotment table for digital television (DTV) represents a bold plan that will 

challenge the resources and capabilities of many broadcasters.  Although the Commission 

appears to have addressed most of the issues in a seemingly well thought out manner, some in 

the industry have already found flaws and weaknesses in it.  

        Most complaints so far have centered around the "wide void" between the FCC's theoretical 

plan and the realities of its implementation. The basic strategy for the allotment table, as stated 

by the Commission, is that it was designed to closely replicate the existing NTSC service area.

        Dr. Oded Bendov, vice president of Antenna Engineering at Raymond, Maine-based 

antenna manufacturer Dielectric, has long asserted that the quest for equivalent Grade B 

coverage with DTV would result in ERP's (effective radiated power) far in excess of the FCC 

plan.

        Low-channel VHF stations assigned to the UHF band, in particular, will find replication of 

the existing service area  an impossible task because "the grade B contours extend well beyond 

the radio horizon," he said. "[This makes it] impractical to replicate at UHF frequencies without 

resorting to extreme power."  

        Bendov has suggested--both in public and during a session presentation he gave at NAB--

that "replication by brute power may require as much as five megawatts to cover the existing 

area.  Given the fact that the FCC has limited ERP's to only one megawatt, replication would 

appear impossible for some broadcasters."

        Then there are the financial realities of replication.  "The last ten percent of the viewers are 

typically in areas characterized by poor NTSC service and significant cable penetration," Bendov 

explained. "It won't be 

economical for broadcasters to attempt true replication. "Some factors not considered by the 

FCC include interference with mobile radio.  Bendov says that "the protection ratios published by 

the FCC should be applied at the receiver, not at the transmitter" and that interference with 

mobile radio "has not been fully researched."

        VHF stations that once complied with RF radiation restrictions may also find that they have 

an additional burden when DTV transmissions begin.  If VHF broadcasters are forced to transmit 

"extreme power" levels to meet their goal of replication "the actual RF hazard levels may yet 

prove to be in conflict with the FCC's own guidelines," Bendov added.

        Art Allison, senior engineer for the NAB Science and Technology department, points out 

that "there are more complexities of the real world that must be considered in DTV 

implementation. such as the number of receivers in a household."  The FCC model assumes only 

one receiver, according to Allison, but the reality is generally far different.

        Dr. Bendov agrees and suggests that "loading a second 

receiver on the same downlead cable will typically reduce coverage by about three miles."

        The long established TASO (Television Allocations Study Organization) system for 

measuring subjective picture quality provides a range from one to six, where one represents an 

excellent picture and six is an unusable image.  In the NTSC world there is a gradual 

degradation of signal strength and picture quality for receivers located at the limits 

(approximately 60 miles from the antenna) of the Grade B and fringe areas.  A DTV signal, 

however, has only two TASO grades, one and six.  A change of only one dB of signal loss makes 

the difference between seeing a picture and not.

        Engineers are also aware that a drop in signal strength due to a simple three dB TV splitter 

in households nearing the edge of the DTV service area, for example, could cause a complete 

loss of signal.  This is due to the "Cliff Effect" inherent in digital transmissions when the signal 

strength drops one dB below the receiver's ability to reproduce a bit error rate that does not 

exceed the prescribed limit.

        "But that's not all," Allison continues. "The FCC plan assumes rooftop antennas at 30 feet 

height above ground.  Indoor antennas with less gain and high building attenuation are not even 

a consideration in the FCC plan."

        He is also concerned with the fact that there are "adjacent channel implementation issues 

where spurious signals from the DTV signal in adjacent channels will be the main source of 

interference. This can be caused by Third order and Fifth order intermodulation products and will 

show up as a variable impact across the NTSC band."

        Due to a lack of tower space in some NTSC facilities, many broadcasters are also faced 

with the possibility of having to mount their DTV antennas on towers up to three miles away from 

their existing site.  

And, in attempting to resolve their tower space problems, some stations could possibly create 

other transmission anomalies.

        "All of the signal strength predications assume co-location of the NTSC and DTV antennas," 

Allison said, adding that some studies show that 

multiple nulls may be created in their radiation patterns if stations are not co-located. "[while] co-

location is a must, it does not assure NTSC protection."