Russell Brown /
04.03.2009 08:43 AM
Expanding DTV coverage Part I

Three basic systems are commonly used today to expand TV station coverage: translators, boosters and single-frequency networks (SFN). The reason for expanding coverage remains the same (increased viewers), but the areas of today’s expansion have reached into places that traditionally have not been of great concern. Now every neighborhood needs good reception for home viewers as well as for mobile viewers, who could be viewing in just about any place their cell phone works. It’s the engineer’s job to figure out how to do this without breaking the budget.

Coverage issues

Within any TV station’s coverage area, there are many places where the signal from the transmitter is either weak or totally blocked. This is caused by many different factors, but the main one is terrain shielding, which can be a mountains, hills or even buildings. While the station has no control over the terrain, it can take steps to ensure its signal does get through to these places. The “cliff effect” of DTV does not help in this matter, because it does not gradually degrade, but is either there or not there making reception more problematic.

Most stations have switched to the UHF band where these frequencies have an even harder time going through or around obstructions than VHF did. Even the FCC propagation curves F(50/90) are based on only 50 percent of the viewers getting your signal 90 percent of the time, which are not very good odds.

Just as there are several methods to increase coverage, there are sometimes more than one name for each of these systems. (To be sure everyone is talking about the same system, it’s best to actually describe what you want to accomplish.) We will be using the most common names for the various methods and systems.

Boosters

Boosters have been used in TV and FM broadcasting for many years. Basically, they receive an off-air signal, amplify it and feed it to a transmitter antenna. Boosters require shielding and are limited to very low power to reduce the possibility of feedback. The requirements of terrain shielding between the receive and transmit antennas can lead to the use of a separate feed from the studio to the booster to eliminate the feedback problem and provide for higher amplification. If the area to be covered by the booster is not itself completely terrain shielded, the boosted signal will act as a multipath interference signal to the main transmitter’s signal. The advantage of an on-channel booster is the low cost involved because it is merely amplifying the off-air signal. (See Figure 1 above.)

Some boosters, known as dual-conversion heterodyne boosters, actually convert the received off-air signal down to IF and then back to the on-channel frequency. The advantage to these types of boosters is the ability to perform some signal processing on the received signal before broadcasting it, including SAW filtering and distortion correction. With this comes the ability to use a higher power output, but terrain shielding is still required. (See Figure 2.)

The most sophisticated booster is a digital signal processing (DSP) booster. These boosters are similar to the dual-conversion heterodyne variety but are capable of more sophisticated RF signal correction.

The thing about DTV is that even if one bit is changed in the bit stream between two otherwise identical 8-VSB signals, the altered signal becomes a jamming signal if it is received with the original one. The DTV receiver’s equalizers only work on multipath, which means two identical signals. When a DSP booster alters the signal passing through it, only the RF is changed, so the RF out of this type of booster does not become a jamming signal. Again, while this allows for higher power, if still requires terrain shielding. (See Figure 3.)

A remodulation booster actually receives the off-air DTV signal and demodulates it down to the transport stream; then forward error correction can be performed to correct bit errors, which none of the other boosters are capable of. The problem with this is that the bit stream is changed and the output of this type of booster will jam the original signal in a multipath situation. These would be used at the outer edges of your coverage area where the original signal would be at its weakest and require error correction before being retransmitted. Once again, it requires terrain shielding between the receive and transmit antennas. (See Figure 4.)

All of the boosters mentioned require terrain shielding due to their use of a receive antenna to pick up the signal from the main transmitter, but there is another way. If the transport stream is sent to the remote booster site via microwave or fiber optics, then the addition of a DTV exciter on the same frequency would eliminate the need for terrain shielding to protect the receive antenna at the booster site because there is none. (See Figure 5.)

Because of how DTV exciters work, the signal out of this system will not be the same as the main signal, and it will act as a jamming signal. This is also the most expensive booster and is not used very often.

Translators

Translators solve many of the problems that boosters have, namely, terrain shielding — they don’t require any. Because a second channel is being used, there can be no feedback or interference with the main signal. The problem is getting that second channel. All of the processing in the various boosters described can be done in translators, ranging from simple RF to IF with signal processing on the received signal, including SAW filtering, distortion correction and the like. Translators mix the IF with a different frequency to obtain a new RF channel to then broadcast, but the PSIP can’t be altered for the new frequency when this method is used. But if a demodulator and remodulator is used to obtain a perfect non-interfering digital signal, PSIP can be changed to reflect the new channel, and viewers will more easily find the station. (See Figure 6.)

Conclusion

Out of all the methods mentioned, only the translator using a second TV channel would not cause interference with the main RF signal — all the others require careful planning to reduce signal blocking in multipath setups, but they can’t eliminate it. All of the methods discussed require careful planning and installation and, in most cases, an experienced engineering firm that can advise a station’s engineering department on the best and most cost-effective solution.

Next time

In the next “Transition to Digital,” the recently approved SFN for DTV will be examined. SFN has now emerged as a way to increase coverage and virtually eliminate signal blocking in multipath setups.

Acknowledgments

Richard Schwartz of Axcera contributed to this tutorial.



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