INDOOR 8VSB RECEPTION:
I've Seen The Holy Grail
by Peter H. Putman, CTS HDTVexpert Home
Regular readers of this column may recall the December 2000 edition, in which I detailed my efforts at improving antenna designs for indoor reception of 8VSB digital television signals. At the time, I was nearing the end of a year-long exploration of 8VSB propagation, both indoors and outdoors. I had accumulated hundreds of spectrum analyzer screen captures and photos; torn apart and rebuilt dozens of antennas, and had surprising results with reception of digital TV signals in previously "difficult" locations.
That December 2000 column wrapped up with this statement: "In short, it is possible to receive 8VSB signals indoors with multipath (or outdoors with multipath, for that matter). It does take a three-way effort between the antenna, the DTV receiver's equalizer, and the DTV receiver's 8VSB demodulator. All of these pieces of the puzzle have to be improved, and they will with time."
It's now exactly one year later, and there have been some interesting developments to communicate. While my tests proved that a good antenna for UHF DTV reception need not cost an arm and a leg, I had also wondered when I would see significant improvements in the performance of DTV set-top boxes.
At Winter CES 2001, Samsung introduced an all-new, over-the-air (OTA) ATSC DTV receiver known as the SIR-T150. This new box incorporated many Broadcom receiver chips, and was designed with improved sensitivity, greater equalization performance, and a super-simple user interface in mind. Although it didn't support DBS, it looked promising for OTA reception (as well as 8VSB cable reception).
I got one of the first 11 boxes shipped into the U.S. for some extensive testing, and right away it was obvious that the SIR-T150 was a "hot" receiver. In side-by-side comparisons with older STBs from Panasonic and RCA, the SIR-T150 had on average 6 dB more sensitivity to signals than either of the first-gen STBs. It also locked up much faster on "valid" 8VSB carriers - sometimes in as little as 1.5 seconds.
Further tests of the SR-T150 took it to Hollywood, New York City, and Philadelphia. Software versions were upgraded a few times, and a bizarre problem with PSIP (Program and System Information Protocol) on PBS stations needed to be debugged. I found this tuner sensitive enough to watch DTV stations from as far away as New York City (64+ miles) with no drop-outs, day or night.
As spring wound into summer, a full complement of DTV signals came on-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 and WCBS-56 from the Empire State Building, were rock solid. Add to them the five Philadelphia stations, two Allentown stations, and one Trenton, NJ station I already received, and I could choose from 15 different DTV carriers for antenna testing over a variety of terrain.
In the 12/00 column, I mentioned my trip to the famous "Schubin Site", the apartment of broadcast/video trade magazine columnist Mark Schubin located on the upper west side of New York City. This site is a "black hole" for indoor TV reception, and tests by numerous engineers and industry groups had not resulted in reliable reception of any OTA DTV stations.
During my brief visit, I had some success in locking up one of the two NYC stations on Mark's RCA DTC-100 set-top box (1st generation). But time ran out before I could finish analyzing the signal from WCBS-56 and get reliable reception of its signal. So, I decided to make a return trip in early August of this year - then postponed that trip until the middle of September, when my schedule would have more free time.
Of course, we all know now what happened on September 11, 2001. The collapse of the twin towers of the World Trade Center not only took five DTV stations (and numerous analog TV and FM stations) off the air, it also killed and injured more than 6,000 people. My tests seemed somewhat trivial at this point, so I postponed them for several weeks.
As September turned into October and we all tried to get back to our normal lives, I thought that a trip to Mark's place might still be worthwhile, even though I'd have only two DTV stations to test - the original two I tried to receive indoors last October. So much had changed since then with the equipment - I had the latest off-the-shelf version of the Samsung tuner (v1.63 software) and had fabricated my experimental antenna into a more finished version.
Not only that, I thought it would be fun to try several commercially-made antennas, starting with a $2.99 Radio Shack UHF bow tie. The success of the ATSC 8VSB DTV system would really hinge on how easy it would be to receive with basic antennas by the average consumer - not guys like me who were armed with a signal strength meter and two spectrum analyzers.
To get the most mileage from my tests, I also scheduled a visit to the Philadelphia home of columnist Jon Takiff, who writes about music and consumer electronics for the Philadelphia Daily News and Philadelphia Inquirer. His first-floor location presented an equally touch challenge for indoor 8VSB reception with low signal levels and lots of multipath.
They say "three's the charm", so I rounded out my field tests with a series of measurements made in my own office. This location, about 23 miles north of the Philadelphia TV antenna towers, is shadowed to the south by a ridge running through town that is about 50' higher than my office. Signal levels are "okay to weak" here, with lots of multipath.
The test equipment line-up was pretty basic. Pete Gaglio of AVCOM sent along another of his PSA-65C portable AC/battery spectrum analyzers with PC interface for frame-grabs, and a 10 kHz resolution option. (I'm awaiting the newest version with an LCD screen!) The Samsung SIR-T150 provided the reception capabilities, and a Channel Master #7775 UHF preamp was available to boost signal levels as needed.
Figure 1
The tests in each location proceeded in this order: (1) Using the Radio
Shack UHF bow tie, without and with the UHF preamp; (2) using a Radio
Shack 15-1862 amplified VHF/UHF antenna, (3) using the Antiference Silver
Sensor UHF log-periodic antenna without and with the preamp, and finally
(4) using my 3-element yagi design without and with the preamp.
The spectrum analyzer was set up with my notebook PC to export real-time screen displays of analog and digital waveforms. By using the AVCOM software, I could freeze the waveform at any time and export the screen (plus all of the settings) as a bitmap file. The first step was to establish the noise floor at each location (typically -96 to -98 dBm), then look for and optimize any 8VSB waveforms for strongest signal by moving the antenna as needed.
Now, the average consumer won't have access to a spectrum analyzer, or even a signal strength meter. But they will have access to the SIR-T150, and its tri-state red-orange-green LED system works like a charm for aiming antennas. In many cases, that's all I needed myself to pick up the signal, and the spectrum analyzer just confirmed my instincts.
Technical note: Depending on
the resolution bandwidth (RBW) setting of the analyzer, digital signals
will not appear to be as strong as analog TV signals. A correction factor
must be made to correctly interpret the waveforms shown in this article.
Since the PSA-65C has an RBW of 150 kHz in the 5 MHz span setting, add
in about 16 dB to determine the actual signal strength levels. Remember
that the ATSC standard calls for 15.3 dB C/N for successful reception
of 8VSB carriers.
Figure 2
Figure 3
Right off the bat, the strongest signals were the easiest to receive with
minimal antennas. Compare Figure 2 and Figure
3, which show KYW-26 and WCAU-67 received on the bow tie with
the preamp - WCAU's carrier is almost 20 dB weaker than KYW, yet both
came in rock-steady with this set-up. I should point out that KYW-26 was
also received with the bow tie antenna and NO preamp.
Figure 4
Figure 4 shows the receiving set-up, with the bow tie clipped
to a camera tripod and KYW's signal displayed on a Loewe widescreen TV.
The open-wire line from the bow tie to the balun was lying alongside the
metal legs of the tripod, yet reception didn't seem to be a problem as
a result. I should mention that the windows seen have vertical iron security
bars on them, resulting in a virtual Faraday cage shielding the antenna.
Similar encouraging results were seen with the amplified antenna, Silver Sensor, and 3-element yagi. Antenna positions were similar in most cases, except for the bow tie - but that's because one of the station received (WTXF-42) transmits from a slightly different location. Two local stations - WHYY-55 and WPVI-64 - were not receivable under any circumstances, due to a combination of low signal levels and high multipath.
On to New York! Once I had all the equipment up and running at Mark's apartment, I quickly detected and locked up a signal from WCBS-56 with just the bow tie and no preamp. Similar results were had with the rest of the antennas, and in more than one location. Most fittingly, the traditional placement atop the TV worked just fine for WCBS-56.
WNYW-44 was a different story - its signal peaked near the
floor, or atop a nearby bookshelf near the ceiling. This difference may
be attributable to the different antennas used by both stations. WCBS'
antenna is on the north side of the Empire State mast, while WNYW's antenna
sits at the top. I have noticed a slightly different radiation pattern
for each out my way in Doylestown, with WNYW slightly stronger than WCBS.
Figure 5
Figure 6
Even so, WNYW-44 was received with the UHF bowtie and the preamp by dangling
the bow tie about a foot off the floor. The 15-1862 amplified antenna
also picked up both stations from the floor, but required a move of about
12" and rotating the UHF element 90 degrees. Figures 5 (WCBS,
no preamp) and 6 (WNYW, preamp) show the waveforms for
each station as received by the bow tie antenna and SIR-T150.
Figure 7
Figure 7 shows the placement of the bow tie for WCBS reception,
while Figure 8 shows the placement of the bow tie on
the floor for WNYW reception. Again; while I could use the analyzer to
peak up the waveforms for best signal, I often just rotated the antenna
slowly until the Samsung's green LED lit up. The resulting waveforms didn't
look too good, but the Samsung's equalizer was up to the job.
Figure 8
Additional tests in my office supported what I'd seen at the other locations.
Low-level DTV signals were locked up with minimal antennas and no preamp.
Figure 9 shows the received waveform for WPVI-64 as received
with the Radio Shack bow tie by itself - note that the signal appears
to be about 5 dB C/N. Applying the correction factor results in a signal
level about 21 dB C/N, or -79 to -80 dBm overall. Figure 10
shows WPVI's signal on my Princeton monitor.
Figure 9
Figure 10
So - what have we learned? First of all, DTV receivers are getting better.
For those people who don't have DTV available over cable TV, or aren't
equipped for DBS, it may be practical to get "free" DTV in your
home, apartment, or other dwelling by using nothing more than a late-model
set-top box and compact antenna.
Secondly, it doesn't take much signal to lock up these tuners. The ATSC standard of 15.3 dB CN is applied to a pure, theoretical Gaussian RF environment, not a more real-world environment with impulse and other noise. Still; you can see that signal levels weren't tremendously strong in order to enable reception. The catch is, signals must be high enough so that any notches in the signal caused by multipath don't fall into or below the RF noise floor. (No equalizer is that good!)
Third, any antenna can be called a "digital TV antenna". Some manufacturers are going overboard with marketing and labeling their products "tested and approved for HDTV", while others are trying to convince consumers that special designs are required for the reception of DTV. Not true! With enough signal strength and a good tuner, a $3 antenna does the trick, and does it in some pretty tough environments.
Antennas are format- and modulation-agnostic anyway. A good antenna for directional reception of VHF or UHF signals will work fine for DTV signals. What's needed is a good impedance match over a wide range of frequencies, plus good front-to-back gain ratio and good sidelobe rejection performance. My first attempt at such an antenna delivered as promised and was built for $10 worth of hardware store parts. Pricey antennas don't mean a thing when it comes to DTV reception.
Finally - the ATSC 8VSB system works. Maybe not as well as some folks would like, but it certainly isn't the "doomed to fail, tragic mistake" many pundits were claiming a year ago. While I have not been able to receive any 8VSB signal in any location at any time, my batting average has been better than 75%. When I've not had success, it's usually due to extremely low signal levels - and comparable reception of analog UHF signals has also been abysmal during my tests.
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Interested in seeing more details? I have prepared three reports in Word format with captured waveforms and photos of the antenna set-ups. These reports are available on a CD-ROM for $30 postpaid (check or money order only) by writing to me c/o ROAM Consulting, Inc., 200 North Street, Suite D, Doylestown, PA 18901.