The DTV reception debate
Aug 1, 2003 12:00 PM
DTV Reception
Dear Mr. Arland,
In Broadcast Engineering magazine’s June 30 issue of the
e-newsletter,
Beyond the Headlines, you are quoted as saying that
broadcasters need to move to high power now in order for consumers to
take advantage of new, more sensitive tuners developed by Thomson. You
blame poor over-the-air DTV reception on broadcasters who have a
“‘lack of commitment’ to full-power
broadcasting,” and cover the improvements Thomson has made to
increase the reception selectivity of its recent DTV tuners. That
you’re planning on utilizing multipath cancellation technology
from LINX Electronics in future generations of digital TV receivers is
good news across the board.
 |
“Notwithstanding these advances in receiver capabilities and performance, accurate and robust reception is inevitably and directly related to the strength of the signal transmitted by the broadcaster,” Arland continued. “Regrettably, most local broadcasters are not transmitting their digital TV signals at full power. In fact, the commission’s most recent figures indicate that only 25 percent of commercial broadcast stations are ‘on-the-air’ with a digital television transmission signal that covers their analog station service areas. This raises the prospect that a very significant number of homes that receive a station’s analog signal cannot receive that station’s digital signal.
You should probably research your claims about current levels of reception more thoroughly, however. I have done extensive testing of terrestrial DTV signal propagation in all kinds of terrain: in canyons, in apartments, through basement walls, over substantial hills, and in many places where I was told there would be no reception. Lo and behold, I received signals in many of these locations, even though they were not all that strong. Reception does not depend on stations broadcasting at full power. The trick is to make sure multipath is handled better. If a weak signal has severe multipath echoes that drop below the receiver’s noise floor, nothing will be picked up. But if multipath is moderate, you would be amazed at how well 8-VSB signals get out. I know – I have hundreds of spectrum analyzer plots and digital photos as documentation.
You said that many homes that are able to receive a station’s analog signal would not be able to receive a digital signal if that station is broadcasting at lower power. I would propose the opposite hypothesis: In many cases, the homes and apartments where I tested DTV reception had marginal to poor analog TV reception. Yet, once they locked up the DTV station, the picture quality was as good as encoded.
This is a common misconception about a ‘threshold’ DTV system. The carrier-to-noise levels don’t need to be as high as they do for analog. In many cases, the problems with reception have nothing to do with RF — they have more to do with decoding PSIP, and broadcasters encoding PSIP and MPEG correctly.
Thomson would be better advised to take a strong corporate stance that broadcasters should be sending our correct PSIP info, including System Time Table, Virtual Channel Table, and Master Guide Table info. Many broadcasters don’t do this, and PSIP is what makes DTV more user-friendly. The FCC doesn’t even mandate PSIP, which is a mistake, in my opinion.
Many of the emails I get talking about DTV reception problems turn out to be PSIP-related problems (PMT conflicts, PID conflicts, etc) and not RF at all. You may have problems with store returns of 8VSB set-top boxes as a result. The tuner may actually be picking up a signal, but cannot decode the PSIP stream correctly or is finding PID and table conflicts.
I’m not providing a justification for broadcasters staying off full power, but even with lower power levels these signals get out much better than most would expect. And in some markets, going to high power means raising the carrier level perhaps 3-6 dB at best. That can easily be made up in an antenna system with a low-noise preamp.
“The availability of only a low-power signal can significantly hamper the ability of any terrestrial receiver-regardless of input sensitivity-to properly receive, tune, and decode digital TV signals. For example, with the prevalent use of low-power TV transmitters, there are instances where the adjacent channel interference ratios will be well in excess of planning factors used to design the receiver components. We designed our receivers assuming, appropriately, that broadcasters would fully engage in their responsibilities in this transition and send signals at full strength in compliance with the transition plan embodied in FCC regulations.” I have done adjacent channel tests with older Panasonic STBs and found no interference problems even with adjacent channel 8VSB carrier variations as great as 20 dB. That would be equivalent to a carrier from a 500 kW station next to a 5kW station. In this case, both stations (WPHL-54 and WHYY-55) are operating at full licensed power, There just happens to be about a 20 dB differential in received signal strength at my location.
Suffice it to say that current generation STBs do a whole lot better in this regard.
The #1 problem with receivers is the use of cheap components in the Front-end RF stages and mixer. Compression and poor third-order intercept performance here will doom any receiver. The best bet is to use a moderate gain, low-noise FET first RF stage driving a passive diode ring mixer, which has the highest dynamic range of any receiver front end. Such a set-up ought to have a noise figure under 3 dB and a 1db compression point in excess of 0 dBm.
I would be more than happy to put your RCA set-top boxes through many of the same terrestrial DTV reception tests I have conducted on Panasonic, Sony, Samsung, and Zenith STBs.
Arland told the FCC that the issue is further exacerbated by spurious transmitter radiation.
How do you know this for a fact? Has RCA analyzed the spectral output of many terrestrial broadcasters to verify spurious emissions? ‘I’d like to see the results of those tests, if they were indeed conducted. Perhaps you are alluding to problems with MPEG jitter on an encoded signal? Or adjacent channel interference due to poor IF filtering or compression of RF amplifier and mixer stages? “Thus, the suggestion by many broadcasters that ‘insensitive receivers’ are somehow to blame for poor consumer reception of digital TV signals misses the real problem, which, Thomson respectfully suggests, lies not with receiver sensitivity but rather by a lack of commitment of the broadcasting community to transmit their digital TV signals at full power,” Arland said.
If broadcasters are ‘guilty’ of anything in the DTV transition, it is not paying attention to PSIP and MPEG encoding issues. A broadcaster who does not implement even static PSIP correctly is shooting his or her digital channel in the foot. Manufacturers should be jumping all over this issue as it can affect future sales of terrestrial STBs.
Thomson is suggesting that the FCC establish an interim deadline of July 1, 2004 (concurrent with the first digital tuner/decoder deadline), by which all broadcasters must transmit a digital signal of sufficient strength to serve their entire Grade A contour.
The received signal is WCDC-36, broadcasting at low power from Mt. Greylock in western Massachusetts. It was picked up off the roof at Plasmaco in Highlands, NY (about 80 miles) and from the rear deck of my brother’s house in Saugerties, NY (about 55 miles). A simple suburban UHF yagi was used with a Channel Master off-the-shelf UHF preamp. Both locations are well outside the predicted DTV contour for WCDC (there are no “A” or “B” contours for ATSC, since it is a threshold system), and yet the station is running less than half power.
In other words, more viewers than predicted can watch ABC HDTV programming via WCDC digital (a sister station of WTEN in Albany, NY), even with the station on low power. Most of those viewers would have a tough time at that distance with WCDC’s analog channel on 19. In fact, my brother in Saugerties, who has terrible off-air analog TV reception where he lives (except for two local independent UHF stations), can now opt to watch something he cannot currently receive on cable - network HDTV programming, and he can get it for free starting this fall when WCDC encodes 720p.
If your STB design engineers would like to discuss this further, I’d be happy to oblige.
Best,
Pete Putman
Response
Dear Pete,
If every market in America was like Philadelphia (or even Indianapolis), there would be no reason to point out shortcomings.....because there wouldn’t be any!
We stand by our technical report and recommendations made in our submission. We HAVE been beating the drum for an FCC requirement on broadcasters to use the A/65 PSIP standard for many years. Still — the biggest issue in this transition has NOTHING to do with broadcasting. It’s cable.
The average consumer will expect HDTV to work with cable.
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- Dave Arland
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