New Year's RF Ramblings

To start out the New Year, I thought I'd share my opinion on where we're headed in the DTV transition. I've also included a report on the smart antenna described at the IEEE Broadcast Technical Symposium.
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To start out the New Year, I thought I'd share my opinion on where we're headed in the DTV transition. I've also included a report on the smart antenna described at the IEEE Broadcast Technical Symposium.

INDOOR ANTENNAS AND PORTABLE TVs

Broadcasters have made a huge investment in building out terrestrial DTV facilities at various power levels. So far, there aren't a large number of over-the-air DTV viewers so it's hard to tell how well each is actually working from a consumer standpoint. Many of the people currently viewing DTV over the air are willing to make some effort to put up an outside antenna and make the adjustments necessary to receive DTV. It is probably unrealistic to consider this will be the future of DTV. The number of consumers willing to make this effort is small compared to those used to making a cable or satellite connection and having hassle-free reception.

Everyone generally agrees few consumers will go to the trouble of installing an outdoor antenna for DTV. Large TV sets in the living room are likely to be hooked to cable or satellite. Over-the-air DTV will provide a signal to smaller TVs, second sets or maybe even in-vehicle sets that keep the children occupied on long trips.

Most sets being sold today are going to more tech-savvy consumers or as part of a satellite HDTV system with broadcast as an add-on for those willing to make the effort to put up an antenna. Consumers buying big screen sets are likely to look to cable and satellite to provide their HDTV programming. At this point in the DTV transition, transmitting a DTV signal that requires an outdoor antenna may be acceptable.

The one area where terrestrial TV has a monopoly -- delivery of signals to portable and second sets not connected to cable or satellite -- won't transition to DTV until lower cost DTV sets are available. The technology used to produce small, inexpensive cell phones and hand-held PCs will find its way into DTV reception. On a recent flight I sat next to a representative from a manufacturer of tuners for TVs and VCRs who said his company was looking at DTV tuner modules they could sell these manufacturers. I urged him to keep them low cost!

The difference between typical analog TV reception and DTV reception is dramatic. Just as digital compact discs quickly replaced phonograph records, once the price of DTV sets drop low enough I believe customers will switch to them as well. Seeing the difference in quality on larger sets, they will want smaller digital sets for other rooms and possibly for portable use.

This poses a bit of a dilemma for the broadcaster. HDTV material will be needed to attract viewers to the big screens, but won't be appropriate for small screens. Some of the robust mode methods under consideration by ATSC may provide a solution. Given different audiences with the two types of sets, it is possible stations will want to broadcast two different streams of programming. This, of course, could have implications for cable DTV must-carry, which is currently limited to the "primary" DTV signal.

TRANSMISSION DESIGNS

Low-power DTV transmitters can provide an acceptable solution at this part of the transition, but will not be acceptable when and if low-cost, smaller screen sets for use with indoor antennas appear. No one knows when this will happen, but the TV tuner manufacturer I met on my flight to Dallas thought DTV receivers could see widespread acceptance as early as 2004 to 2005. In that case, any low-power DTV facility should be planned with the idea of eventually increasing power.

Stations have been building out full-power DTV facilities with two goals in mind -- protecting coverage areas and providing robust DTV signals. The FCC, except for the top four network affiliates in the top 30 markets, has protected coverage area for facilities authorized but not constructed. During the next review of the DTV rules, the FCC is likely to set some "build it or lose it" deadlines for maximized and possibly even allocated facilities. Therefore, it is important to obtain as large a protected contour as possible.

This could conflict with the goal of providing the best signal for indoor TV antennas and portable sets. For a given amount of transmitter power, the maximum protected area will be achieved using a high gain horizontally polarized antenna with minimal beam tilt to put at least 80 percent of the power on the radio horizon. However, to provide the strongest signal to less than optimum indoor and portable antennas, the antenna will usually need more beam tilt, have some amount of vertical polarization (at least fifteen percent or more of the horizontally polarized power) and have less gain to provide greater field strength over a wider area and closer to the antenna.

There are no easy answers here. Transmitter power and the electricity needed to provide it are not cheap. Replacing antennas later is not cheap. Indoor/portable antenna coverage may not be important now, but if there is any future to terrestrial DTV it will be. For guidance on the technical decisions, re-read some of my past columns and keep an eye on new TV Technology articles.

A good way to start would be to set up a spreadsheet with your ideal antenna and transmitter configuration and then look at how you can build into it. Antenna replacement may not be a big issue if a site move is being planned, so an antenna optimized for maximum protected area with lower transmitter power requirements may work fine. However, if protecting a large coverage area isn't important and you don't want to face a future antenna replacement, a lower gain antenna with vertical polarization but less transmitter power may be a good starting point. I'd be interested in knowing what decisions your station is making in this area.

SMART ANTENNAS FOR DTV RECEPTION

Given the "cliff edge" effect associated with DTV reception, consumers may be frustrated if they get a picture when they fiddle with the antenna but it disappears as soon as they walk away and sit down. A smart antenna system can automatically fiddle with the antenna to get the best reception. At the 2002 IEEE Broadcast Technical Symposium, Andrew Youtz from ATI Research, the company that bought NxtWave last summer, described its implementation of the EIA/CEA-909 smart antenna interface.

The antenna design presented in the ATI paper used four log periodic dipoles with one amplifier/attenuator for the north and south dipoles and another for the east and west dipoles. The two switches, amplifiers and attenuators are then combined. It is possible to get away with only two paths because there won't be a case where both the north and south antennas (or the east and west antennas) would be used simultaneously. Using these antennas individually or combining one with an antenna 90 degrees away and varying the gain between the two antennas, it is possible to select sixteen cardinal positions.

The problem, of course, is figuring out which of those sixteen positions is best. The ATI paper did not address the algorithms used to make the decision, but did describe the two metrics it looked at. One metric is the AGC voltage. Allowing for AGC settling time, it can scan the positions in about 30 to 40 milliseconds.

The other metric is the signal to noise ratio (SNR). This metric is needed because in the presence of multipath the SNR is sometimes better at a position with a slightly lower signal level but with less multipath. ATI says their method can search all sixteen positions for the best SNR in approximately 800 milliseconds.

Once the best position is selected and the signal locked, SNR is used to track any changes in the signal.

ATI conducted tests on the antenna to see how well it works. Antenna uniformity, search accuracy and track accuracy were tested. The last two are the most interesting. The 909 interface found the best SNR in most cases. In the few cases where it didn't, the SNR was already high, with multiple peaks and the difference between the selected position and the optimum one was only about 1 dB. The antenna was slowly rotated through 360 degrees to test the tracking accuracy. As it was rotated, the electronic antenna position rotated the opposite way, tracking the signal.

If the EIA/CEA-909 standard actually starts appearing in TV sets and indoor antennas it could make indoor DTV reception easier than indoor analog TV reception, provided there is enough signal strength. For portable use, however, the smart antenna will have to get a lot smaller than what was shown at NAB and described at IEEE.