re: AM radio reception

February 20, 2006 6:57 AM

when scanning AM radio stations in my car, why does seeking UP always catch fewer stations than seeking DOWN? this has been the case with each of the 3 cars i have owned. anyone else get this?
posted by jmccw to technology (11 comments total)

Has this been the case in your local market, or in other locations too? My guess is that it has to do with most of the stations being concentrated towards the low end of the am band; not sure beyond that.
posted by ParisParamus at 7:06 AM on February 20, 2006

I think they mean more like, they're at station 750, press up and the next station they arrive at is 850. But, if they press down from 850 they find a station at 820, 790, 770 etc.

Perhaps I'm just talking out my ass.
posted by cloeburner at 7:34 AM on February 20, 2006

I've seen this sort of thing when the seek command entails scooting through the first few frequencies adjacent to the current frequency. So if you have stations close together followed by a gap, the seek command can bypass an adjacent station, and catching stations would depend on starting frequency and seek direction. Bad radio design, I think.
posted by exogenous at 8:01 AM on February 20, 2006

no you're right cloeburner. that is the behavior i am asking about. exogenous may have the best answer so far but i'll wait a few hours to see what others think
posted by jmccw at 8:11 AM on February 20, 2006

Yes - I have the same issue with my radio. No explanation, though.
posted by shifafa at 8:44 AM on February 20, 2006

For standard AM broadcast band transmissions in the U.S., the "intelligence" that is "carried" on the signal is all in the sidebands to the carrier frequency. Thus, a station with a carrier frequency of 650 kHz is producing energy in a 10 kHz band from 645 to 655 kHz. Ideally, exactly 1/2 of the sideband energy should be in the band from 645 to 650 kHz, and exactly half of the remaining sideband energy should be in the 650 to 655 kHz band. But modulation is rarely exactly symmetric, for a lot of reasons, including the fact that at least some AM stations broadcast stereo. So scanners coming "down" the band at some pre-determined rate, will tend to identify more weak stations when tuning than will scanners going "up," particularly when the scan rate is high (takes less than 10 seconds to scan the entire band). As the scan rate is lowered (20 seconds to a minute to scan the band) the ratio of stations "missed" due to to "direction" of scanning will drop, until at some point, every station that can be picked up scanning in one direction will be picked up the other way, too. Those of you interested can verify this by manually tuning at different rates in different directions, and I think you will find that as you slow down, you eventually get the same stations going up or down, but if you go very fast, you get more going "down" than "up."

Unfortunately, most car radios can't be easily set to automatically scan more slowly, and most people are willing to sacrifice weak station selectivity, for getting rapid automatic selection of strong local stations. So, if you want to get Del Rio, TX or clear channel stations like WSM out of Nashville at night, tune manually.
posted by paulsc at 9:22 AM on February 20, 2006

paulsc, I don't see how your answer explains a consistent difference, assuming it actually is as consistent as reported.

Given paulsc's speed of scanning issue, it is remotely possible that it is designed behaviour. Since there are two ways to do it, might as well make them different, that would give a useful features to the user. Or, to put it another way, you get fast seeking going up, but slow seeking going down - best of both worlds. I've seen alarm clocks that adjust quickly in one direction, and slowly in the other.

You could imagine that it is a channel spacing vs. step size issue, but I don't think so. I haven't played with a digital tuner in a long time, but I think the step size is small enough that you could never jump over a station.

I've experienced similar behavior with cheap dial tuners. I think that is explained by friction differences in the variable capacitor - it just moves more smoothly in one direction. I don't get that behaviour on good radio tuners, such as the old marantz gyroscopic tuners.
posted by Chuckles at 10:04 AM on February 20, 2006

I'll chime in with support of this behavior as a "design feature." I think it's by design that the threshold detection of going up is lower than going down. That way, if you *know* there's a station at some point, you can come back to it after settling on the more powerful station above it. When you don't know where the stations are, the browse feature still works.

Evidence: I had an older Mercedes wagon. It had a weird stereo in it. One of its weirdest features was that it had this behavior -- it would winnow to smaller and smaller stations, but only if you went all the way around the dial. So, it' worked liked:

1. Start up, find 3 stations. Get to 107.9 and start again...
2. Find 6 stations. Go around a third time.
3. Find all 9 stations.

HOW UNBELIEVABLY ANNOYING!! How does one know about this god-forsaken "feature" without reading in the manual? How do you know which go-round you're on without any feedback?

All this is to say that it is possible with tuners to make different thresholds for lock-in. My Mercedes had a digital tuner, btw.
posted by zpousman at 10:43 AM on February 20, 2006

Um, let me try again.

The C-QUAM system of modulating a stereo signal on a broadcast AM channel inevitably results in a slightly greater amount of sideband energy being produced in the upper 5 kHz sideband of the 10 Khz channel, than the lower sideband. This is due to, among other things, the presence of the 25 Hz stereo pilot signal shifting the average noise floor very slightly from a dead centered symmetric position. Thus, for receivers of otherwise symmetric response, tuning in the "down" direction will deliver, on average, a slightly greater "blip" of signal energy, earlier in the signal reception period, in the total time the receiver is tuned anywhere in the selected stations total signal band, than is received when the direction of tuning is in the "up" direction.

That difference is significant, because the response of the tuner is intentionally not instantaneous to weak signals (otherwise the scan would stop constantly on weak static bursts and distorted skywave signals at night from distant stations). In scanning mode, the receiver output is quieted until such time as a strong enough signal is present to get above the quieting threshold long enough for the receiver to accept the signal as a valid station. Because of the asymmetry of energy distribution in the upper and lower sidebands, that will favor a scenario in which the sideband with greater energy is received first, as is the case with higher average energy being present in the upper sideband, and the "direction" of tuning being "down."

Again, if the rate of change of the tuned frequency is low enough, ("slew" rate approaches zero), the difference in selectivity of the scan due to direction of scan will drop towards zero as well, all other things being equal.
posted by paulsc at 10:51 AM on February 20, 2006

zpousman, implementing the feature as you describe would be quite a bit more difficult. Assigning different behaviour to buttons is easy, they already have different behaviour after all. Keeping track of how long the user has been seeking a station is a whole new thing. It could be done, it isn't exactly hard, just harder...

paulsc, thanks for clarifying, I can see what your saying now. I don't have enough experience in this area, but my understanding has been that betting rid of symmetry is the hard part... I mean, I would expect the 25Hz tone to have positive and negative parts, just like every other frequency component in the modulated signal.
posted by Chuckles at 12:41 PM on February 20, 2006

My car does the same thing, but it's better at catching stations going up than going down.
I always figured it was a byproduct of design, because hitting the "Scan" button always goes up.
It is however, incredibly annoying when you hear something you like, but can't get to the radio before it continues on, then hit "Seek" to go back and the radio happily cruises by the station it had just found.

However, my car has the world's worst stock radio tuner, so I've gotten used to it over the years.
posted by madajb at 3:18 PM on February 20, 2006