[Elecraft] [K3] Passband "flatness" for FSK441

[Bill W5WVO]

David Woolley (E.L) wrote:

> The reason that the decoder can cope with up to 400 Hz off frequency
> is probably because there are receiver filters around that will allow
> that, but I don't see that that is a reason for always making the
> receive filter that wide.  The system has been clearly designed  to
> work with filters that are well behaved over a much narrower
> frequency range.

Probably so, but it's useful, operationally, to start out with the filter 
tolerance set to 400 (its max value), because transmitters and receivers are 
not usually exactly on the same frequency. This TX/RX frequency offset problem 
is typically worse on the higher-frequency bands (144 MHz, 222 MHz, even 432 
MHz). Once a signal has been detected, RIT can be used to bring the received 
signal close to the transmit frequency, and the WSJT tolerance figure is then 
decreased to a more appropriate level. This filters out a lot of false decodes 
on noise, especially when working with a very marginal signal.

> If the decoder is good, it will have an adaptive filter that will
> automatically equalise the signal for good digital decoding.

All of this is somewhat speculative.  :-)  Joe Taylor, to the best of my 
knowledge, isn't on this list. (If you are, Joe, speak up!) And I certainly 
don't pretend to understand either filter physics or decoding algorithms as 
well as you obviously do, so I really appreciate your technical explanations. 
What I am looking forward to, once I get my K3, is experimenting with filter 
width and equalization settings (versus no equalization, of course) to see if 
I can improve the decoding of extremely marginal signals, where the worst-case 
upper channel is at or just above the noise floor.

It could be that Joe's algorithms are so powerfully adaptive that nothing one 
can do at the "analog" RX end will make things any better. That wouldn't 
really surprise me. But meteor-scatter is extreme weak-signal work, and every 
half-dB you can gain on the noise floor is effort well-spent.

Of course, the very best thing you can do in that respect is move QTH out to 
the middle of Kansas somewhere. :-)

Bill W5WVO