How do you define DTV coverage? Since 1997 the FCC has defined it as the 41 dBu F(50/90) contour for UHF stations, 36 dBu for Channels 7-13 and 28 dBu for Channels 2-6. In a Report and Order released earlier this year, the FCC defined a "City Grade" DTV field strength contour and also set minimum signal levels for reception of broadcast DTV signals at cable headends.
This month I’ll explain the FCC’s new coverage definitions and present some tables to allow you to easily see what kind of coverage can be expected from a DTV facility.
PLANNING FACTORS
The signal levels the FCC used to define the DTV service area in 1997 were based on the ATSC planning factors outlining specific antenna gains and receiver performance expectations. They are described in FCC OET Bulletin 69, which I’ve discussed in previous columns.
DTV allotment power levels were chosen to best match a station’s DTV service contour with its NTSC Grade B contour. The field strength levels were calculated using the FCC F(50,50) and F(50,10) propagation curves to define a service availability in 50 percent of the locations 90 percent of the time.
The F(50,90) field strengths were derived by subtracting the field strength difference between the F(50,10) and F(50,50) curves from the value obtained using the F(50,50) curve: F(50,90) = F(50,50) — [F(50,10) — F(50,50)].
In the Report and Order and Further Notice of Proposed Rule Making – Review of the Commission’s Rules and Policies Affecting the Conversion to Digital Television (MM Docket 00-39, FCC-01-24), the FCC found a higher field strength was required to provide more-reliable coverage of a station’s principal community. The FCC set the DTV City Grade signal levels 7 dB higher than those used to define the service area.
Why did the FCC pick 7 dB? The Report and Order noted that the NTSC Grade A service is based on service availability at 70 percent of the locations inside the contour. By increasing the field strength in the DTV service area by 4 dB, the FCC Report and Order said service would be available at the best 70 percent of the locations in the area, if all other planning factors remained the same.
The FCC also assumed viewers inside the community of license would use lower gain antennas than those at the edge of the service area. The field strength was increased by 3 dB to cover this, resulting a total increase of 7 dB. Commercial DTV stations are required to provide this level of service over their community of license by Dec. 31, 2004. The deadline for noncommercial DTV stations is one year later. This Report and Order is available in Adobe Acrobat format at www.fcc.gov/bureaus/mass_media/orders/2001/fcc01024.pdf.
DTV CABLE CARRIAGE
In late January the FCC released the First Report and Order And Further Notice of Proposed Rule Making — Carriage of Digital Television Broadcast Signals… (CS Docket 98-120, FCC 01-22). The First Report and Order outlined criteria for must-carry and retransmission consent, as well as many other nontechnical issues. I won’t address those here. The FCC document is available in Adobe Acrobat format at www.fcc.gov/bureaus/cable/orders/2001/fcc01022.pdf.
Briefly, the FCC allowed DTV-only TV stations to chose must-carry or retransmission consent on cable systems and allowed them to elect whether their signal was carried in digital or analog format. Stations with both NTSC and DTV channels were not given must-carry rights for their DTV signal, but they may negotiate retransmission consent.
Cable systems are not required to pass through 8-VSB – they can remodulate it as 64QAM or 256QAM. Although the broadcast DTV signal must be on a basic tier, the cable system is not required to provide a set-top box capable of converting the digital signal for viewing on an analog set.
The data rate the station is carried on the cable system can be less than the ATSC 19.4 Mbps without the signal being considered "materially degraded." In addition, the cable system is not required to carry a broadcast DTV signal at a higher resolution than nonbroadcast digital programs carried on the same system.
However, the broadcaster’s DTV resolution cannot be reduced below that of any nonbroadcast digital program. If a DTV broadcaster is transmitting multiple program streams and ancillary or supplemental data, the broadcaster must chose which program stream is the primary stream, and that is the only one the cable company is required to carry.
Except for program-related material such as closed captioning, Nielsen ratings data, V-chip data and PSIP, the cable system is not required to carry a station’s ancillary or supplemental data.
SIGNAL LEVELS
As with analog TV signals, broadcasters are required to supply the cable company with an adequate signal level. The FCC set this level at -61 dBm for both VHF and UHF DTV signals. This compares with a level of -49 dBm for analog VHF-TV signals and -45 dBm for analog UHF-TV signals. The -61 dBm level was derived by starting with the thermal noise level of -106.2 dBm in a 6-MHz bandwidth.
Because the ATSC 8-VSB system planning factors specify a minimum 15.2 dB carrier-to-noise ratio for successful reception, if there were no other losses a -91 dBm signal would be required. The FCC allows an additional 10 dB for receiver noise figure plus a 20 dB "propagation and implementation margin," resulting in the -61 dBm signal level requirement.
It is important not to confuse -61 dBm with dBmV or dBu. In my Sept. 1993 RF Technology column (available on my Web site at www.transmitter.com), I showed how to convert between dBm, dBmV and dBu/m. The dBm to dBmV conversion is easy because we are simply converting a power measurement to a voltage.
Add 48.75 dB to the signal power level in dBm to obtain dBmV if the impedance input of the cable preamplifier or converter is 75 ohms, as is common. In other words, the received signal voltage must be at least -12.25 dBmV or 244 microvolts to meet the FCC signal level requirement in a 75-ohm system.
RESONANT DIPOLE
How does this relate to the field strength contours described earlier? Remember that the FCC TV propagation curves give the signal level in decibels above 1 microvolt per meter. As a resonant dipole will be shorter at UHF than at VHF, higher frequencies require more field strength to give the same power at the output of the antenna. For a 75-ohm antenna, the formula is: Voltage = 48.38 * Field / Frequency.
Where Voltage is in millivolts, Field is in millivolts/meter and Frequency is in Megahertz.
If a cable system is inside a DTV station’s "City Grade" contour, will it receive enough signal level? It depends on the DTV station’s channel and the antenna at the cable headend.
If the DTV station is on Channel 10, the center channel frequency is 195 MHz. The City Grade field strength is 43 dBu/m, or 0.1413 mV/m. Voltage at the 75-ohm dipole antenna will be: 48.38 * 0.1413/195 = 0.035 (or -29.12 dBmV).
An additional 17 dB of gain is required to meet the minimum signal level requirement. This can be achieved with increased transmitter power and additional antenna gain at the cable headend.
Consider another example with a DTV station on Channel 18, center frequency at 497 MHz and a required City Grade signal level of 48 dBu/m or 0.251 mV/m: 48.38 * 0.251/497 = 0.024 (or -32.24 dBmV).
In this case, an additional 20 dB of gain is required. Note that as the frequency increases, the field captured by the shorter dipole drops proportionately. The implications are clear – the higher your DTV channel, the more effective radiated power you will need to meet the minimum signal requirement at the cable headend.
Before you get too discouraged by these numbers, note that these calculations were based on the signal level at the edge of the DTV city grade contour. In addition, the antenna at a cable TV headend is likely to be substantially higher than the 30 feet receive antenna height used when determining FCC coverage.
This is a good time to look at the signal levels expected from a DTV facility. Table 2 shows the field strength at various distances for a 10 kW effective radiated power (ERP) VHF DTV station as predicted using the derived FCC F(50,90) propagation curves for Channels 7-13. Table 3 shows field strength values for a 100 kW ERP UHF DTV station using the F(50,90) curves Channels 14-69. Results are shown for antenna heights above average terrain of 500, 1,000 and 2,000 feet.
The antenna elevation and azimuth patterns were not taken into account when calculating the field strength. This may not make much difference at VHF, but at UHF a high-gain antenna may have a significantly lower ERP near the transmitter site and reduced ERP in some directions.
Although I’ve only provided 15 points in each table, it should give you a quick way to estimate DTV coverage for common configurations. Any increase in ERP will cause the same increase in signal level. For example, a 1,000 kW UHF station has 10 dB more power than a 100 kW station. Using an antenna at 1,000 feet above average terrain, at 1,000 kW the signal level at 60 miles will increase by 10 dB to 41 dBu/m.
In the earlier example, where we calculated the field strength required to meet the FCC minimum for DTV cable carriage at Channel 18, if the cable company used an antenna system with 10 dB gain the field strength would have to be 58 dBu/m. Using Table 3, you can see where a 58 dBu/m signal would be available with 100 kW ERP. If the headend was 40 miles away and the antenna was at 1,000 feet above average terrain, 6 dB more effective radiated power, approximately 398 kW, would be needed to deliver the necessary signal to the cable headend.
If you want to calculate either the distance to a signal level or signal level at a distance, the engineering consulting firm of Hammett and Edison has an FCC curves calculator online at www.h-e.com.
Contact Doug Lung at dlung@transmitter.com.
