Video Projections; DISTRIBUTION of digital television signals
Peter H. Putman
In last month’s column, I covered the problems inherent in trying to receive dependable 8VSB DTV signals and mentioned the significant problems presented by signal multipath. Indeed, multipath is the biggest problem confronting the ATSC 8VSB (Eight Level Vestigial Sideband Modulation) standard, adopted after much debate and political maneuvering in 1995.
Although 8VSB works well in suburban and even some rural environments, it is not proving to be as robust in the concrete canyon locations of large cities – areas where signal multipath is a fact of life. For analog TV transmissions, that is not as much of a problem. Many city-dwelling TV viewers have grown accustomed to fiddling with the rabbit ears to try to get a better analog picture. Otherwise, they have simply signed up for cable or MATV service.
As we learned last month, however, 8VSB is an all-or-nothing-at-all broadcast system. Either you get a perfect picture, or you do not get a picture at all. Severe multipath that causes signal distortion of more than 4 dB to 6 dB will result in a tiled, pixilated image with intermittent or no audio. That’s exactly what many early adopters of DTV are getting on their receivers.
There is only one way to know for sure if you will be able to receive a clean DTV signal at your desired location, and that is to beg, borrow or steal a spectrum analyzer. For those readers not familiar with this device, it is basically a souped-up oscilloscope with an RF front end that allows the user to actually see parts of the radio frequency spectrum. Spectrum analyzers can be used to determine signal levels, calculate signal gain or loss, determine noise figure, establish a noise floor and reveal the presence of harmonics or other unwanted RF energy.
Although a simpler signal strength meter will give you an indication of how much DTV signal level you can receive, it will not tell you if the signal has been affected by multipath or cancellation. As a rule, if your desired DTV location has a good path to the transmitter site with good signal levels, then a signal strength meter will probably be all that is needed for setup.
If, however, your desired reception site is in an area with a lot of tall buildings, hills, trees or other obstructions nearby, you may have fits with multipath. Only a spectrum analyzer can show if the received signals are distorted. Even with severe multipath, signal strength meters will still lock on to the DTV station’s pilot signal, which is located at the leading edge of the broadcast carrier wave.
I recently checked out a variety of TV antennas to see how well they received DTV broadcasts in both line of sight (LOS) and medium multipath locations in the northern Philadelphia suburbs. The LOS location was on a large open field, adjacent to the National Shrine of Our Lady of Czestochowa, New Britain, PA, which is at 500 feet (152 m) above sea level and has no obstructions in the direction of the Philadelphia DTV transmitting antennas located in the Roxborough section of northwest Philadelphia.
At the other location, the rear deck of my home, I set up a temporary mast and a couple of UHF yagi antennas to watch the Super Bowl in HDTV this past Jan uary. This location is surrounded by several houses and trees, but tests with my Sadelco Mini-Max 800 signal strength meter show that there is plenty of RF available from three of the five DTV Philadelphia broadcasters.
Table 1 shows the results of my tests at the LOS location. While in the midst of operating field day, a national test of emergency radio communications for amateur radio operators (and for you readers who are hams, my Extra Class call sign is KT2B), I sneaked over to an unused area of the Field day site and quickly erected 15 feet (4.6 m) of antenna mast.
I tested four Channel Master yagis, representative of four antenna designs for close-in, suburban, rural and deep-fringe reception. At this location, I noticed a small amount of multipath, but it was not enough to cause problems with reception. As the table shows, the antenna I currently use for DTV reception, Channel Master’s 3021 four-bay screen antenna, performed quite well against its smaller and larger brothers.
As expected, the largest antenna, the 3028 eight-bay UHF screen, had the greatest gain on the lowest DTV channel (26). Higher in frequency, Channel Master’s 3023 deep suburban yagi was the best performer in terms of gain, outdistancing the eight-bay by at least 3 dB. At the upper end of the UHF TV spectrum, however, the four-bay 3021 was just a hair better on channels 64 and 67. Frequency response is definitely something that you will want to keep in mind as you select a terrestrial DTV antenna.
Although it is true that gain and directionality go hand in hand, not all high-gain antennas reject multipath equally. A long yagi, like Channel Master’s 3023, is quite adept at handling signal reflections in a low-multipath setting with a fairly unobstructed reception path, but it can be overwhelmed by secondary and even tertiary signal reflections when used in an urban environment.
Consequently, our DTV antennas should have both a high front-to-back gain ratio as well as excellent sidelobe signal rejection. Checking Table 1 again, you will notice that, oddly enough, the small 3022 yagi had the best front-to-back ratios on channels 26 and 42, while the 3021 4-bay screen won out on channel 64, and the 3028 was tops on channel 67.
In terms of sidelobe signal rejection, the 3028 worked best on channel 26; the 3023 had the edge on channel 42, and the 3021 was at least 3 dB better on 64 and 67. I should qualify these results by stating that I did observe some signal multipath on channels 42 and 64, which may have been caused by adjacent antenna towers or a reflection from a microwave tower about a mile to the northeast.
Most of the differences among antennas are not substantial. I primarily use the 3021 four-bay screen because it has the highest gain above channel 60. If, however, the DTV stations in your area broadcast on lower frequencies as in New York City where allocations include 28, 44, 45 and 56, then the 3022 or 3023 may do the trick just fine.
Table 2 shows the results of my deck antenna tests. The degree of multipath made taking accurate front-to-back and sidelobe measurements difficult, so I concentrated instead on how much headroom I had with a given antenna – that is, how much extra signal I had available before the DTV transmission broke up and dropped out. Only three stations (26, 42 and 64) were strong enough to pick up at this site.
Although I did not have the 3028 eight-bay screen to test, I did have two city-dweller antennas on hand – Channel Master’s STEALTHtenna and SMARTenna. Both are low-profile designs with built-in signal amps, and both are certified as DTV-compatible. Channel Master’s 3021 gave the most signal headroom on channel 26, but the SMARTenna did even better with channels 42 and 64. This antenna not only handles multipath well, but it also gives you a little extra signal.
Just how much signal is needed to get a set-top DTV tuner to lock up? Trying to get this information from manufacturers has been like pulling teeth. Fortunately,I have also been compiling data on those magic numbers simply by using a precision 75 V step attenuator (from JFW Industries) that can dial in as much as 30 dB of attenuation in 1 dB steps.
Remember that under the 8VSB standard, the theoretical minimum S/N ratio is 12 dB to 15 dB to hold a steady picture. That assumes, of course, that you have no multipath. What I did was put the 75 V attenuator in series with the set-top box’s antenna input and just kept throttling the signal back until I started to see any evidence of tiling or pixilating.
Table 3 shows the results. Two 1999-vintage set-top boxes were used – Samsung’s SIRT-100 and Panasonic’s TU-DST51. A 26 dB preamp (Channel Master Titan 2 #7775) was installed at the antenna, and each received signal was measured at the input of the set-top box to establish a baseline. Next, I recorded the total attenuation at the point where the signal started to drop out. Finally, I calculated the minimum discernable signal (in dBmV) for each channel.
It becomes apparent from the data that the MDS remains pretty constant across the UHF band for both tuners. It takes a signal of at least -26.2 dBmV to hold KYW-DT’s signal (channel 26) on the Samsung consistently and at least -24.1 dBmV to maintain the status quo with WCAU-DT (channel 67). The results were similar on the Panasonic tuner, except for 4 dB more headroom on channel 26.
Interpolating from earlier signal level tables I prepared, those MDS figures work out to about the specified threshold for DTV set-top boxes. Using a spectrum analyzer, I calculated my antenna system’s noise floor to be -89 dBm (the preamp I use has a 2 dB noise figure). WTXF-DT’s amplified signal measures -44 dBm at the set-top box, so 32 dB of attenuation drops the signal level down to -76 dBm at the input to the Samsung tuner. This works out to 13 dB S/N, which is just a hair more than the ATSC standard.
On the Panasonic tuner, I could hold WTXF-DT’s signal with another 3 dB of attenuation, giving me a minimum S/N ratio on Channel 42 of 9 dB. That is 3 dB better than the theoretical minimum for 8VSB reception. In effect, either tuner can do the job when it comes to sensitivity. The question lies in feeding both tuners a steady diet of signals that are free of multipath.
Distant signals can also be received without too much trouble using the Titan 2 UHF preamp. Both WNYW-DT (channel 44) and WCBS-DT (channel 56) come in pretty steadily at my house, although their transmitting site is 65.5 miles (105 km) northeast of me. Their signals at the antenna terminals are too weak for reception, measuring -84 dBm for WCBS and -85.5 dBm for WNYW, and they are just 8 dB and 6.5 dB above the noise floor without a preamp (-92 dBm).
By adding the preamp, the noise floor is raised to -89 dBm, but both signals are boosted to -64 dBm and -62 dBm, respectively. Now, the S/N ratio is at least 25 dB, which, assuming that multipath is not a problem, is more than enough for reliable reception. I can also detect the DTV signal pilot from WITF-36, the PBS station in Harrisburg, PA, which is almost as far in the opposite direction from me (west-southwest) as the two New York City stations.
I should add that if you tried to receive analog TV broadcasts with some of these signal levels, you would have a hard time getting a noise-free picture. Thanks to digital signal processing, broadcasters will be able to run a lot less power to achieve the same coverage area. As I write this, WHYY-DT 55, running 87 kW ERP, is getting steady reception reports from the New Jersey shore area, about 60 miles (96.6 km) to the east of its Philadelphia antenna site. Its analog signal on channel 12 would be extremely noisy at that same power level.
Installing a terrestrial DTV system involves a little detective work on your part. Before promising anything to corporate, educational or home theater customers, you will need to do some antenna tests on your own. If you do not have access to a spectrum analyzer, it would pay to buy several UHF antennas (like the four I mentioned) and run some tests of your own with a DTV set-top box to see which antenna provides the best reception.
You may be surprised at the results. You may also find that you cannot get a clean signal, no matter which antenna you use or which direction it is aimed. Unfortunately, that is the way things are with 8VSB DTV at the moment. If you have further questions, I am compiling data on multipath performance for several UHF antennas this summer, and I should have the results by September’s CEDIA show. Send me an email at firstname.lastname@example.org, and I will send you a copy as an Excel spreadsheet.
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