Not one of those things you’d tend to need for your average home theater system, you still might wonder from time to time what a notch filter is. Very simply, a notch filter very effectively blocks a very narrow range of frequencies and it does so very well. In other words, it’s like it creates a little “notch” in the signal where nothing gets through.
Notch filters are used when two or more signals are combined to make sure that you don’t create interference. A perfect example is in an apartment where cable television or satellite TV comes in on the same line as internet service. If the two services come from different providers, a notch filter is used to isolate out a small range of frequencies where the internet signal can operate. The notch filter is used on the cable or satellite incoming signal before it goes into a combiner. If the notch filter is built into the combiner, it’s referred to as a diplexer.
If you’re familiar with the term “band stop filter” this is sometimes used interchangeably with the term “notch filter” but “notch filter” really tends to refer to a very narrow band of frequencies with very high isolation where a “band stop filter” could talk about a large range of frequencies, such as hundreds of megahertz.
The opposite of a notch filter (or band stop filter for that matter) is a band pass filter. Where a notch filter lets through every frequency except a small range, a band pass filter lets in only a small group of frequencies. If you were really going to be super careful to avoid interference, you would put a notch filter on one cable and a band pass filter on the other before they were combined. In reality this is usually considered overkill, even though the parts are relatively inexpensive.
Notch filters can be used to filter out extremely narrow ranges if you are trying to isolate interference from a very strong local source. In some cases they are used to isolate out strong cellular signals that could interfere with weak antenna signals. This may become a more common use of notch filters if the FCC’s plans to reassign some of the frequencies above UHF channel 31 come to pass. Those frequencies could be used for cellular internet and it’s possible those new signals could cause a little problem in some situations.