Path: utzoo!utgpu!ugw.utcs.utoronto.ca!CUVMA!SWL-L Date: Fri, 2 Feb 90 13:38:52 EST Reply-To: Will Martin Sender: Short Wave Listener's List From: Will Martin Subject: Re: Selectivity vs. audio quality X-To: arritt%kuhub.cc.ukans.edu@UUNET.UU.NET X-cc: swl-l@cuvmb.cc.columbia.edu To: UofToronto LAN redistribution Message-ID: <90Feb2.162703est.58696@ugw.utcs.utoronto.ca> Newsgroups: bitnet.swl-l Distribution: ut Approved: devnull@gpu.utcs.toronto.edu 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