Thanks Bob. I understand now. The AFX provides the effect of both narrowing the bandwidth and increasing the audio gain. That increases the effect of having the signal "pop" out of the noise.
Any time we listen to a single-frequency or narrow-band signal surrounded by white (or pink) noise and then reduce the bandwidth to remove the off-frequency noise it will seem as the audio level has dropped too. That's because our hearing reacts to the total audio power coming down the audio channel which greatly varies with the bandwidth when noise is present. So, after decreasing the bandwidth to kill much of the noise, we raise the gain to bring the signal back up to a normal listening level. When we do that with filters in the I.F., the receiver's AGC system will bring the gain up automatically as the selectivity is narrowed. AFX provides the same effect when more filtering is wanted at audio frequencies - after the AGC loop. I doubt if I'd notice the difference since I normally copy weak signals with the AGC off. I'm used to riding the gain manually anyway. The first "active" audio filter I worked with that provided both gain and selectivity was the Selectoject from the 1950's. It was very popular back then. Ron AC7AC -----Original Message----- ...But what really makes it so great (and distinguishes it from just an audio filter) is that it adds another dimension. In addition It peaks or boosts the narrowed down audio signal. That is why we users say that it pulls the signal up out of the mud. The FT-1000's APF and my TS-930S (I wish I still had her, sniff, sniff) did this very effectively. The CW signal can be right down at the noise level, you know it is in there but cannot copy. By adjusting the APF you can pull the signal up out of the mud and copy the signal. Bob K6UJ ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
