DSP / filtering thoughts.

[Darwin, Keith]

Something bugs me about the implementation of DSP filters.  It seems
we're using DSP to duplicate crystal filters by having them do bandpass
functions.  The other thing we do with DSP is noise reduction which I
hear works best at wide bandwidths so the algorithm has some noise to
work with.

But what about other DSP things?

How about a specific CW filter that takes into account CW speed and CW
elements length along with pitch?  Imagine a filter that selects a 12
wpm signal in the presence of 15 - 20 wpm signals.  Or how about a
filter than can de-flutter a signal traveling over the north pole.  Or
maybe one that blocks strong signals!

How about a filter that corrects bad CW spacing?

Or a filter that uses the CW signal as a trigger to drive a synthesized
CW signal for virtually infinite S/N ratio!  It would be like listening
to a code practice oscillator controlled by the other op.

I guess I'm thinking that there are other filtering techniques and
approaches that could be far more novel than simple band pass or low
pass and would really set DSP apart from the tried and true crystal
filters.

- Keith KD1E -

-----Original Message-----
From: [hidden email]
[mailto:[hidden email]] On Behalf Of Solosko, Robert B
(Bob)
Sent: Tuesday, May 09, 2006 9:30 AM
To: '[hidden email]'
Subject: [Elecraft] DSP & XTAL filter settings?


Hello All,

        I just added the DSP processor to my K2 and adjusted both the
DSP and XTAL filters to the same center frequency using Spectrogram. The
DSP and XTAL filters work fine together, except that when using the
default filter settings, the DSP and XTAL filters essentially duplicate
each other and it's not obvious what the best combination of filter
settings should be and how the XTAL and DSP filters should be used in
combination, particularly for SSB.

        I found a posting from Tom Hammond from January in which he
recommends that for CW, because of the signal loss of the XTAL filters
below 200Hz, use the DSP filters below 200 Hz. and the XTAL filters
above 200 Hz. Thus, it seems that I should set the CW DSP filters to
something like 200 Hz, 100 Hz, 50 Hz and LOPASS, and the XTAL CW filters
to something like 1.5 kHz, 1.0 kHz, 700 Hz and 400 Hz.  Then, when I
want the wider bandwidth, I'd use one of the XTAL filters with the DSP
filter set to LOPASS, and when I want the narrower bandwidth, I'd use
one of the DSP filters (and since the XTAL filters all would have wider
bandwidths, it wouldn't make any difference which XTAL filter is
selected).

        That seems like a reasonable approach to the DSP and XTAL filter
settings and combinations on CW. (Comments?)

        However, I have no idea how to set up the DSP and XTAL filters
and what filter combinations to use for SSB. It seems to be a waste of
the K2 capabilities just to have the DSP and XTAL SSB filters
essentially duplicate each other, as their default settings seem to do.
So, is there some optimum way to setup the DSP and XTAL filters and use
them in combination for SSB that makes best use of the K2 capabilities?

        Thanks for any advice and suggestions that you can give me.


Bob Solosko
W1SRB
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[Darwin, Keith]

I said:

How about a filter that corrects bad CW spacing?

To which Geoff replied:

      Pass.

-----------------

Pass?  Oh, but it would be great!  I could finally work those guys who
use bugs! :-)

I could also see a filter that would change the speed of the CW.  It
would be great during a contest or DXpedition to reduce the speed on
things now and then.  So after sitting in the pile-up for 30 minutes
when you finally get through and the op sends you QSL info (which you
miss 'cause he's too darn fast) you can ask your rig for a QRS replay
and voila, you have the data.

And I wonder what sorts of things a neural network could learn about CW
and what sorts of cool processing it could bring to the table.  A NN
based signal tracker might have an easier time following a signal
through the QRM / QRN / QSB jungle as it could learn things like the
op's speed and keying characteristic (swing).

Of course, the biological neural network can do a pretty good job of it
as well.

- Keith KD1E -
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[Bill Coleman-2]

On May 9, 2006, at 10:07 AM, Darwin, Keith wrote:

> Something bugs me about the implementation of DSP filters.  It seems
> we're using DSP to duplicate crystal filters by having them do  
> bandpass
> functions.

Crystal filters are just one way of rejecting unwanted signals. Other  
types of receiver designs have used tuned circuits, ceramic  
resonantors, and mechanical filters. Why would DSP be any different  
from these?

There are certainly some issues in using an audio DSP to implement  
receiver filtering. Fortunately, the K2 has a pretty good crystal  
filter design, and an excellent mixer.

My only beef with the K2's crystal filters is the poor bandpass shape  
and slope factor, especially for the OP1 (SSB) filter. The DSP  
filtering really helps to clean up this filter.

> The other thing we do with DSP is noise reduction which I
> hear works best at wide bandwidths so the algorithm has some noise to
> work with.

This works pretty well with the OP1 (SSB) filter.

> But what about other DSP things?

Don't forget about the automatic notch filter! It makes 40m SSB  
tolerable.

> How about a specific CW filter that takes into account CW speed and CW
> elements length along with pitch?  Imagine a filter that selects a 12
> wpm signal in the presence of 15 - 20 wpm signals.

That would be pretty tough to do. Properly detecting an OOK (on-off  
keyed) is really difficult, because the off state can be easily  
confused by noise, especially on weak signals.

And synchronizing to a particular speeds works better if the speed is  
known and fixed. But CW speeds are highly variable, especially when  
hand-keyed. K6STI had a RTTY decoder that did something like this --  
it synchronized to the sending and could fill in bits that were  
mostly or partially missed.

>   Or how about a
> filter than can de-flutter a signal traveling over the north pole.

This is possible. CocoaModem implements something like this for RTTY.

> Or maybe one that blocks strong signals!

This is tough. Rejecting strong signals is what we attempt to do with  
frequency-based filters.

> How about a filter that corrects bad CW spacing?

Certainly possible, but you have the same problem with building an  
OOK detector, and you have the speed-variance problem.

> Or a filter that uses the CW signal as a trigger to drive a  
> synthesized
> CW signal for virtually infinite S/N ratio! It would be like listening
> to a code practice oscillator controlled by the other op.

This has been done! They call it "regenerated CW". Basically, it  
requires an OOK detector driving a tone oscillator.

> I guess I'm thinking that there are other filtering techniques and
> approaches that could be far more novel than simple band pass or low
> pass and would really set DSP apart from the tried and true crystal
> filters.

Of course, another thing we could do is to use modulation techniques  
other than CW (OOK) that are easier to digitally detect. (eg FSK,  
multi-FSK, PSK, etc)

Bill Coleman, AA4LR, PP-ASEL        Mail: [hidden email]
Quote: "Not within a thousand years will man ever fly!"
             -- Wilbur Wright, 1901

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