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Dual Pass Band

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k0wa@swbell.net

Dual Pass Band

Dear Elecrafter RTTY Enthusiasts....

Ran RU this weekend and was experimenting with Dual PB. I found that the sensitivity of the receiver would be much less that using the standard 500 Hz Filter with the bandwidth set at 350. What causes that or is something wrong? I could even see the difference on the P3. The K3 rtty abilities are pretty darn good...

Lee
K0WA

In our day and age it seems that Common Sense is in short supply. If you don't have any Common Sense - get some Common Sense and use it. If you can't find any Common Sense, ask for help from somebody who has some Common Sense. Is Common Sense divine?

Common Sense is the image of the Creator expressing revealed truth in my mind. - John W. (Kansas)

Never interfere with anything that isn't bothering you.
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k0wa@swbell.net

Re: Dual Pass Band

Is this designed this way? That does seem to be something that needs to be fixed. Or, am I not seeing the whole picture here?

Lee - K0WA

________________________________
From: Mark <[hidden email]>
To: Lee Buller <[hidden email]>
Sent: Monday, January 6, 2014 10:17 AM
Subject: Re: [Elecraft] Dual Pass Band

I found the DPF degraded all but the strongest most stable signals, unless I really opened
up the bandwidth. You could see a good signal but the decode was bad. I stopped using
DPF after that. There is more support for not using these filters as it degrades the system,
unless you are using the k3s internal rtty decoder.

Look at
http://www.nccc.cc/pdf/RTTY-Receive-W0YK.pdf
For good tips.

Mark. N2QT

> On Jan 6, 2014, at 10:52 AM, Lee Buller <[hidden email]> wrote:
>
>
>
> Dear Elecrafter RTTY Enthusiasts....
>
> Ran RU this weekend and was experimenting with Dual PB. I found that the sensitivity of the receiver would be much less that using the standard 500 Hz Filter with the bandwidth set at 350. What causes that or is something wrong? I could even see the difference on the P3. The K3 rtty abilities are pretty darn good...
>
> Lee
> K0WA
>
>
> In our day and age it seems that Common Sense is in short supply. If you don't have any Common Sense - get some Common Sense and use it. If you can't find any Common Sense, ask for help from somebody who has some Common Sense. Is Common Sense divine?
>
> Common Sense is the image of the Creator expressing revealed truth in my mind. - John W. (Kansas)
>
> Never interfere with anything that isn't bothering you.
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html

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Mike K8CN

Re: Dual Pass Band

In reply to this post by k0wa@swbell.net

Lee,

Ed, W0YK, has a commentary on this very subject in the current issue of NCJ (subscription required). Perhaps he will weigh in directly here, but his recommendation was to use the normal filter rather than dual passband. This is not an issue unique to the K3.

73,
Mike, K8CN

Joe Subich, W4TV-4

Re: Dual Pass Band

That information is based on input from K7AY (cocoaModem) and G3YYD
(2-Tone). The software decoders do a better job when supplied with
audio at least 400 Hz wide without frequency shaping from the IF (DSP)
filter. The Dual Band Pass filters in general introduce significant
phase delays and amplitude inequality which results in poorer decoding
accuracy in modern FFT based demodulators - although the frequency
shaping may be of value in older amplitude (filter/detector) style
demodulators.

73,

... Joe, W4TV

On 1/6/2014 1:26 PM, Mike K8CN wrote:

> Lee,
>
> Ed, W0YK, has a commentary on this very subject in the current issue of NCJ
> (subscription required). Perhaps he will weigh in directly here, but his
> recommendation was to use the normal filter rather than dual passband. This
> is not an issue unique to the K3.
>
> 73,
> Mike, K8CN
>
>
>
> --
> View this message in context: http://elecraft.365791.n2.nabble.com/Dual-Pass-Band-tp7582564p7582570.html
> Sent from the Elecraft mailing list archive at Nabble.com.
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>

Carl Clawson-3

Re: Dual Pass Band

If a digimode decoder is good enough then it can only be hurt, not helped,
by additional pre-filtering. This is because in the presence of noise there
is a statistical limit ("optimal detection") to how faithfully you can
extract the desired signal. Which is why I've said before: If the dual
filter were such a great idea, it would already be written into the
software for your RTTY decoder, at least as part of one of the decode
methods you can select. It's possible that some less capable decoders will
be helped by it, but if it doesn't help you then don't use it. I use MMTTY
and have not noticed it to help. Never hurts to try, though. There are so
many varied impairments to over-the-air signals that no simple collection
of decoders will have one that's optimal in every case.

73, Carl WS7L

On Monday, January 6, 2014, Joe Subich, W4TV wrote:

>
> That information is based on input from K7AY (cocoaModem) and G3YYD
> (2-Tone). The software decoders do a better job when supplied with
> audio at least 400 Hz wide without frequency shaping from the IF (DSP)
> filter. The Dual Band Pass filters in general introduce significant
> phase delays and amplitude inequality which results in poorer decoding
> accuracy in modern FFT based demodulators - although the frequency
> shaping may be of value in older amplitude (filter/detector) style
> demodulators.
>
>
> 73,
>
> ... Joe, W4TV

Mark-3

Re: Dual Pass Band

In reply to this post by Mike K8CN

Ed has a presentation on decoders he did for NCCC available at

http://www.nccc.cc/pdf/RTTY-Receive-W0YK.pdf

Also the rtty reflector
http://lists.contesting.com/mailman/listinfo/rtty

Has had multiple discussions on this and related topics.

Mark. N2QT

> On Jan 6, 2014, at 1:26 PM, Mike K8CN <[hidden email]> wrote:
>
> Lee,
>
> Ed, W0YK, has a commentary on this very subject in the current issue of NCJ
> (subscription required). Perhaps he will weigh in directly here, but his
> recommendation was to use the normal filter rather than dual passband. This
> is not an issue unique to the K3.
>
> 73,
> Mike, K8CN
>
>
>
> --
> View this message in context: http://elecraft.365791.n2.nabble.com/Dual-Pass-Band-tp7582564p7582570.html
> Sent from the Elecraft mailing list archive at Nabble.com.
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html

Jim Brown-10

Re: Dual Pass Band

In reply to this post by Joe Subich, W4TV-4

On 1/6/2014 10:34 AM, Joe Subich, W4TV wrote:
> That information is based on input from K7AY (cocoaModem) and G3YYD
> (2-Tone). The software decoders do a better job when supplied with
> audio at least 400 Hz wide without frequency shaping from the IF (DSP)
> filter. The Dual Band Pass filters in general introduce significant
> phase delays and amplitude inequality which results in poorer decoding
> accuracy in modern FFT based demodulators - although the frequency
> shaping may be of value in older amplitude (filter/detector) style
> demodulators.

Actually, it's based on things a lot more fundamental than that. In the
world of pro audio, we learned nearly four decades ago that non-flat
amplitude response is accompanied by phase shift (distortion). The ear
does not like phase distortion, and neither do decoders. Filters produce
distortion, not only when their amplitude response is changing (the
slopes), but also in the passband.

When i first encountered the dual-filter approach separate from the
decoder itself, it looked to me like a bad idea for this reason. It was
good to see it confirmed by G3YYD in the doc for his 2Tone RTTY decoder.
BTW -- Nobel Laureate Joe Taylor, K1JT, author of the WSJT digital
protocols, said exactly the same thing during a lecture I heard him do
at Pacificon several years ago.

73, Jim K9YC
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Bill Coleman-2

Re: Dual Pass Band

On Jan 6, 2014, at 5:16 PM, Jim Brown <[hidden email]> wrote:
>
> Actually, it's based on things a lot more fundamental than that. In the world of pro audio, we learned nearly four decades ago that non-flat amplitude response is accompanied by phase shift (distortion). The ear does not like phase distortion, and neither do decoders. Filters produce distortion, not only when their amplitude response is changing (the slopes), but also in the passband.
>
> When i first encountered the dual-filter approach separate from the decoder itself, it looked to me like a bad idea for this reason. It was good to see it confirmed by G3YYD in the doc for his 2Tone RTTY decoder. BTW -- Nobel Laureate Joe Taylor, K1JT, author of the WSJT digital protocols, said exactly the same thing during a lecture I heard him do at Pacificon several years ago.

That said, the dual pass band filter is often useful for squashing QRM that also confuses the decoder. Like, when someone starts tuning up between your mark and space frequency….

Just don’t run it all the time. Use it for those brief times when you need it.

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

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ab2tc

Re: Dual Pass Band

Hi,

I'll chance another entry on this topic. I am sure that those who are experts in statistical communications theory can prove analytically that the optimal filter ahead of the decoder in the presence of plain white Gaussian noise is some form of Gaussian response with bandwidth slightly wider than the frequency shift. The presence of "QRM" is a different matter. It's hard (impossible?) to quantify mathematically and it is quite possible/likely that the double hump filter can help in those cases.

AB2TC - Knut

Bill Coleman-2 wrote

On Jan 6, 2014, at 5:16 PM, Jim Brown <[hidden email]> wrote:
>
> Actually, it's based on things a lot more fundamental than that. In the world of pro audio, we learned nearly four decades ago that non-flat amplitude response is accompanied by phase shift (distortion). The ear does not like phase distortion, and neither do decoders. Filters produce distortion, not only when their amplitude response is changing (the slopes), but also in the passband.
>
> When i first encountered the dual-filter approach separate from the decoder itself, it looked to me like a bad idea for this reason. It was good to see it confirmed by G3YYD in the doc for his 2Tone RTTY decoder. BTW -- Nobel Laureate Joe Taylor, K1JT, author of the WSJT digital protocols, said exactly the same thing during a lecture I heard him do at Pacificon several years ago.

That said, the dual pass band filter is often useful for squashing QRM that also confuses the decoder. Like, when someone starts tuning up between your mark and space frequency….

Just don’t run it all the time. Use it for those brief times when you need it.

Bill Coleman, AA4LR, PP-ASEL Mail: [hidden email]
Web: http://boringhamradiopart.blogspot.com
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
<snip>