10.05.2004 12:00 AM
FCC Releases Rules for Digital Class A, LPTV and TV Translator Stations
Last week the FCC released a Report and Order establishing rules for digital LPTV and TV translator stations and amending the rules for Digital Class A TV stations. In the Report and Order, the FCC concluded that it would be better to hold off completing the low power digital transition until after the transition deadline for full-service television stations, when the FCC expected a sufficient number of channels would be returned to allow most LPTV, TV translator and Class A stations to have a second channel for digital service "for some period of time." The Report and Order also allows the use of channels 52-69 for low power digital TV, although significant conditions were attached to the use of these channels to avoid interference to public safety and other wireless operations on these channels.

The Report and Order adopted the Class A DTV protected signal contours for all digital TV low power/translator stations--43 dBu for stations on channels 2-6, 48 dBu for stations on channels 7-13 and 51 dBu for stations on channels 14-69. The Report and Order said the signal levels, which are significantly higher than those for full service DTV stations, are appropriate because the yield digital service areas comparable in size to LPTV/translator analog service areas.

The FCC rejected calls for different D/U ratios for low power DTV stations. Except for some adjacent channel D/U ratios, low power DTV interference analysis will use the same D/U ratios listed in Section 73.623 of the rules for full-service DTV interference. The Report and Order gives low power DTV stations the option to elect to use either a "simple mask" or "stringent mask" and sets different adjacent channel D/U ratios for each mask. Low power DTV stations using the "Simple Mask" will have to meet D/U ratios of 10 dB for DTV into analog interference and -7 dB for DTV into DTV interference. If the "Stringent Mask" is used, the D/U ratios are relaxed to 0 dB for DTV into analog and -12 dB for DTV into DTV interference. The Report and Order defines the attenuation requirements for each of these masks, but said, "we will make some accommodation to station operators converting existing analog transmitters for 'on-channel' digital operation where the analog transmitter falls short of the Simple Mask (i.e., at the mask "shoulders"), due to limitations of the transmitter's RF power amplifier. In this event, we will apply the adjacent channel D/U ratios for the Simple Mask."

The FCC decided to adopt most of the recommendations of the Association of Federal Communications Consulting Engineers (AFCCE). AFCCE recommended use of the OET-69 interference method with 1 km maximum grid resolution, reflecting the smaller coverage area of low power DTV stations. The de minimis criteria for determining unacceptable interference was kept at 0.5% of the population served by the interfered with full-service stations. However, for protection of other secondary services such as analog and digital LPTV and TV translator stations, the FCC adopted a 2% de minimis criteria, which it said would give proposed new digital low power DTV stations the flexibility to serve more people.

As I've illustrated in my recent RF Technology articles, there are many cases where the actual vertical patterns of antennas differ substantially from the patterns in Table 8 of OET Bulletin 69. The FCC said they were convinced that the OET-69 patterns are not appropriate for LPTV and TV translator stations, but said they do not have suitable replacement patterns to adopt. As a temporary measure, the commission said that it "will assume (for predicting both service and interference) that the downward relative field strengths for digital and analog LPTV and TV translator stations, and digital and analog Class A TV stations is double the values specified in OET Bulletin 69, Table 8, up to a maximum of 1.000."

The FCC gave a hint that it might consider actual vertical radiation patterns for full-service DTV in the future, noting "If in the future we develop an ability to account for actual vertical radiation patterns and related beam tilt in the full-service DTV context, we will consider applying that ability in the digital LPTV and TV translator context." As a further indication of its willingness to consider different vertical radiation patterns, the FCC said it's "inclined to adopt a revised procedure in the future if parties develop and propose realistic alternative vertical patterns, including the shifting of relative field strength values to account for electrical antenna beam tilting." The FCC will continue to allow contour overlap analysis for process analog LPTV, TV translator and Class A TV application, but will also allow an optional showing based on "DTV methodology" (OET-69) without requiring a rule waiver. The FCC said it was not prepared to replace analog spacing requirements with DTV methodology standards.

The FCC decided that requiring a digital LPTV or TV translator licensee to maintain the pilot frequency of its DTV signal to specified offsets with respect to the visual carrier of an analog LPTV or TV translator station wasn't worth the cost as it would be unlikely to significantly improve the service quality or coverage area of the analog station. However, if interference does result, it expects licensees to cooperate to use precision offset to reduce the interference. Some LPTV and TV translator stations operate without maintaining a precise frequency (within 1 KHz) or offset from other stations. The Report and Order states, "Where analog LPTV and TV translator stations operating without a nominal frequency offset prevent the proposed service of a new or modified LPTV, TV translator or Class A station, we agree that the time has come to require that station to maintain a designated offset."

The Report and Order outlines permissible services and other operating rules that I won't go into here except to mention that the FCC did adopt new call sign suffixes for low power DTV stations. Digital LPTV stations will be identified with the suffix "-LD" and digital Class A stations will use the suffix "-CD." Since PSIP generators are limited to a six character call sign, the single letter "D" suffix will be used for identifying digital translators and digital LPTV stations that have a five character letter/number call sign. LPTV and Class A DTV stations are prohibited from using the suffix "-DT," which has been reserved for use by full-service DTV stations.

The FCC maintained the power limits it adopted in its DTV proceeding for low power DTV stations. These are 300 W average DTV effective radiated power (ERP) for channels 2-13 and 15 kW average DTV ERP for UHF channels 14-69.

These are a few highlights of the rules described in Report and Order. Refer to it for more detailed information on topics including low power DTV station filing requirements, emission mask requirements, and permitted operations.

At least one of the rule changes in this Report and Order will affect full service stations. Software used for evaluating coverage and interference based on OET-69 will have to be modified to take into account the new vertical (elevation) pattern specified for low power TV, TV translator and Class A stations. As the new elevation pattern has double the relative field (capped at 1.0), this will result in increased signal strength from low power stations in many areas. Since the relative field at and above 0.75 degrees (for UHF antennas) will remain at 1.0, this wouldn't affect interference calculations near the edge of the protected contour but could allow higher interfering signal levels closer to the low power transmitter site. The new low power elevation pattern therefore will likely have more of an impact on low power filings than it will on full-service interference to low power or Class A TV stations.


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