# Cell Phone Filter / Interference?

How do cell phone filters filter out all the extraneous EM waves and noise (especially noise) when you get a call?

I would expect a phone to have a bandpass filter to get rid of most noise but even then there is noise at still in the resulting signal.

• @SDsolar You stated that this question is off-topic since it doesn't involve Amateur radio. Are you sure it's off-topic? The tour states that "Questions are expected to be about Amateur radio specifically, OR about the technology of radio. And this section of the help page states that questions about "the science and technology of radio" is OK here. Maybe you or someone else can correct me if I'm wrong. – Mike Waters Jul 8 '17 at 2:13
• OK, @Mike. You are absolutely correct. I will repost my comment with that part omitted. – SDsolar Jul 8 '17 at 7:44
• The simplest answer is that they are digital. So there can not be any fading in of other signals. That means that if the tower signal becomes too weak it drops the call entirely. Or, if there are other waves blocking the signal (which would be illegal) the calls can't work. Yes I am sure they have filters so they only listen to their assigned frequencies, so that rejects most of the interference. Welcome to Amateur Radio SE! Please take the tour at ham.stackexchange.com/Tour to get the best out of this site. I see that your question generated a quality answer for the database. – SDsolar Jul 8 '17 at 7:47

How do cell phone filters filter out all the extraneous EM waves and noise (especially noise) when you get a call?

Short answer: they don't. Of course, they use a bandpass filter to reject EM radiation outside the designated channel. But every radio does that. Modern cell phones also dynamically adjust their antennas so they are more sensitive in the direction of the cell tower, and less sensitive to other directions where there's only noise. But after that, there's still plenty of noise mixed with the signal.

If you're wondering why you don't hear the noise in the call, why it doesn't sound like a distant AM radio station in a thunderstorm, it's because the transmissions are digital. It's the same reason when you download an MP3 over WiFi it doesn't sound noisy. It's the same reason I can show you a number:

And you see forty-two, not fcrtv-lwo.

There is a point at which there's enough noise that the numbers can't be reliably decoded, and you do hear noise in the call. But it doesn't sound like static, because a wrong number in a digital audio doesn't sound like static. Because cell phones are used for voice, the digital coding is based on what voices sound like. The digital signal contains instructions for reproducing a human voice not unlike a human would do it. So when the bits are wrong, it doesn't sound like static: it sounds like a voice making nonsense sounds.

When the signal isn't great, a few of these errors slip in, but you don't notice them because you're paying attention to what's being said, not the audio quality. And, the codec is designed to make such things difficult to notice. For example, when a few bits of data are missing, the codec may continue making the same sound that it was making before the error to fill in the gap. Such things are difficult to notice unless you listen carefully.

When a lot of errors slip in, the phone just drops the call so you hear nothing at all.

• You crack me up, @Phil. Spot on as always and I love the visual. – SDsolar Jul 10 '17 at 1:41