[Elecraft] K3 APF (audio peaking filter) progress report #2

[Wayne Burdick]

We now have our basic APF function in place, and I'm amazed at how  
well it works. Lyle (KK7P, our DSP guy) surprised even himself :)  It  
improves signal to noise ratio significantly, as evaluated both by  
measurement (S+N/N increase of ~3 to 5 dB using the K3's built-in dBV  
meter) and by ear (makes many weak signals buried in noise easier to  
copy). Because we're using DSP, we've been able to improve on analog  
implementations by providing constant gain over the full pitch range  
and ideal passband characteristics.

One explanation for why APF improves copy is that it is *not* a brick- 
wall filter: it has a narrow peak, but the skirts are very broad,  
leading to a more "natural" sound. It has fewer of the artifacts you  
get with a really narrow filter.

To borrow terminology from our existing DUAL PB function, the K3's  
first and second I.F. filters provide the "context" bandwidth (along  
with a very high level of ultimate attenuation), while APF provides  
the "focus." The combination of the two sometimes works better than  
brick-wall filters at an equivalent narrow bandwidth, at least in some  
QRN/QRM scenarios. For example, I was listening to 40 meters last  
night with S5 background noise. Copy on many signals was better with  
WIDTH=400 and APF on, than with WIDTH set to 100 or 50 Hz and APF OFF.

Many of you sent suggestions for how APF should be turned on and  
controlled (thanks). We're weighing various alternatives.

Wayne
N6KR