The power of television
One misconception about digital transmissions is the old thinking that more transmitter power is better. Fact is, the FCC determines how much power television stations should transmit, and only the FCC can allow stations to change their power. The key for over the air digital TV is using a good antenna at the receiver, and doubling the transmit power will not make any difference if your antenna isn’t efficient. Here’s a look at the NBC 17 analog and Digital coverage maps from the FCC database:
The left map shows the coverage area of the current NBC 17 analog signal and the map on the right is the current NBC 17 DTV coverage (on ch. 55). The two circles are drawn at different signal level thresholds, or -64 dBu for analog, and -41 dBu for digital. The minus 41 number is the cliff that differentiates that a receiver can no longer tune that signal. With analog, the signal can be 20 dBu lower before it is totally gone. Even at those levels, the DTV signal outperforms the analog by a few miles.
But assuming that any household within the circle can pick up the signal perfectly does not hold up in the real world. The very reasons you picked your house, surrounded by trees and hills and valleys are the very things that kill a DTV signal. I’m not saying you have to move, but you do need to pay attention to the antenna and get it right before you will be satisfied.
While RF Engineers have formulas and software to calculate what you need, there are many times it comes down to trial and error. Try the cheap rabbit ears, then move to the next step. Don’t give up because something didn’t work, just keep trying.
The squiggly lines on the right is the RF signal for a typical analog TV station. The left peak is the video signal, and the right peak is the audio signal. When you tune in an analog signal, you are peaking up those narrow spikes and the receiver creates the picture and audio. Each channel is 6 MHz wide, and there are mathematical formulas to determine how the signals are put together in just the right order. Multi-path will cause another peak near those, and can sometimes overpower the main carrier. Analog sets will display them all with multiple images, crazy colors, etc.
This is the DTV signal similar to what every station is broadcasting to the left. To a farm-boy like me, it looks like a haystack, but others think it looks like Bart Simpson’s head. When tuning in a DTV signal, the receiver uses the jagged carriers on top of the pedestal. The top should be flat, and multi-path will cause the ends to round off, or dips and bows in the middle causing the receiver to break lock. It is still 6 MHz wide, but the actual digital data is only on the very top. The squiggles at the bottom of both graphs is terrestrial noise, frequencies floating around in the air from other services, etc. The top of the haystack needs to be a certain level above the noise floor for the receiver to decode all the data. It’s a pretty fast system with a lot of data, and getting the signal into the receiver flat is the job of the antenna. If you have items I mentioned around you like trees and hills, then try a more directional style antenna, try the attic or outside.
The signals are floating around you all the time. Capturing the good ones and eliminating the bad ones is the job of the antenna and receiver. Once you get more good signal over the unwanted signals, you will be fine. I just do not accept defeat easily. I’m pretty sure the average viewer can get a clean signal, but you have to get off your can and try. People will watch snowy ghosting and unstable analog, but that is not an option in digital. You either get it or you don’t, and the don’ts just have to try a little experimenting. We are 8 years into providing DTV over the air and it is still in its’ infancy. Color TV took 10 years to gain popularity. Stereo was also slow to arrive in most homes, but digital is different.
DTV looks great even on older TV sets. It is awesome in HD on a new TV sets. Sports sells more TV’s than anything else, and before you make the jump to a new HD TV set, you better have this antenna issue resolved. Men see the difference, and usually want it. Women are not such an easy sell, and if you turn it on and it freezes, breaks up, or is anything but perfect, you’ll have a lot of explaining to do to the misses! Getting the cheaper converter box, and the right antenna, will help convince the family a home theatre system is required. The Olympics on NBC is a great time to showcase HD to the non-believers since it has universal appeal to everyone. The opening ceremonies alone will convince any skeptic that HD was a great investment. And like the early pioneers settling America, or in the ’40’s watching test patterns on the air before the network programs begin, or blazing undiscovered territory, you can have that American pride feeling that i did it myself!
Or you can pay someone to do it for you.
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