Choosing a Final DTV Channel

This month, I'll look at some of the things to consider when deciding on a DTV channel, and also relay some information I received from readers about low VHF DTV.
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If you have been following my articles, you know that TV stations may soon be asked to choose the channel they will keep when analog broadcasting ends.

At this time it appears that date could be as soon as Jan. 1, 2009, less than five years away. This month, I'll look at some of the things to consider when deciding on a DTV channel, and also relay some information I received from readers about low VHF DTV.

CHOOSING A CHANNEL

Some channels can be eliminated immediately. Channels above 51 have already been allocated to other services. It also appears Channels 2 through 4, and likely Channel 5 as well will not be suitable for DTV in most markets due to antenna size and electrical interference. Channel 5 may work better, but if broadcasters avoid other low VHF channels, there may be little incentive for people to erect antennas for Channel 5. Channel 6 will probably be unsuitable for DTV in most areas due to FM radio interference.

This leaves Channel 7 through 13, and UHF Channels 14 to 51, excluding Channel 37. In larger markets, some UHF channels between 14 and 20 are reserved for land-mobile uses, including public safety, and can't be used.

If your analog station is on a high VHF channel and your DTV is on a UHF channel, you have probably thought about keeping the high VHF channel for DTV at the end of the transition. Before you give up the UHF channel, it is worth doing a study to see how interference may affect coverage. This is because the desired-to-undesired interference ratios are different for analog-into-DTV than DTV-into-DTV.

A CO-CHANNEL SCENARIO

Let's look at what would happen to two VHF co-channel stations, one analog and one digital.

Under FCC OET Bulletin 69, if the peak analog signal is more than 2 dB below an average DTV signal on the same channel, the DTV signal receives no interference. However, if that analog station were to switch to DTV on the same channel, its signal level has to be more than 15 dB below that of the co-channel DTV signal to avoid interference, a drop of 13 dB from when it was transmitting an analog signal. The drop in maximum ERP from a 316 kW peak to 100 kW average accounts for 5 dB of this, but another 8 dB will be needed to avoid interference to the co-channel DTV station, reducing DTV ERP down to 16 kW!

This is only an example of how a station could be limited in ERP if it switches to digital on the same channel it used for its existing analog operation. A real-world analysis would be much more complex and would have to take into account factors such as the number of people predicted to receive interference inside the first DTV station's protected contour.


(click thumbnail)Fig 1. Contours of all authorized stations, excluding translators and non-Class A low-power stations, on VHF Channels 7 (blue) 8 (red) and 9 (green) in the Northeastern U.S.Also, an FCC OET 69 analysis may show interference from other stations is masking interference from the co-channel station, allowing more power.

To get an idea how congested TV channels are in the Northeast, I decided to plot the contours of all authorized stations, excluding translators and non-Class A low-power stations, on VHF Channels 7, 8 and 9 and UHF Channels 29, 30 and 31 in the northeastern U.S. The results are shown in Fig. 1, VHF and Fig. 2, UHF.

The contours show the Grade B (56 dBu at VHF, 64 dBu at UHF) coverage for analog stations and the noise-limited threshold coverage for DTV (36 dBu at VHF, 41 dBu at UHF). The contours for Channels 7 and 29 are shown in blue and Channels 9 and 31 in green. I used red for Channel 8 and Channel 30 because other channels are adjacent to them.

(click thumbnail)Table 1. VHF Channels
For more information on the stations shown, see Table 1 for the VHF channels and Table 2 for the UHF channels. This data was taken from the ComStudy database extracted from the CDBS files and while the technical data is current as of July 1, 2004, the call letters may not be up to date. The FCC does not update call letters in the TV engineering database table as frequently as in other CDBS tables.

When I was preparing these maps, I saw complications MSTV is dealing with in trying to come up with an optimized final DTV table of allotments.

There were many applications pending, especially near the Canadian border. In addition to pending applications, some stations had multiple authorizations including STAs, construction permits, licensed facilities, auxiliary facilities and, of course, the original allocation. Many Canadian allocation listings lacked technical data. You will notice that no Canadian DTV channels are shown in the table or on the map.
(click thumbnail)Fig 2. Contours of all authorized stations, excluding translators and non-Class A low-power stations, on UHF Channels 29 (blue) 30 (red) and 31 (green) in the Northeastern U.S.


(click thumbnail)Table 2. UHF ChannelsOne thing that should be clear after looking at these maps is that broadcasters will have to work together if they want to maximize coverage. One station's channel election can have an impact on multiple surrounding stations, creating a ripple affect that could go well beyond its coverage area as surrounding stations change their choice based on other stations' choices.

The MSTV plan for DTV channel elections provides a way for this to happen. See http://www.mstv.org/ and the sidebar in my June 9 TV Technology column for more information.

While I saw some limitations in it, especially for stations that weren't able to maximize power due to interference constraints that would go away after analog ends, this structured approach is clearly better than the chaos that will result if each station has to make an independent final DTV channel election, then try to modify it if it causes or receives excessive interference from another station's DTV channel choice.

LOW VHF CHANNELS

WBBM-DT on Channel 3 in Chicago continued to generate a lot of comments, including some from CBS engineers. The two big problems people mentioned when talking about low VHF reception were picture break-up due to impulse noise and the need for a large outside antenna.

Jim Steffey, director of engineering for WWMT-DT, Channel 2 in Kalamazoo, Mich., said that even though the station is operating at its full authorized power, "During thunderstorms, the signal becomes totally unwatchable with frequent pauses in the audio and heavy pixelation and picture freezes."

Power line noise is also a problem. He noted that upper VHF and UHF DTV stations in the same market can be received "without any trouble at all, with not much more than a coat hanger for an antenna."

George Morisette in DeKalb, Ill. ,wrote that as a broadcast engineer, he's always had an antenna farm on his roof. With his array of antennas, filters and amplifiers, he can get good reception of the Rockford analog stations and fair reception of the Chicago analog stations, with low VHF analog "noticeably noisy anywhere further than 40 miles out" and "just about unusable here."

When he hooked up his first DTV receiver, he got reliable reception of all but Fox stations from both cities on UHF, but WBBM-DT on Channel 3 was non-existent.

Larry Davis, the transmitter supervisor at WBBM-TV/DT, said that the signal is receivable in the market, provided you have an outdoor antenna. The need for an outdoor antenna to receive low VHF when an indoor Silver Sensor works fine on the other channels is a problem.

However, he noted that noise is a problem at more distant sites where the signal is weaker and people are used to using outdoor antennas. Viewers have no idea what's going on when the picture drops out randomly due to impulse noise. He confirmed that WBBM-DT is operating at full power-3.7 kW at 1,500 feet.

Derek Justmann, an antenna engineer with Winegard, said his company measured the VSWR performance of Winegard's SquareShooter antenna. He explained that Winegard uses A/B port measurements in a range system instead of a conventional network analyzer one-port S11 measurement. Calibrated S11 network analyzer measurements fail to take into account the reflections in the down-lead transformer module to the actual antenna elements, resulting in inaccurate measurements unless a specially designed test fixture is used. He said the average UHF band VSWR was 1.53:1. For VHF high-band Channels 8 and 9, the VSWR was measured at 3.0:1 and at Channel 3, it was 7.0:1.

Justmann said, "It is unfortunate that a physically large antenna is required for well-matched reception of low-band channels, which as of late are considered eyesores by a good portion of the general public."

In the real world, he said DTV consultant Steve Zahn found the SquareShooter could receive WBBM-DT, and all other Chicago DTV signals, about 20 miles from downtown Chicago. Zahn, a professional who used a DTV field strength meter to find the best location and orientation for the antenna, installed it himself. Gain measurements at Winegard found the antenna does meet the gain standards for digital VHF reception in the CEA's yellow map at www.antennaweb.org

Pete Putman, a DTV consultant who has a wealth of information on his Web site at http://www.hdtvexpert.com said low VHF DTV is not a good idea due to impulse noise and e-skip. He also said his tests have shown that "UHF DTVs do not always have to run full power if directional-receiving antennas are used; even moderately directional antennas, like a folded dipole with a reflector. UHF can work for DTV with lower-power transmissions so the electric bills don't get out of hand. Three dB is a lot easier to make up at the receiving end of things and cheaper, too."

Considering all the problems with Channels 2 and 3, could Channels 4, 5 or 6 work better?

They might. I heard that WHP-TV was happy with the DTV coverage it was getting on Channel 4. I was not able to confirm this. The CDBS, which as we know from WBBM-DT may not be up to date, shows WHP-DT with an STA at 570 Watts ERP and a license for 2.3 kW.

Philip Smith wrote to mention problems he was having receiving WBBM-DT. At 46 miles from downtown Chicago where WBBM has its antenna, his reception using conventional antennas was similar to the others I've mentioned. What I found interesting was his experience with a USB-2.0 based DTV tuner and a laptop.

I've always wanted one of those to check out DTV reception as I travel from market to market. However, the electrical noise from the tuner and computer seemed to be creating RFI (radio frequency interference) that interfered with low VHF DTV reception. The USB 2.0 tuner is made by Sasem in Korea and is supposed to use the "latest" LG Electronics (Zenith) 8-VSB demodulator chip. More information is available at www.sasem.com

Your comments and questions are always welcome. Drop me an e-mail at dlung@transmitter.com.