K3 Filters for digital work

[Dave Barr-2]

Which two of the 200/250/400 and 500 hz filters are best for digital
use, primarily RTTY, both contests and dx.  I understand that for JT-65
and PSK the standard ssb filter, or even the AM filter is useable.  I am
concerned that the 200 hz might be too narrow for RTTY.  I work some CW
but almost no ssb.

73, Dave, K2YG
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[Joe Subich, W4TV-4]

 > I am concerned that the 200 hz might be too narrow for RTTY.

The 200 Hz filter is too narrow (mine measure about 190 Hz at -6dB).

The "250 Hz" filter - nominally 370 Hz at -6dB - is a good match to
the minimum necessary bandwidth for MMTTY, 2-Tone, etc. to function
well (particularly if the DSP bandwidth is set for 400 Hz).

I generally use the 400 Hz filter (435 Hz @ -6dB nominal) with good
results.  If I had the 250 Hz filter I would pair it with the INRAD
500 Hz filter as the difference between the 400 (435 Hz) and 250 (370
Hz) are not enough to be significant.  The 500 Hz and 250 Hz filters
also make good "narrow filter" for PSK31 or JT65/JT9 if necessary to
get rid of strong interfering signals on the "other end" of the
passband.

 > I work some CW but almost no ssb.

If you work no SSB but do work CW and data modes, you might consider
the 1000 Hz ("Wide CW/DATA"), 500 INRAD ("normal" CW/RTTY) and 250
("optimum RTTY") filters.  Unless you do a lot of very weak CW in
very crowded bands (DX contests) and need the 200 Hz for CW, I would
consider 500 Hz INRAD and 250 Hz the best all around combination.

73,

    ... Joe, W4TV


On 11/27/2013 12:58 PM, Dave Barr wrote:

> Which two of the 200/250/400 and 500 hz filters are best for digital
> use, primarily RTTY, both contests and dx.  I understand that for JT-65
> and PSK the standard ssb filter, or even the AM filter is useable.  I am
> concerned that the 200 hz might be too narrow for RTTY.  I work some CW
> but almost no ssb.
>
> 73, Dave, K2YG
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[Bill Frantz]

I have a 250 Hz filter I use for PSK and RTTY. It helps when
there is a strong nearby station that overloads the AtoD
converter between the K3 and my computer. During a RTTY contest,
I found it helped, but the dual frequency DSP RTTY filter was
more than enough for most contacts.

When I use the filter with PSK, I lose the computer waterfall
and the computer noise reduction because of the narrow
bandwidth. I use full bandwidth to scan for signals and the 250
filter after establishing contact. Usually a narrow filter works
better than the computer noise reduction.

I would worry about 200 Hz for RTTY. While the deviation is
nominally 170 Hz, there is energy out wider than that which may
be affected by the skirts of the 200 Hz filter.

IMHO, RTTY is like AM -- a classic mode which uses more
bandwidth than necessary, but is nice to keep around for
nostalgia and to relive history.

Cheers - Bill, AE6JV

On 11/27/13 at 9:58 AM, [hidden email] (Dave Barr) wrote:

>Which two of the 200/250/400 and 500 hz filters are best for
>digital use, primarily RTTY, both contests and dx.  I
>understand that for JT-65 and PSK the standard ssb filter, or
>even the AM filter is useable.  I am concerned that the 200 hz
>might be too narrow for RTTY.  I work some CW but almost no ssb.

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[Mike Harris-9]

I work quite a bit of RTTY and have 223 DXCC using only the stock 2k7Hz
filter squeezed down to 2k0 with DSP for general tuning and running the
split channel on a DXped.  The 500hz five pole is used for the DX
station and if I'm handing out a few points in an RTTY contest and need
to suppress nearby strong signals.

The stock 2k7Hz is used when I'm at the sharp end running split and need
to see the spread in the waterfall.

FWIW.

Regards,

Mike VP8NO

On 27/11/2013 14:58, Dave Barr wrote:
> Which two of the 200/250/400 and 500 hz filters are best for digital
> use, primarily RTTY, both contests and dx.  I understand that for JT-65
> and PSK the standard ssb filter, or even the AM filter is useable.  I am
> concerned that the 200 hz might be too narrow for RTTY.  I work some CW
> but almost no ssb.
>
> 73, Dave, K2YG
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[Joe Subich, W4TV-4]

> IMHO, RTTY is like AM -- a classic mode which uses more bandwidth
> than necessary, but is nice to keep around for nostalgia and to
> relive history.

RTTY is not like AM in using more bandwidth than necessary.  *ANY*
FSK signal will have a minimum bandwidth (as defined by ITU) of
(1.2 * shift + baud rate).  For 45.45 baud, 170 Hz shift (traditional
RTTY) that works out to about 250 Hz

73,

    ... Joe, W4TV


On 11/27/2013 2:00 PM, Bill Frantz wrote:

> I have a 250 Hz filter I use for PSK and RTTY. It helps when there is a
> strong nearby station that overloads the AtoD converter between the K3
> and my computer. During a RTTY contest, I found it helped, but the dual
> frequency DSP RTTY filter was more than enough for most contacts.
>
> When I use the filter with PSK, I lose the computer waterfall and the
> computer noise reduction because of the narrow bandwidth. I use full
> bandwidth to scan for signals and the 250 filter after establishing
> contact. Usually a narrow filter works better than the computer noise
> reduction.
>
> I would worry about 200 Hz for RTTY. While the deviation is nominally
> 170 Hz, there is energy out wider than that which may be affected by the
> skirts of the 200 Hz filter.
>
> IMHO, RTTY is like AM -- a classic mode which uses more bandwidth than
> necessary, but is nice to keep around for nostalgia and to relive history.
>
> Cheers - Bill, AE6JV
>
> On 11/27/13 at 9:58 AM, [hidden email] (Dave Barr) wrote:
>
>> Which two of the 200/250/400 and 500 hz filters are best for digital
>> use, primarily RTTY, both contests and dx.  I understand that for
>> JT-65 and PSK the standard ssb filter, or even the AM filter is
>> useable.  I am concerned that the 200 hz might be too narrow for
>> RTTY.  I work some CW but almost no ssb.
>
> -----------------------------------------------------------------------
> Bill Frantz        | If the site is supported by  | Periwinkle
> [phone]      | ads, you are the product.    | 16345 Englewood Ave
> www.pwpconsult.com |                              | Los Gatos, CA 95032
>
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[Bill Breeden]

Dave,

I have 200 Hz and 400 Hz filters in my K3.  My K3 copies RTTY fine through the 200 Hz filter.  That being said, I normally use the 400 Hz filter unless the band is crammed with signals during a contest.  The 200 Hz filter also reduces a the apparent signal to noise ratio between my ears when working CW on the low bands during very noisy conditions.

73,

Bill - NA5DX
 

>Which two of the 200/250/400 and 500 hz filters are best for digital
>use, primarily RTTY, both contests and dx.  I understand that for JT-65
>and PSK the standard ssb filter, or even the AM filter is useable.  I am
>concerned that the 200 hz might be too narrow for RTTY.  I work some CW
>but almost no ssb.

>73, Dave, K2YG

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[Joe Subich, W4TV-4]

 > My K3 copies RTTY fine through the 200 Hz filter.

This is much like the old ads "I worked 150 countries with my
Gotham vertical" ...  When one tries to pass a FSK signal through
a "too narrow" filter it doesn't just stop working.  However, the
tight skirts of the filter cause the energy in the mark and space
channels to smear (pulse stretching) into the next bit period and
cause errors in decoding.

If one is trying to copy a strong, stable signal in the presence
of strong adjacent channel interference, the "too narrow" filter
may provide improved copy by reducing AGC capture from the QRM.
However, if one is trying to copy a weak signal with flutter or
selective fading in flat noise, copy will be much worse with the
"too narrow" filter.

Chen, W7AY has written a lot on FSK decoding.  I strongly recommend
his web site (and following some of the bibliography) for understanding
RTTY, required bandwidth and filters.

73,

    ... Joe, W4TV


On 11/29/2013 9:04 AM, Bill Breeden wrote:

> Dave,
>
> I have 200 Hz and 400 Hz filters in my K3.  My K3 copies RTTY fine
> through the 200 Hz filter.  That being said, I normally use the 400 Hz
> filter unless the band is crammed with signals during a contest.  The
> 200 Hz filter also reduces a the apparent signal to noise ratio between
> my ears when working CW on the low bands during very noisy conditions.
>
> 73,
>
> Bill - NA5DX
>
>
>> Which two of the 200/250/400 and 500 hz filters are best for digital
>> use, primarily RTTY, both contests and dx.  I understand that for JT-65
>> and PSK the standard ssb filter, or even the AM filter is useable.  I am
>> concerned that the 200 hz might be too narrow for RTTY.  I work some CW
>> but almost no ssb.
>
>> 73, Dave, K2YG
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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> Post: mailto:[hidden email]
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[Jim Brown-10]

On 11/29/2013 6:25 AM, Joe Subich, W4TV wrote:
> > My K3 copies RTTY fine through the 200 Hz filter.
>
> This is much like the old ads "I worked 150 countries with my
> Gotham vertical" ...  When one tries to pass a FSK signal through
> a "too narrow" filter it doesn't just stop working.  However, the
> tight skirts of the filter cause the energy in the mark and space
> channels to smear (pulse stretching) into the next bit period and
> cause errors in decoding.

Yes.  Those of us who worked extensively in pro audio in the last 30 or
so years are well aware of the significance of the phase response of
filters and systems. We learned, for example, that varying amplitude
response has an associated variation in the phase response. Thus, I was
quite wary of using the "double humped" response provided by the K3

Quoting from W7AY's website,  "With a software demodulator that includes
a matched filter or a raised cosine filter that matches the baud rate of
the RTTY signal, the receiver's filter should be only as narrow as
needed to keep interference from clipping the sound card. "

David Wicks, G3YYD, author of the new and highly regarded RTTY decoder
called 2Tone, advises use of a filter no narrower than for RTTY
decoding. I was not at all surprised to read it in his documentation.

73, Jim K9YC
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[Carl Clawson-3]

My first thought on seeing that was "If that filter is such a great idea,
wouldn't it be built into the RTTY demodulator to begin with?"

73, Carl WS7L

On Friday, November 29, 2013, Jim Brown wrote:

> Thus, I was quite wary of using the "double humped" response provided by
> the K3.
>
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