Digital TV Follies

April 21, 2007 4:06 PM

Why are there intermittent dropouts in over-the-air digital TV signals?

I'm curious as to why digital TV signals would vary so much from hour to hour, or from day to day.

TV set has a channel-tuning option that lets you view "bars" on each channel to get a rough estimate of signal strength. Why does the signal vary so much from day to day? One day it's fine, another day it's crap, without being touched or adjusted in the interim.

And why would the signal seem to waver so much? The variation in bars can go like this from second to second:

8 9 8 8 8 9 9 8 8 0 1 0 9 8 8 9 9 9 8

Why would that little glitch pop in for a second, when the signal appears to be so strong otherwise? Or is this just an illusion from the way the "channel strength" is being displayed on my set?

I'm getting digital cable anyway...just curious about this. Tales of weather, multipath, migrating flocks of geese, whatever, are welcome.

(Me: about 6 miles from monster antenna farm, relatively flat terrain between here and there, UHF antenna mounted in window.)

posted by gimonca to technology (7 comments total)

I have no idea. Although, I have noticed that Chinooks can play havoc with the tv.
posted by popcassady at 4:22 PM on April 21, 2007

Clouds, changes in air density due to ambient heating and cooling, solar activity, Magnetic fields, can all affect the signal.
posted by kanemano at 4:33 PM on April 21, 2007

OTA digital TV plays at very high frequencies because that was the only part of the spectrum left to play in. Frequencies that high are extremely linear and are easily interfered with. Effectively they're line-of-sight, and both the transmitting and receiving antennas are very small.

As Kanemano says, air density changes can cause the signal to refract, which will louse you up. Weather (fog, clouds) between you and the transmitter can attenuate the signal. Quick dropouts can be caused by birds.

Nothing can be done about any of this. It's a consequence of the frequencies being used.
posted by Steven C. Den Beste at 4:58 PM on April 21, 2007

Whenever a plane goes over my house, certain channels fall over.

I also have a lot of interference in some of the signals due to bad antenna placement and large amounts of active amplification, so even though the signal might be strong on some channels, there can be a lot of glitchiness because the noise overcomes the decoder's ability to see through the error correction.
posted by tomierna at 5:18 PM on April 21, 2007

Multipath interference is a problem, too, and passing airplanes or big trucks can cause that.
posted by Steven C. Den Beste at 6:33 PM on April 21, 2007

OTA UHF signals generally require significant height above average terrain, at both the transmitting and recieving antennas, for reasons SCDB mentioned. In a home antenna situation, nothing makes up for antenna height, which is why the consumer AntennaWeb site does not even discuss indoor antennas for DTV reception. I'm in a similar situation to you (5.9 to 7.1 miles from DTV stations, over flat terrain, without intervening tall buildings, major airport, large stands of trees, etc in my signal path), and medium directional VHF/UHF antenna on a 30' mast is barely adequate for reliable reception.

In heavy rain, I still see occasional drop out. And because I live near military installations, I get situational drop out from aircraft, too. Despite the FCC's ideas on the matter, OTA DTV reception is rarely stable or effective.
posted by paulsc at 6:39 PM on April 21, 2007

Generally speaking, the newer ATSC decoders do a better job of dealing with multipath than older ones, but the reason for your intermittent dropouts is almost certainly multipath.
posted by wierdo at 3:10 PM on April 22, 2007