Expanding DTV coverage Part I
Apr 3, 2009 8:43 AM, By Russell Brown
Figure 3: A DSP booster
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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.)
Figure 4: A remodulation booster
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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.)
Figure 5: A direct feed booster
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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
Figure 6: A translator using a second channel
Click to enlarge
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.
AcknowledgmentsRichard Schwartz of Axcera contributed to this tutorial.
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