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

[Guy Olinger K2AV]

Goes without saying that a lot of us would love to have this for SS CW
and especially CQWWDX CW, even if it's only a field test.

73, Guy.

On Wed, Oct 27, 2010 at 1:47 PM, Wayne Burdick <n6kr at elecraft.com> wrote:
> 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
>
>
>
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