Guy, your missing the whole point of what APF is and what is does, has nothing to do with selectivity, nothing to do with skirts at all, it is in the audio chain and there for no effect or cause with AGC Is rarely if ever used on a signal that is loud enough to copy, its mainly used for signals that are at the noise level or very weak. Narrow bandwidth can be the enemy of APF. Many times copy at wider width is the best with APF. APF never drops off the signal, it enhances it. APF = Audio Peak Filter not DSP or some other manipulation to try and fake it. To say that DSP beats APF in the proper use of APF by 10 or 20 db is proof that there is a lack of understanding of what APF does or even is. APF is not used as a selectivity tool. Its not a Q-multiplier type function, or a filter as in xtal filter or DSP filter. There were many reiterations of APF in various radios, the FT-1000 being one, and with the 1000 there were various versions even, the one that works correctly is the old original circuit, many ops had to change components to the older circuit to get the APF to function, so unless you had one of the correct versions you have not heard APF work. > After watching spectrogram on a noise signal and noting a much sharper shape > on 50 Hz DS as an audio peak filter, I began to wonder about some of the > posts I was seeing asking for APF. What was going on? After some > measurements... > > 1) the 50 Hz DSP bandwidth is *narrower* at the top than any APF I have > used. The 100 or 150 Hz DSP is more like it. > > 2) the 50/100 Hz DSP has *far* steeper skirts than the APF. > > 3) the APF *sounds to the ear* to drop off a signal more, but that is > because the signal as it moves outside the APF is still controlling the IF > AGC off the APF center, holding the input to the APF constant. In the > 50/100 Hz DSP the AGC opens up the gain when one tunes off the signal, > giving the *appearance* of no skirts. Watching the S meter will show what is > going on. This effect dissappears entirely with the AGC *off*, where the > signal falls off the table on the DSP skirts as one tunes away. > > If one simply measures the amount of discrimination between two close > signals, the 50 Hz DSP can beat the APF by 10, 20 db or more. Listening to > two S9 signals only 150 Hz apart, the 50 Hz DSP can completely isolate > them. In run situation, the shift and 50 Hz DSP would allow me to pick up > just one and then the other by remembering their respective tones, without > changing my transmit or RX reference. > > The SOUND of tuning across a signal with APF in a 500 Hz IF bandwidth is > illusory in close signal discrimination, even if it is one's favorite analog > sound. Isn't this just dumbing down a superior DSP function to "sound like" > an analog result? > > And reversing myself in one of my earlier posts, the modulation "pops" > produced by the sharp skirt drops on 10 Hz shift increments off a strong > signal are annoying, and perhaps the shift can be damped just a tiny bit to > remove them. >
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