Got HDTV? Page 4

Unlike antennas, there's a direct correlation between price and quality when it comes to preamplifiers. A good preamp can accept a wide range of signals and amplify them in a linear fashion without adding excessive noise. Lower-quality preamps are often noisy and unstable. Strong signals of all kinds can cause them to oscillate, which produces spurious signals and interferes with DTV reception. Winegard, Channel Master, and Blonder Tongue all make high-quality VHF, UHF, and combo-VHF/UHF preamps with low-noise figures.

I use the Channel Master Titan 2 7775 UHF-only preamp in my home. This two-piece, mast-mounted preamp develops about 24 to 26 dB of signal boost in the middle of the UHF TV band and doesn't generate too much noise of its own. It works well with all of my old and new DTV set-top receivers and costs about $55. There are also VHF-only (7777) and dual-band (7778) models of the Titan 2 that are used for DTV reception with good results. Power for all three of these preamps travels up the coaxial feedline through a small power supply located near your set-top receiver or TV set.

RadioShack sells a cool-looking $30 UHF cartridge preamp, but you should avoid it for DTV reception, as it's quite unstable and strong signals can easily overload it. TERK's $48 PDMA is actually a combo VHF/UHF preamp and satellite/terrestrial diplexer. At just under $50, it's reasonably priced and quite stable, although it doesn't have as much gain as a single-band VHF or UHF preamp would. In conjunction with the TV35, it provided fair reception of low UHF channels.

To date, I haven't found any problems with an external preamp overloading a set-top receiver like Sharp's TU-DTV1000, RCA's DTC-100, Panasonic's TU-DST50/51 or TU-HDS20, Sony's SAT-HD100, Samsung's SIR-T100/150/151/165, Princeton Graphics' HDT-2000, or Zenith's DTV1080.

The rule of thumb with coaxial-cable feedlines is, the larger the diameter, the more signal you'll have at your receiver. RG-59 coax is acceptable for short runs, as its attenuation is rated at 6.5 dB per a 100-foot run at 700 megahertz (700 MHz is around channel 52 or 53). At 200 MHz (channel 10 or 11), RG-59 clips off 3.5 dB of signal for every 100 feet, or about half the signal you started with.

RG-6 is better for outside or attic antennas. Its loss specification is around 5 dB per 100 feet at 700 MHz. At 200 MHz, RG-6/U trims about 2.8 dB of signal for every 100 feet of cable. For really long runs and/or weak signals, RG-11U is the best choice, as it only cuts signals 3.5 dB per a 100-foot run at 700 MHz. This cable is about 0.375 inches in diameter and a little more difficult to work with. RG-11/U will only attenuate 1.7 dB for each 100 feet of cable at 200 MHz.

A note about shielding: Most store-bought coaxial cable is quad shield. This type of shielding simply cuts down on the signal radiation from the cable and helps prevent the coupling of unwanted signals into the cable. The amount of shield coverage has nothing to do with signal loss through the cable. Identical lengths of coaxial cable with different degrees of shielding but the same overall diameter, dielectric, and center conductor will attenuate the same amount of signal.

Most splitters are cheaply made. If you have enough signal present, they do an acceptable job of dividing up the signal to feed two or more set-top receivers. The rule of thumb is a 3-dB reduction (about 50 percent) in signal strength each time you add another port. For example, if you have 24 dB of signal from your UHF preamp and connect it to a two-way splitter, you'll get about 21 dB from each splitter port. A three-way splitter knocks signal levels down to 18 dB at each port, and a four-way splitter brings it down further to around 15 dB at each port. Cheap splitters can cause additional signal loss. I've measured "el cheapo" models that have as much as a 4.5-dB reduction per port.

If the DTV signals you want to receive aren't strong to start with, a preamp is a must if you wish to use a splitter. As I mentioned earlier, the DTV signal threshold is about 20 dB in the real world (and that's without much multipath and echoes), so the use of a splitter will cut your usable signal levels down quickly.

The final link in the reception chain is often the most vexing. DTV set-top receivers vary quite a bit in signal sensitivity and also in how they handle constantly changing signal echoes, otherwise known as multipath. Multipath causes big problems when the signal you want to receive and an echo of it arrive at your DTV set-top receiver's antenna at the same time, but out of phase with each other. If the phase difference is sufficient, the two signals will simply cancel each other out, and you won't see or hear a thing.

Most indoor and outdoor DTV reception problems I've investigated are caused by a combination of low signal levels and high multipath. This is often a fatal combination. Usually, the fix involves changing to another antenna with higher gain or a more-directional pattern and/or adding a preamp to boost overall signal levels. If a DTV set-top receiver gets a strong signal with a fair amount of multipath, its internal equalizer circuits can often clean up the signal enough to ensure good reception. Similarly, a weak signal with little multipath can also be locked up reliably.

Other problems that can affect reception are excessive jitter and errors in the digital data that the broadcaster is transmitting. These errors can cause an image to break up or freeze, in addition to causing audio dropouts. Sometimes, the set-top box may not translate the digital data (known as PSIP, or program and system information protocol) correctly, again resulting in no reception. No amount of antenna or preamp work can fix problems with data. I recently checked out a problem with a brand-new DTV set-top receiver that would absolutely not pick up a strong, clean signal from a local broadcaster. The culprit? The receiver didn't understand the broadcaster's PSIP data, even though it received the signal. A software fix by the broadcaster cured the problem, and all ended well.

If you're having reception difficulties, the best thing to do is borrow another set-top receiver and see if you can duplicate the problem. If so, it's either a broadcaster problem or an antenna/preamp/feedline problem. If not, you may have a squirrely set-top box, and a trip back to the manufacturer's warranty service center is probably necessary for a fix or software upgrade.