Doug Lung /
01.05.2009 02:15 PM
DTV Transition Survival Guide
While it's probably too late to make any major changes before the analog shutoff next month, there are steps broadcasters can take to avoid unpleasant surprises.

MAKE SURE YOUR SIGNAL DOESN'T DISAPPEAR

In most markets, more viewers watch TV over cable or satellite than over-the-air. While DirecTV, Dish and the larger cable MSOs are already taking feeds from over-the-air DTV or fiber to feed, their headends, there is a good chance many master antenna systems in apartment buildings and condominiums have not prepared for the shutdown.

Because viewers in these buildings have their TV sets hooked up to an outlet in the wall, they may assume that they are getting cable TV and feel they are OK even if they see snow during analog "soft shutdowns." In older buildings, it's possible the company that did the original installation is no longer in business. Someone, however, has to maintain the system. If you are lucky, you may be able to find someone responsible for several systems. Don't be surprised if they are working for a company selling telephone systems or DBS subscriptions.

Stations changing DTV channels after Feb. 17 have a special challenge. I suspect many stations will drop off some systems for a few days until someone changes the receiver tuning. The outage could be worse if the new channel can't be received due to a weaker signal, the wrong antenna (using a UHF antenna on VHF, for example) or interference. The FCC recently allowed a station to broadcast on two DTV channels simultaneously so it could transition cable and satellite companies to its new channel before shutting off its out-of-core pre-transition DTV channel. While my limited tests showed no problems on current DTV sets and older converter boxes, a reader did a more extensive test with newer converter boxes and found some had problems with the same stream on two different channels.

Simulcasting isn't an option for stations moving back to their analog channel post-transition, but it may be worth sacrificing some analog airtime to test reception at headends now. If there are problems, it will give the cable or satellite company time to fix it.

Most DTV sets sold include ClearQAM tuners that provide reception of unencrypted digital cable channels. While some systems carry the broadcaster's PSIP data, many don't. This will make it hard for subscribers to find your channel on sets not hooked up to a set-top box. Try hooking a DTV set directly to a cable outlet. Punch in your major/minor channel number. See what happens. If nothing shows up, do a channel scan see where your station is located. There is little time to fix the problem, but it is worth the effort, especially if cable subscribers can punch in your competitors' major-minor channel number and see its station. Most cable operators willing to work with broadcasters on PSIP issues. FCC rules Section 74.640 requires systems with 750 MHz or more activated channel capacity carry broadcasters' PSIP according to the ATSC A/65 standard, available at www.atsc.org/standards.

It may be difficult to find time to locate and contact every MVPD (multichannel video programming distributor) and master antenna system to make sure they are ready for the shutdown. If that's the case, prioritize MVPDs based on the number of subscribers. Enlist the help of employees and other viewers to find out what apartment buildings may have problems. Station giveaways (hats, T-shirts, coffee mugs, etc.) might help in recruiting viewer volunteers.

Paul Sokoloff (WCAU) adjusts antenna for signal measurements.
KNOW YOUR DTV COVERAGE

Sometimes viewers receiving a station's analog channel have problems receiving its DTV signal. Viewers that are getting good reception from a station's UHF DTV channel may not have as much luck if it switches to a VHF DTV channel post-transition. Alternatively, some viewers that aren't getting a station's UHF DTV signal may find they are able to get the post-transition VHF DTV signal, if they have the right antenna and re-scan their DTV set or converter box. Knowing what your DTV coverage looks like now and what it will look like on Feb. 18 is important. Coverage can be determined using computer propagation models or by field measurements.

John Magliacane's SPLAT software makes it easy to calculate DTV coverage using the Longley-Rice propagation model and the shuttle radar terrain mapping mission three second elevation data. See my past articles on SPLAT and my SPLAT resource page at www.xmtr.com/splat for details on installing and running SPLAT. A computer model is only as accurate as the input data. Use the most accurate antenna azimuth and elevation pattern data available, including tower scattering if possible. SRTM elevation data includes buildings and trees present during the shuttle mission in February 2000. When using SRTM topographic data, you may want to use a lower receive antenna height to obtain more realistic results.

SPLAT now provides the ability to save the study as a PPM image and a Google Earth kml file. PPM images are uncompressed, very large, and slow to load. Use a graphic program to convert them to PNG and then modify the image filename in the kml file to match for faster map response. After loading the file in Google Earth, zoom in to see how terrain and building impact signal strength. Remember that the signal strength is based on an antenna 4 meters (or whatever height was specified in SPLAT) above the terrain/foliage/buildings.

The zoomed area of the Google Earth map with WNJU-DT coverage shows the impact of buildings in lower Manhattan on the signal from the West Orange transmitter site based on an antenna 4 meters above terrain. (See Fig. 1.) When viewing this map, keep in mind that the cells are three seconds on a side. This means there could be significant variation inside the cell—from 4 meters above the rooftop of a building to the street below. The color scale for the map ranges from dark orange at 98 dBuV/m through yellow to dark green at 68 dBuV/m and then cyan at 58 dBuV/m and ends up at violet for 41 dBuV/m. The underlying color of the Google Earth map has some impact on the signal strength overlay color. The www.xmtr.com/splat Web site has links to the files used to create this map and the home of SPLAT at John Magliacane's Web site.

Field measurements provide a way to find major differences between calculated and actual antenna performance. One procedure for field strength measurements is outlined in the FCC rules. Gary Sgrignoli, founder of Sgrignoli Consulting, details procedures for DTV field measurements in his VSB measurements seminar. Many stations won't have the time, equipment or money to do detailed measurements or to hire someone that can. I've come up with a simpler, faster, easier method that's sufficient to show any major coverage abnormality.

My method uses an antenna at 12 to 15 feet above ground. At 12 feet, a light stand provides sufficient stability in light winds. For greater heights, a drive-over mount like the one shown in the photo works well. I've had good luck with the Antennas Direct C1 and C2 antennas for UHF measurements. They are compact, fairly rugged and the broad azimuth pattern makes aiming easy. The antenna is connected to a splitter feeding a USB tuner and laptop to check reception and to a spectrum analyzer capable of channel power measurement.

(click thumbnail)
Fig. 1: A Google Earth map with WNJU-DT coverage shows the impact of buildings in lower Manhattan on the signal from the West Orange transmitter site. Courtesy of Google Earth
Since the goal is to identify problems that may require more detailed coverage studies, I focus on the difference in signal strength between multiple stations after correcting for the difference in antenna pattern. I don't recommend this technique if nearby "reference stations" are not available. To minimize the impact of reflections, buildings and foliage, only use sites with a clear view to the tower where a SPLAT coverage study or terrain profile shows no blockage. Try to find sites equal distance from the antenna in the directions of interest.

At each site, orient the antenna for best reception. If the signals seem too weak, one station is much weaker than expected or the optimum orientation is not in the direction of the transmitter tower, move the antenna or find another site nearby. For example, in a recent test I saw signal levels much lower than expected. Looking at the antenna orientation, we saw the antenna was pointing at a large pine tree! Moving the antenna few feet to clear the tree provided more realistic results. Once the antenna is aimed, take a wide-span spectrum analyzer shot of channels in the market. After doing this, take channel power measurements of the station being measured and as many reference stations as possible. Stations using omnidirectional patterns make comparisons easy.

Back in the office, calculate the difference between the predicted signal level and the measured signal level for each station. Ideally, the difference should remain constant. If it doesn't, as may be the case at a site with a lot of foliage around or other unavoidable obstruction, the difference should be close for all the stations, after taking into account differences in antenna gain and greater foliage loss at higher frequencies.

I've used this technique at three different locations on the East Coast and results were as expected. The impact of the tower and transmission lines on side-mounted antennas was obvious. I've also taken readings along one radial to attempt to verify the null in the elevation pattern but haven't had a chance to analyze the data to see how it worked.

CHECK THE TRANSMITTER

As a final check, if any changes to the DTV transmission plant are required on Feb. 18, as noted earlier, it is worth some time off air to test the transmitter, filter and antenna in the exact configuration it will be switched to on Feb. 18. Coordinate with other broadcasters if necessary to do some on-air testing, even if it has to be overnight and be sure to have people monitoring the signal in enough locations to know it is working. Compare the signal with other stations and, if possible, your current DTV signal. The FCC requires proof of performance for DTV transmitters. Most of the proof involves checking out-of-channel emissions (see the IEEE P1631/D3 draft recommended practice for how to do this with a spectrum analyzer and notch filter, www.ieee.org/organizations/society/bt/DTVEmissionsD3.pdf), but the output power calibration and signal-to-noise ratio measurements are also important. Test the transmitter into a dummy load for a few days to reduce the chance of component failure in new or converted transmitters.

Good luck! If you uncover some interesting problems or unique solutions, let me know by e-mail.

Even though time constraints may make it difficult to respond to all e-mail, I read all comments. Your question could provide the seed for a future RF Technology column. E-mail me at Dlung@transmitter.com.



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