By: O. Bendov

The long anticipated Proposed DTV Table of Allotments ¹ ("Table") was expected to clarify the RF transmission equipment issues faced by the broadcaster. However, for many VHF broadcasters, the extraordinary ERP's indicated in the "Table" raises a host of implementation issues:

• Is the equipment available?
• How much will it cost?
• What are the tradeoffs of ERP vs. Coverage vs. Plant Costs?

In the following text, "Replicated Coverage", the basis for the high ERP's is discussed. Additionally, the transmitter power issue is examined by considering the "peak" vs. "average" power requirements. Finally, we address the importance of DTV system design studies to assist the broadcaster in evaluating the implications of the "Table".

Replicated Coverage

Dr. Bendov ² of Dielectric has long asserted that the quest for equivalent Grade B coverage would result in ERP's far in excess of the Grand Alliance suggested -12 dB power levels (DTV average power relative to NTSC peak power). The "Table", using different assumptions, bears this out. Whether or not "replicated coverage" is actually achieved is another question.

Replicated coverage, as a percentage area match, depends on certain assumptions regarding the propagation and receiver models and additional issues. In particular,

• The desired service area in terms of picture quality and range from the Tx.
• The desired percentage of time that the signal may be permitted to fall below the specified picture quality without losing viewers.

For the first assumption, the FCC Grade-B contour for NTSC was used which provides CCIR-3 picture quality with outdoor antennas. The FCC's system design uses a 90% time availability for a "passable" picture (CCIR-3) as the second assumption.

Unfortunately, the FCC's Grade-B contours are based on an outdated propagation model and, especially in mountainous terrain, the distance to the Grade-B contour is unrealistically extended. The propagation model used to generate the "Table", which is more realistic than the NTSC model, simply determined the power at which the UHF-DTV signal carried to the Grade-B contour. As a result, extraordinary powers are required to "replicate" the Grade-B contour of VHF stations.

Transmitter Power: Peak vs. Average

At first look, the ERP's in the "Table" may not seem unusual. After all, UHF stations have been broadcasting at ERP's of up to 5 MW for some time. However, NTSC power levels are relative to peak sync while DTV powers are average. DTV has peak signal levels 4-5 times (6-7 dB) greater than average. The transmitter must be sized to pass these peak levels with minimum distortion. Therefore, to equate DTV transmitter sizes to NTSC transmitters it is necessary to multiply the DTV power by 5.

Let's look at a specific example taken from the "Table". NTSC channel 5 at 100 kW ERP with allotted DTV channel 48 at 3,162 kW ERP.

How did we get the 640 kW for the DTV TPO?

TPO = (ERP x 5)/(30 x .823) = 640 kW

where:

5 (=7 dB) is the peak/average ratio for DTV
30 is estimated gain of practical omnidirectional UHF antenna
823 is the efficiency of 1600' of 15" circular waveguide @ ch 48

Applying these assumptions to the "Table", the following statistics are generated:

• 33% or more than 550 stations will need a transmitter larger than 120 kW
• 30% or more than 500 stations will need a transmitter larger than 240 kW
• 17% or more than 280 stations will need a transmitter larger than 480 kW
• 3% or more than 50 station will need a transmitter larger than 960 kW

You can see the complete distribution on the graph.

Now, consider that the largest UHF NTSC transmitter in the US at present is 280 kW.

Implementation Considerations

Assuming that future alterations to the "Table" will be minimal, the broadcaster is faced with difficult RF system implementation issues.

• Is the equipment available to transmit the proposed high powers?
• How would the DTV and NTSC antennas be configured?
• What would be the costs?
• What would be the loss in coverage by implementing more practical and affordable power levels?
• What is the realistic, reliable DTV coverage area?

Dielectric DTV System Studies

Dielectric is uniquely positioned to assist the broadcaster with the implementation questions. Design studies, performed by Dr. Bendov, have been completed for Mt. Sutro in San Francisco and the World Trade Center in New York. Additionally, Dielectric is under contract for more than 50 studies in major markets around the country. Dr. Bendov has developed pioneering software and methodology for assessing the true coverage of HDTV transmissions accounting for the power penalties due to frequency response variations and equalizer compensation. Applying NTSC methodology for coverage analysis will not provide representative results for DTV.

Before contracting for a DTV Design Study, compare the issues examined with our Study checklist.

¹ Sixth Further Notice Of Proposed Rule Making, FCC 96-317, released August 14, 1996.
² Bendov, O. "The Effect of Channel Assignment on Transmitter and Receiver Requirements for Equivalent HDTV/NTSC Coverage," NAB Proceedings, 1994.

For additional information contact Dr. Bendov by phone at 1-609-354-0776, fax at 1-609-354-0686 or email at DTV@dielectric.com