arritt@YALE.EDU (02/01/90)
I have a Sangean ATS-803A with a wide/narrow selectivity switch. On
occasion I have tried using the narrow selectivity setting, but this
makes the audio very muffled, often to the point of un-intelligibility.
Turning the treble all the way up and the bass all the way down often
helps, but the audio still isn't as good as with the wider selectivity
setting. (Turning the treble up seems to emphasize the interfering
station as well.) Usually I just go ahead and use the wide selectivity.
Is this (1) a problem with my particular set; (2) a problem with the
803A; or (3) a natural consequence of narrow selectivity? Would a more
expensive set having even narrower selectivity have even worse audio
quality? Any suggestions for improvement?
P.S.: Antenna is a dipole, but use of the internal whip gives the same
general behavior.
________________________________________________________________________
Ray Arritt |
Dept. of Physics and Astronomy |
Univ. of Kansas |
Lawrence, KS 66045 |
arritt@kuhub.cc.ukans.edu |
arritt@ukanvax.bitnet |Will Martin <wmartin@STL-06SIMA.ARMY.MIL> (02/02/90)
Basically, you trade off selectivity and audio bandwidth. When you select a narrower IF filter, which is what the bandwidth switch is doing, you cut off the higher audio frequencies too. You narrow the window your receiver is looking through and thus can avoid interference from other close-by stations, but the higher audio frequency modulation is at the edges of the signal you are tuning, and it can get chopped off, too. In a higher priced receiver, one of the things you pay for is better filters, which have a better "shape factor". They pass a given width of signal, but the cutoffs on each side are sharper. If graphed, the bandpass for an ordinary ceramic filter like is in the Sangean looks like a very weathered mesa, with long sloping sides and a rounded top. A high-quality filter looks like a sharp-edged rectangular building in comparison, with only a litlle sloping down at the bottom. With a filter with a better shape factor, you can get better audio, because you can use a filter with somewhat wider nominal bandwidth. This lets more of the desired-signal audio modulation through, but the abrupt cutoff at the band edges still reject interference from adjacent signals. A poor-shape-factor filter, in order to achieve a rated bandwidth, will be attenuating the desired-signals edges quite severely in order to achieve enough rejection to prevent an adjacent-channel signal from getting through. That is what you are hearing. This is easy to explain with a drawing or graph, but somewhat difficult to do in words. Just buying an expensive receiver, unfortunately, doesn't always get you the better or best filters you might expect. Thus there is a trade in aftermarket filters, or better-quality filters as a manufacturer's added-cost option. You can spend $200 and up for high-quality Collins mechanical filters, for example. Some shops specialize in replacing the filters in standard radios. "Radio West" is one such company. However, I don't know if the Sangean is one of the radios they (or others) install improved filters in; in some radios the circuit layout makes doing this difficult or produces poor results. Regards, Will Martin
liud@RUTGERS.EDU> (02/03/90)
In article <22115.25c84581@kuhub.cc.ukans.edu> arritt@kuhub.cc.ukans.edu writes: >I have a Sangean ATS-803A with a wide/narrow selectivity switch. On >occasion I have tried using the narrow selectivity setting, but this >makes the audio very muffled, often to the point of un-intelligibility. >Turning the treble all the way up and the bass all the way down often >helps, but the audio still isn't as good as with the wider selectivity >setting. (Turning the treble up seems to emphasize the interfering >station as well.) Usually I just go ahead and use the wide selectivity. > >Is this (1) a problem with my particular set; (2) a problem with the >803A; or (3) a natural consequence of narrow selectivity? Would a more >expensive set having even narrower selectivity have even worse audio >quality? Any suggestions for improvement? This has always been a challenge to radio engineering. Actually modern receivers is much better in selectivity/audio quality than those 20 years ago. The problem is to make a good frequency filter. In shortwave communications the audio signal is used to control the magnitude of the carrier frequency. Audio frequency is not a constant, it contains frequency componaents up to 5khz (in AM systems). Because the magnitude of the carier is altered at the audio frequency, the carrier itself is nolonger a pure frequency. Infact, there is a frequency "band" centered at the nominal carrier frequency. Higher audio frequency causes wider side band. The frequency can be expressed by f=f0+-fa If stations are spaced 2fa apart, then theoretically all 5khz audio can be recovered at the reciver without interstation interference. However, this requires a perfect filter which only passes frequencies from f0-fa thru f0+fa for a nominal station frequency f0. In reality there is no such filter. A typical filter has a quite flat passband and very sharp attenuation beyond. Although the attenuation can be vary sharp, the adjacent station still has a certain frequency amount that goes into the receiver's amplifier together with the selected station frequencies. One way to improve the seperation is to narrow the passband. This way the adjacent channel is further attenuated. At the same time the high audio frequencies of the selected station is also lost (see formula above). THat is exactly what Sangean is doing. This is not a problem with Sangean because it is already using state-of-art technologies by using multiple stage crystal resonator filters. Even with perfect filter you may also have interstation interference because the frequencies gets changed (off from its center) due to long distance and wheter/atmosphere conditions so as to invade the adjacent station frequencies.