What is the “shelf effect?”

Have you heard of “the shelf effect?” In some ways, digital broadcasting just doesn’t make sense to most folks. It’s much more sophisticated than old-school TV. Of course that’s on purpose — the whole goal of digital broadcasting is to get a high-quality, high-definition signal to your TV in the same space that was originally designed for a black-and-white standard definition one. That’s going to take some magic.

If you think of the old-school antenna TV of your youth, you remember a snowy picture like this one:

In this image, it’s possible to barely make out a person standing in front of something. You don’t get a lot of detail but you at least get that. Old-school broadcasting (we call it “analog”) takes away a little bit of the picture at a time, replacing it with something random we call “snow” until the whole picture is gone. That makes sense to people

That’s not how digital TV works… on purpose

Digital broadcasting came to us in the early 2000s with cellular phones. The advantage of digital broadcasting is that you’re sending a bunch of ones and zeros, with a lot of “error correction” so that if part of the signal is lost it can be reconstructed. Believe it or not due to the way digital signals are constructed, a signal can be up to 80% lost before you notice a significant problem over time.

As a digital signal degrades, you don’t notice it. As it gets worse, or as you get further from it, you might notice a quick blip. The TV picture might look distinctly weird for a second or two. These are called MPEG artifacts and come from not having enough data to decode the signal. Eventually enough data comes in that the missing pieces are filled in.

And then… “the shelf effect.”

At some point a digital signal is so weak that there isn’t enough of it to decode anything. The difference between a signal that can be decoded and one that can’t isn’t very big, and as you’re watching TV you may not even know how degraded the signal is. Then, all of a sudden the signal stops. It’s like it fell off a cliff. They call this “the shelf effect.”

(Which may have you wondering why they don’t call it the “cliff effect.” I agree.)

This is actually a good thing. With digital, you’re getting crystal clear picture … until you’re not. While some may argue that a dim, shadowy, snowy picture is better than no picture at all I would disagree. More importantly, most broadcasters would disagree. The purpose of TV broadcasting is to show ads to people in a very specific area, and digital TV makes it possible to show people crystal-clear ads. The “shelf effect” generally doesn’t happen until you’re outside the area that the broadcaster is supposed to serve.

Let me back up and unpack that a little:

  • Broadcasters charge money for commercials.
  • The amount they charge is based on ratings.
  • Those ratings are measured in a specific geographic area.

If you live out in central Michigan, you might be able to get TV in the Detroit area or you might not. But it’s a sad and true fact that even if you do, that doesn’t benefit the broadcaster. He or she only cares about whether people in the Detroit Metro area and suburbs are watching, because the amount of money he charges for ads is based on people in Detroit and the area around it.

Broadcasters actually tune their signals so the shelf effect mostly happens outside their service area, so they don’t have to worry.

Seeing the shelf effect and you’re close to a city?

If you’re getting poor reception and yet you’re fairly close to a major city, you may need a new antenna or you may need to put the antenna you have somewhere else. A larger outdoor antenna will usually help the situation, and placing it up higher will also make a difference since up to 90% of a TV signal can be blocked by a building or tree.

If you’re not sure what to do, call the antenna experts at Solid Signal. We can help recommend a new antenna or give you advice on where to place the one you have. One call is all it takes… 888-233-7563.


About the Author

Stuart Sweet
Stuart Sweet is the editor-in-chief of The Solid Signal Blog and a "master plumber" at Signal Group, LLC. He is the author of over 9,000 articles and longform tutorials including many posted here. Reach him by clicking on "Contact the Editor" at the bottom of this page.