Having worked on HDTV and DTV to one degree or another since the late 1980s, it may or may not surprise the reader to learn that I bought my first HDTV set only some 2 1/2 years ago.
At that time, I installed an 8-bay UHF-only rooftop antenna. Although I live in the foothills of the San Gabriel Mountains in Los Angeles, by some stroke of good fortune my antenna has a good "look" at the transmitting antennas on Mt. Wilson, some 20 miles away as the crow flies.
Prior to the analog shutoff, all the major Los Angeles DTV stations were on UHF channels. At that time, I received signals from all DTV stations except WABC-DT, 7.1, which was on RF channel 53, and KCOP-DT, 13.1, which was on RF channel 66. A check with a spectrum analyzer revealed absolutely no signal from either of these channels, although there were robust signals from the channels adjacent to them.
I can only speculate that the physical location of the transmitting antennas in question somehow "shielded" their signals from my receiving antenna. As I could receive an almost-watchable picture from KABC-TV analog channel 7 on my UHF-only antenna, I held hopes that once the analog signal was shut off and KABC-DT moved to RF channel 7, I would be able to receive it off-air. More on this later.
IN THE OLD DAYS
Way back in the antediluvian past, the UHF band was designated for television broadcast use at least partially for the purpose of experimentation with color television. Back then, there was little pressure for the use of RF spectrum, particularly the ultra-high frequency spectrum, and the original UHF TV broadcast band ranged from channel 14 to above channel 80. As other services began coveting UHF spectrum, the UHF broadcast band shrank until, when the temporary DTV allocations were assigned, it stopped with channel 69.
In the early HDTV/DTV days, there was a strong interest in some quarters in putting all advanced television broadcasting on UHF channels. In Europe and other parts of the world, PAL and SECAM television broadcasting had been, for its history, confined to UHF channels. Initially, the U.S. "core spectrum"—that spectrum to which all television broadcasting would be relegated after the analog shutoff—was to be channels 14–59, completely in the UHF band. This got twisted around several times with the resulting core spectrum ranging from channels 2–51.
The dreams of the supporters of moving U.S. TV broadcasting fully into the UHF band have been shattered, and many old, powerhouse NTSC broadcasting stations have put their digital signals on their former analog RF channels. This has not been an unmitigated success.
|Although the author lives in the foothills of the San Gabriel Mountains in Los Angeles, his antenna has a good "look" at the transmitting antennas of Mt. Wilson, some 20 miles away as the crow flies. |
Low-band VHF stations had been coveted in the analog era because of their long reach, over the radio horizon in many cases, but even in the NTSC days this was not an unmixed blessing.
Old salts will be familiar, for example, with the precise frequency offset used to minimize interference between, for example, WNBC-TV, channel 4 in New York, and its relatively short-spaced sister station, WRC-TV, channel 4 in Washington D.C., whose signals routinely interfered with each other despite the 200 or so miles separating them.
Precise frequency offset was a clever scheme that "interleaved" the two channel 4 signals, minimizing the visible interference between them.
Many are also familiar with the "tunneling" of VHF signals from great distances, sometimes many hundreds or even thousands of miles, caused by certain atmospheric conditions. Add to this man-made impulse noise from a multitude of sources ranging from refrigerator motors to car engines to who-knows-what. This was bad enough in the analog days, but the digital age has added the FCC imposition of radiated power levels too low to overcome it. Then factor in the problems many analog TV set tuners had with low-band VHF signals.
The big story from the analog shutoff has been that when a number of U.S. television stations shut off their analog signals and turned on their DTV transmitters on their former analog VHF RF channels, many of their viewers lost their signals. There are several reasons for this.
VHF DTV power levels are, in some cases, too low to overcome impulse noise (a show-stopper for a digital signal) and occasionally too low to propagate far enough in difficult terrain. Also, much of the viewing public got the notion (not very effectively countered by broadcasters) that a UHF-only antenna was all that was required for DTV reception.
In fact, many DTV stations are now on VHF channels, and not a few of them are on low-band VHF channels. This largely changed the landscape of DTV broadcasting in June.
Returning to your writer's personal experience, it was mixed, but generally more positive than before the shutoff.
I do receive KABC-DT now that it has moved to RF channel 7, and also KCOP-DT now that it is on RF channel 13. However, I have lost KTTV-DT, formerly on RF channel 65, now on RF channel 11. Both KTTV-DT and KCOP-DT operate with effective radiated powers of 13 kilowatts. Both have applied for power increases to 100 kilowatts.
IT'S ALL ABOUT THE POWER
The power issue has been a problem elsewhere. A number of stations, including, for example, WPVI-DT in Philadelphia, which is on RF channel 6 and WABC-DT in New York, on RF channel 7, have also requested power increases.
The power issue is, of course, somewhat complex. RF propagation is affected by frequency, effective radiated power (ERP), antenna height, terrain, and a slew of other factors.
In the case of KTTV-DT and KCOP-DT in Los Angeles, both are high-band VHF stations, operating from Mount Wilson, with similar antenna heights above average terrain (HAAT). When these stations were broadcasting analog NTSC, both had ERPs of 161 kilowatts peak. They now both have DTV average ERPs of 13 kilowatts. Granted, the characteristics of analog and digital TV signals are vastly different.
The analog signal has many peaks and valleys and its amplitude varies constantly with average picture level, giving it a variable, high peak-to-average ratio. Analog radiated power is specified at peak of sync.
The digital signal's peak-to-average ratio is a fixed, statistical property of the digital signal, about 6–7 dB. DTV radiated power is specified as average power. This would imply that a digital signal requires a much lower average effective radiated power (AERP) than an analog signal's peak radiated power to deliver equivalent coverage on a given RF channel.
In the case of KTTV-DT and KCOP-DT, the difference between analog and digital power is nearly 11 dB. If these stations were to increase their powers to 100 kilowatts it would bring the difference to 2 dB.
In 1992, NBC, along with a consortium of several other companies, was a proponent of a DTV broadcasting system. In a demonstration, this digital signal was transmitted from a rather low-mounted antenna on the WRC-TV tower in Washington, D.C., at an ERP of about 5 kilowatts. If memory serves, this signal was on a channel in the 40s. The signal was received and recorded at various locations in the exurban Washington, D.C. area, many of which were on Maryland's Eastern Shore.
The farthest distance from the tower at which a successful recording was made was about 76 air miles, verified with LORAN, using a mere 5 kilowatts AERP (we wanted 100 miles, of course). Granted, we picked our reception spot very carefully. And this experiment was far from being representative of real-world broadcasting to a large metropolitan population.
My personal experience after the analog shutoff was a net gain: gained two, lost one. But many stations lost many viewers in June, and efforts are underway to rectify this problem.
Randy Hoffner, a veteran of the big three TV networks, is a senior consulting engineer with AZCAR. He can be reached through TV Technology.