I thought this was supposed to be static-free

October 4, 2007 4:17 PM

What factors affect my XM radio's reception?

I've had my XM radio in my car for about 10 months now. Most of the time, I only listen to it when driving to and from work (about three miles each way). This means that I'm usually listening to it at the exact same place I was the day before, and the day before that, and so on.

The thing is, sometimes it's staticy and sometimes it's not. There is an underpass that I go under every day--a few weeks ago, there were three days where the sound would short out for a split second when I went under it. Since then, however, the radio has sounded perfectly fine at that part of my route. Similarly, there is an intersection where, sometimes, the static gets so bad that I can hardly hear the music. Other times, there's no static there at all. Most of the time, at that intersection, the static is noticeable but not very loud. I have never noticed any important differences at these locations (and others where the radio behaves similarly) on the different days (though I'll admit I don't know what to look for).

So, my question is, what the heck could be causing these differences? Is it something about the FM frequency I'm using (even though I have tried several different frequencies with the same result)? Does it have to do with the receiver? Something else? Is there anything I can do to stop it?

I'll admit I don't know much about the technology, so sorry if I'm naive in some stupid way.

posted by Ms. Saint to technology (11 comments total)

If it really is static that sounds like a normal radio tuned to the wrong station, it's a problem with you FM transmitter. When XM loses its signal it'll blank out completely or you'll get some berpy artifacts.

My theory on the problem at the underpass: were these three days sequential? XM not only has satellites, it has ground-based stations in urban areas so you don't lose your signal in situations like going under underpasses. The ground station might have been out for those few days.

The static at the intersection: there's something at the intersection that's interfering with your FM radio. If you have clear skies above, you certainly will not lose contact with the XM satellites.
posted by marionnette en chaussette at 4:45 PM on October 4, 2007

We have XM built into our car. The way you describe hardly being able to "hear the music" over the static doesn't mesh with my experience. For our car, we either have a clear signal or no signal .. and the no signal static sound seems artificially generated (kind of like how point and shoot digital cameras make a fake SLR shutter noise).

I do recall reading in the manual that the further north you travel, the more interference you will get. That's due to the satellite being in the southern sky and the angle of the radio waves.
posted by dereisbaer at 4:45 PM on October 4, 2007

Most digital devices, including satellite radios, "buffer" their outputs (i.e. store several seconds of decoded audio signals in memory buffers), so as to give themselves a few seconds of processing time, to catch up, from any internal or transmission errors that occur, in the digital front end. Accordingly, you might never be hearing a "digital radio" signal in "real time," as you must for an analog signal, but neither do you need listen to dead air, if you drive quickly enough under your underpass.
posted by paulsc at 4:56 PM on October 4, 2007

Your problem is with the FM device in your car. Get a pro to install XM so it "line ins" to your radio or read this previous askme.
posted by damn dirty ape at 5:19 PM on October 4, 2007

Just to echo what a couple of other folks have said, satellite radios are designed to have either signal or silence -- no static. So if you're hearing static, there's a problem with whatever is connecting your XM radio with your car's receiver... most likely an FM transmitter.

If it is an FM transmitter, it could be interference from a station that broadcasts on a similar (though not identical) FM frequency to the one your XM transmitter is tuned to.

If, for example, you have your transmitter set to broadcast on 88.7, and there's a station in the next county at 88.9, it might cause interference when the signal from the next county is strong... say when there's a lot of clouds, or there aren't any mountains between you and the transmitter, or whatever. This can also come up when other people have FM transmitters (for their iPod, say) tuned to the same frequency, and you pull up next to them at an intersection.

And yeah, as far as the signal going out completely... paulsc has it pretty much. The satellites are line-of-site, so if you're going under an overpass, you're either relying on terrestrial repeaters or the system's buffer. Both of those can vary in their performance based on various stuff.
posted by YoungAmerican at 5:20 PM on October 4, 2007

I have factory XM in my car and it generally works well, but it will get static if it gets and loses the signal rapidly. This typically happens when driving through trees that block the southern sky and gets better when they lose their leaves during the winter. Other obstacles as well as weather affect the reception as well; I have wondered about solar activity but don't know for sure.
posted by TedW at 6:13 PM on October 4, 2007

I lose the sat signal when I've got big trees to the immediate southwest of me - if I move over one lane of traffic, it's fine.

I've also had the signal clobbered when driving past military installations - thought it was my antenna for a long time, then realized the pattern.
posted by chookibing at 6:15 PM on October 4, 2007

I have factory xm in my car, and I've never heard static on it. Not once. Echoing the comments above, if the signal cuts out it goes silent.
posted by monju_bosatsu at 6:28 PM on October 4, 2007

I've heard 'static' on my directly-connected XM receiver, but it's in fact just synthetic static (white noise), not the same as FM static at all. This is done on purpose; when there are short gaps in the signal, it's better to fill in with a short burst of white noise rather than silence. Studies have shown that interrupted speech is more intelligible this way.
posted by xil at 11:44 PM on October 4, 2007

chookibing, uh. I live on Fort Drum and my car has XM and it works just fine. I also spent 2006 in Afghanistan running satcom, FM, and HF communications. "Military bases" are not jamming your satellite signal. XM operates on a band of frequencies that it has reserved with the FCC and believe me, stateside, the spectrum is sacrosanct for the military. We've got our freqs, and when a unit even wants to use a military-only frequency for, say, training purposes, that unit needs to clear it with battalion or brigade S6.

Now! I have seen and played with some pretty neat commo equipment that works on civilian freqs, but it was all passive, and I've never seen it anywhere outside of a combat zone.
posted by kavasa at 5:12 AM on October 5, 2007

kavasa: I have mucho reception problems with Sirius here in the SF Bay Area, but once I get outside of the Silicon Valley, it works perfectly.

Who knows, maybe there *is* something that's getting in the way.
posted by drstein at 9:37 AM on October 5, 2007