QUEST FOR THE HOLY GRAIL
Dec 1, 2001 12:00 PM,
PETER H. PUTMAN, CTS
REGULAR READERS OF THIS COLUMN MAY recall S&VC‘s December2000 issue, in which I detailed my efforts at improving antenna designsfor indoor reception of 8VSB digital television signals. At the time, Iwas nearing the end of a yearlong exploration of 8VSB propagation, bothindoors and out. I had accumulated hundreds of spectrum-analyzer screencaptures and photos; torn apart and rebuilt dozens of antennas; and gotsurprising results with reception of digital TV signals in difficultlocations.
That column ended with this statement: “It is possible toreceive 8VSB signals indoors with multipath (or outdoors withmultipath, for that matter). It does take a three-way effort betweenthe antenna, the DTV receiver’s equalizer and the DTV receiver’s 8VSBdemodulator. All of these pieces of the puzzle have to be improved, andthey will be with time.”
It’s now exactly one year later, and there have been someinteresting developments. While my tests proved that a good antenna forUHF DTV reception did not need to cost an arm and a leg, I had alsowondered when I would see significant improvements in the performanceof DTV set-top boxes.
PROGRAMMED TO RECEIVE
At Winter CES 2001, Samsung introduced an all-new, over-the-air(OTA) ATSC DTV receiver known as the SIR-T150. This new boxincorporated many Broadcom receiver chips and was designed withimproved sensitivity, greater equalization performance and asuper-simple user interface in mind. Although it didn’t support DBS, itlooked promising for OTA reception (as well as 8VSB cablereception).
I got one of the first 11 boxes shipped into the United States forsome extensive testing, and right away it was obvious that the SIR-T150was a hot receiver. In side-by-side comparisons with older STBs fromPanasonic and RCA, the SIR-T150 had, on average, 6 dB more sensitivityto signals than did either of the first-generation STBs. It also lockedup much faster on valid 8VSB carriers, sometimes in as little as 1.5seconds.
Further tests of the SIR-T150 took it to Hollywood, New York Cityand Philadelphia. Software versions were upgraded a few times, and abizarre problem with Program and System Information Protocol on PBSstations needed to be debugged. In Philadelphia, I found this tunersensitive enough to watch DTV stations from as far away as New YorkCity (over 64 miles away) with no drop-outs, day or night.
As spring wound into summer, a full complement of DTV signals cameon-air from the World Trade Center, including WNBC-28, WPIX-33,WWOR-38, WABC-45 and WNET-61. All of these signals, plus WNYW-44 andWCBS-56 from the Empire State Building, were rock-solid. Add to thosethe five Philadelphia stations, two Allentown stations, and oneTrenton, New Jersey, station I already received, and I could choosefrom 15 different DTV carriers for antenna testing over a variety ofterrains.
In last year’s column, I mentioned my trip to the famous Schubinsite, the apartment of broadcast/video trade magazine columnist MarkSchubin, located on the Upper West Side of New York City. This site isa black hole for indoor television reception, and tests by numerousengineers and industry groups had not resulted in reliable reception ofany OTA DTV stations. During my brief visit, I had some success inlocking up one of the two New York stations on Mark’s RCA DTC-100set-top box (first generation). But time ran out before I could finishanalyzing the signal from WCBS-56 and get reliable reception of itssignal. So, I planned a return trip this past September.
On September 11, 2001, the collapse of the twin towers of the WorldTrade Center not only took five DTV stations (and numerous analog TVand FM stations) off the air, it also killed and injured more than 3000people. My tests seemed trivial at this point, so I postponed them forseveral weeks.
THE ANNUAL DTV TEST
As September turned into October and we all tried to get back to ournormal lives, I thought that a trip to Mark’s place would still beworthwhile. So much had changed since last year with the equipment. Ihad the latest off-the-shelf version of the Samsung tuner (v1.63software), and I had fabricated my experimental antenna into a morefinished version.
Not only that, I thought it would be fun to try several commerciallymade antennas, starting with a $2.99 Radio Shack UHF bow tie. Thesuccess of the ATSC 8VSB DTV system would really hinge on how easy itwould be to receive by the average consumer with a basic antenna, notby guys like me who were armed with a signal strength meter and twospectrum analyzers.
To get the most mileage from my tests, I also scheduled a visit tothe vPhiladelphia home of columnist Jon Takiff, who writes about musicand consumer electronics for the Philadelphia Daily News andPhiladelphia Inquirer. His first-floor location presented anequally tough challenge for indoor 8VSB reception with low signallevels and lots of multipath.
I rounded out my field tests with a series of measurements made inmy own office. This location, about 23 miles north of the PhiladelphiaTV antenna towers, is shadowed to the south by a ridge running throughtown that is about 50 feet higher than my office. Signal levels areokay-to-weak here, with lots of multipath.
The test equipment was pretty basic. Pete Gaglio of Avcom sent alonganother of his PSA-65C portable AC/battery spectrum analyzers with PCinterface for frame-grabs, and a 10kHz resolution option. (I’m awaitingthe newest version with built-in LCD screen.) The Samsung SIR-T150provided the reception capabilities, and a Channel Master #7775 UHFpreamp was available to boost signal levels as needed.
At Jon’s House. Right off the bat, the strongest signals werethe easiest to receive with minimal antennas. The Radio Shack bow tiewas clipped to a camera tripod, and KYW’s signal displayed on a Loewewidescreen television. The open-wire line from the bow tie to the balunwas lying alongside the metal legs of the tripod, yet reception didn’tseem to be a problem. I should mention that the windows here hadvertical iron security bars on them, resulting in a virtual Faradaycage shielding the antenna. Now see Figure 1, which comparesKYW-26 and WCAU-67 received on the bow tie with the preamp. WCAU’scarrier is almost 20 dB weaker than KYW’s, yet both came in rock-steadywith this setup. I should point out that KYW-26 was also received withthe bow-tie antenna and no preamp.
I saw similar encouraging results with the amplified antenna, SilverSensor, and 3-element yagi. Antenna positions were similar in mostcases, except for the bow tie, because one of the stations (WTXF-42)transmits from a slightly different location. Two local stations— WHYY-55 and WPVI-64 — were not receivable under anycircumstances, due to a combination of low signal levels and highmultipath.
On to New York!
Once I had all the equipment up and running at Mark’s apartment, Iquickly detected and locked up a signal from WCBS-56 with just the bowtie and no preamp. There were similar results with the rest of theantennas, and in more than one location. Most fittingly, thetraditional placement on atop the television worked just fine forWCBS-56.
WNYW-44 was a different story. Its signal peaked near the floor, oron top of a nearby bookshelf near the ceiling. This difference may beattributable to the different antennas used by both stations. WCBS’santenna is on the north side of the Empire State mast, while WNYW’santenna sits at the very top. I have noticed a slightly differentradiation pattern for each out my way in Doylestown, with WNYW slightlystronger than WCBS.
Even so, WNYW-44 was received with the UHF bow tie and the preamp bydangling the bow tie about a foot off the floor. The 15-1862 amplifiedantenna also picked up both stations from the floor, but required amove of about 12 inches and a 90° rotation of the UHF element.While I could have used the analyzer to peak up the waveforms for thebest signal, I often just rotated the antenna slowly until theSamsung’s green LED lit up. The resulting waveforms didn’t look toogood, but the Samsung equalizer was up to the job. The waveforms foreach station as received by the bow-tie antenna and SIR-T150 are shownin Figure 2, which compares WCBS with no preamp and WNYW withpreamp.
From the Home Office
Additional tests in my office supported what I’d seen at the otherlocations. Low-level DTV signals were locked up with minimal antennasand no preamp. Figure 3 shows the waveform for WPVI-64 asreceived with the Radio Shack bow tie by itself. Note that the signalappears to be about 5 dB carrier to noise. Applying the correctionfactor results in a signal level about 21 dB C/N, or -79 to -80 dBmoverall.
THIS YEAR’S CONCLUSIONS
The first, obvious observation is that DTV receivers are gettingbetter. For those people who don’t have DTV available over cabletelevision and aren’t equipped for DBS, it may be practical to get DTVby using nothing more complicated than a late-model set-top box andcompact antenna.
Second, it doesn’t take much signal to lock up these tuners. TheATSC standard of 15.3 dB C/N is applied to a pure, theoretical GaussianRF environment, not a real-world environment with impulse and othernoise. Still, you can see that signal levels didn’t need to betremendously strong in order to enable reception. The catch is thatsignals must be high enough so that any notches in the signal caused bymultipath don’t fall into or below the RF noise floor. (No equalizer isthat good.)
Third, any antenna can be called a digital TV antenna. Somemanufacturers are going overboard, marketing and labeling theirproducts “tested and approved for HDTV,” while others aretrying to convince consumers that special designs are required for thereception of DTV. Neither type of hype is true. With enough signalstrength and a good tuner, a $3 antenna does the trick, even in somepretty tough environments.
Antennas are format- and modulation-agnostic anyway. A good antennafor directional reception of VHF or UHF signals will also work for DTVsignals. What’s needed is a good impedance match over a wide range offrequencies, plus good front-to-back gain ratio and good sideloberejection performance. My first attempt at such an antenna delivered onall counts and was built with about $10 worth of hardware. Priceyantennas don’t mean a thing when it comes to DTV reception.
Finally, the ATSC 8VSB system works, maybe not as well as some folkswould like, but certainly enough that it isn’t “doomed tofail,” as many pundits were claiming a year ago. While I have notbeen able to receive all 8VSB signal in any location atall times, my success rate has been better than 75%. When I’venot had success, it’s usually due to extremely low signal levels, andcomparable reception of analog UHF signals has also been abysmal duringthese tests.
Peter H. Putman owns PHP Communications, Doylestown,Pennsylvania. He is the author of The Toastmasters Guide toAudiovisual Presentations and reviews large-screen video displays,scalers and computer/video interfaces.
RUNNING WHERE THE BRAVE DARE NOT GO:
How the testing went down
The tests in each location proceeded in this order:
- Using the Radio Shack UHF bow tie, without and with the UHFpreamp
- Using a Radio Shack 15-1862 amplified VHF/UHF antenna
- Using the Antiference Silver Sensor UHF log-periodic antenna withoutand with the preamp
- Using the homemade 3-element yagi design without and with thepreamp.
The spectrum analyzer was set up with a notebook PC to exportreal-time screen displays of analog and digital waveforms. The Avcomsoftware allowed for freezing the waveform at any time and exportingthe screen (plus all of the settings) as a bitmap file. The first stepwas to establish the noise floor at each location (typically -96 to -98dBm), then look for and optimize any 8VSB waveforms for strongestsignal by moving the antenna as needed.
The average consumer will not have access to a spectrum analyzer oreven to a signal strength meter. But they will have access to theSIR-T150, and its tri-state red-orange-green LED system works like acharm for aiming antennas. In many cases, that’s all we used to pick upthe signal, and we used the spectrum analyzer only to confirm our firstinstincts.
Technical Note: Depending on the resolution bandwidth (RBW) settingof the analyzer, digital signals will not appear to be as strong asanalog TV signals. A correction factor must be made to accuratelyinterpret the waveforms shown in this article. Since the PSA-65C has anRBW of 150 kHz in the 5MHz span setting, add in about 16 dB todetermine the actual signal strength. Remember that the ATSC standardcalls for a carrier-to-noise ration of 15.3 dB for successful receptionof 8VSB carriers.
For Those Who Just Can’t Get Enough
Peter H. Putman has prepared three reports in Word formatwith captured waveforms and photos of the antenna setups. These reportsare available on a CD-ROM for $30 postpaid (check or money order only)by writing to:
c/o ROAM Consulting Inc.
200 North Street, Suite D
Doylestown, PA 18901