[Elecraft] APF For K3: progress report

[Jim Brown]

On 10/26/2010 5:28 PM, The Smiths wrote:
> If find that in Noisy conditions I like to lower my side tone down to 380Hz, yet when I'm in a casual QSO I find that I receive better at about 450Hz.

Yes. There's another psychoacoustic principle at work here, as well as 
an electronic one.  50 Hz is a larger percentage of 350 Hz than of 600 
Hz. This gives the ear/brain more to work with in discriminating tones 
(and noise) that are closer in Hz, and it also makes the filter have 
less percentage bandwidth, which means that, all things being equal, it 
has better performance than a filter designed for the higher audio 
frequency.

Ah, you say, these are digital filters, so it doesn't matter. Wrong -- 
digital filters are simply digital emulations of analog filters, and the 
math that makes analog filters do what they do has the same effect on 
the emulations!  So there's no free lunch -- the digital filters are 
simply far more flexible, because you're building with bits, not Ls and 
Cs. :)

And there's a third VERY important operational principle, which i think 
you and others have alluded to -- all of this must be done VERY QUICKLY 
-- the entire QSO may only last 15 seconds, and you can easily lose a CQ 
frequency while taking too long to dig a weak one out of the mud.

73, Jim K9YC