Brett wrote: On weak signals, I often find narrower filter settings of the K2 make the signal harder to copy... The noise and the signal seem to blend into one pitch...
----------------------------- Like antennas, filters are a perennial subject of interest to many Hams, including me. I've learned a lot just reading the mail here on the Elecraft reflector over the years, especially about the subjective experiences of Hams trying to hear a signal and what it takes to allow them to copy. My interest in filters hasn't been so much about the various parameters that are measured in the lab, but more about how well I can hear and copy signals under various conditions. I'm not sure the two are as directly related as many hope! Like Brett, I've never much liked a narrow CW filter. I almost never use less than a 500 Hz filter and generally stay with a filter at 1 kHz or wider for CW copy. One's first reaction might be, "Oh no! That's backwards! A wider filter lets more noise power through so the signal-to-noise ratio is worse with a wide filter." Measuring the audio voltage at the output of a receiver using a wide filter might well prove that out: the total audio power my be much the same when the distant transmitter key is down as it is with the key up. That is, the S/N ratio might be very poor. It's not only about power levels. Frequency plays a huge role in determining how easily we can copy a signal. In this case, the signal we want to copy occupies essentially one frequency in the audio spectrum while the noise occupies all the frequencies allowed through by the filter in use. Our brains can be extremely adept at focusing on the one keyed tone while ignoring the wide spectrum noise, even when the signal is somewhat weaker than the noise itself. Something bad happens when we use a narrow filter to isolate the signal. As we center a weak signal in the bandpass, the noise frequencies off to the sides of the signal frequency are attenuated. That's the job of the filter! The more narrow the filter, the fewer range of frequencies we hear until, at the extreme, both the noise and the keyed signal have essentially the same frequency. By eliminating other frequencies, the filer stretches out the noise pulses at the filter's frequency. At the extreme we call it "ringing" because a pulse of noise will produce a rather distinct and long tone just like striking a bell makes it ring. Even if a filter avoids this extreme phenomena, the pulse stretching always occurs. It's what a filter does. As that stretching happens we find ourselves listening to what is essentially a tone right on the same frequency as the signal we're trying to copy. Of course, as we make the filter passband narrower the total energy in the noise is reduced as well. That should make the signal to noise ratio better, even though they are both producing essentially the same audio frequency in the speaker or phones. There seems to be some ratio that is easiest to copy between isolating only one frequency, and having the noise and signal producing the same audio frequency in the speaker or 'phones, and not isolating one frequency so that the operator hears noise covering a wide range of audio frequencies surrounding the signal on a single frequency. To say it another way, some people seem to be able to copy a signal better when there's a steady weak signal (the noise) directly on its frequency and others can copy a weak signal buried in broadband noise. For most of us there seems to be some optimum ration that works best to copy a really weak signal. Maybe that ratio varies a lot with individuals. It seems to do with how our brains process audio information. There's another reason for filters. The most common reason cited for using filters is to eliminate QRM from other signals, not noise. Comments here indicate that's a requirement that varies widely from one operator to another. Some can focus on one signal in a passband filled with signals while others are distracted and unable to copy if only two signals are present simultaneously. I suspect that I'm pretty typical: I can copy one signal among several in the passband, but not if one of the unwanted signals is very, very loud. To me, that's like listening to someone talk in a crowded party. The other voices are just babble that I can "tune out" unless someone starts shouting in my ear! If that happens I have to do something to cut down his "signal strength" if I'm going to hear anything but the "loudmouth". When I switch to a narrow filter it's usually for that reason. I need to attenuate a very loud signal nearby. Often that works, but the ideal solution for me is to use a notch filter that attenuates one narrow band of frequencies. Placing the notch on the annoying signal removes - or severely attenuates it - thereby preserving the desirable broadband characteristic of the background noise. Of course, in the middle of a contest for a DX pileup, one might need a basketful of notch filters. That's the tradeoff. After all, it'd take all the fun out of it if we could copy every signal from every where under every condition <G>. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
