[RTTY] 300hz or 500hz IF filter?
W5XB
w5xb at att.net
Mon Aug 26 14:22:23 EDT 2013
I have been enjoying the discussion about filters.
Back at the beginning of the discussion, there were some posts about noise
floor in sound cards.
Chen, Joe, or others, has anyone tested/posted a list of sound card
performance? They can't all be equal!
A couple of computers back, I thought the old Soundblaster had noise in it.
Presently, the card I am using does not seem to become as inaccurate as the
old Soundblaster copying a week signal next to an S9+20, but we are all
looking for improvement, eh?
Thoughts? Numbers?
73s
Grant
W5XB
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Today's Topics:
1. Re: 300hz or 500hz IF filter? (Kok Chen)
2. Re: 300hz or 500hz IF filter? (Tom Osborne)
3. Re: 300hz or 500hz IF filter? (Joe Subich, W4TV)
4. Re: 300hz or 500hz IF filter? (Jeff Blaine)
5. Re: 300hz or 500hz IF filter? (Kok Chen)
6. Re: 300hz or 500hz IF filter? (Bill Turner)
----------------------------------------------------------------------
Message: 1
Date: Sun, 25 Aug 2013 16:17:47 -0700
From: Kok Chen <chen at mac.com>
To: RTTY Reflector <rtty at contesting.com>
Cc: Bill Turner <dezrat1242 at yahoo.com>
Subject: Re: [RTTY] 300hz or 500hz IF filter?
Message-ID: <8BA9A9E8-FEE4-40AB-AA2F-146ECEC89E77 at mac.com>
Content-Type: text/plain; charset=us-ascii
On Aug 25, 2013, at 2:20 PM, Bill Turner wrote:
> Long experience has shown that around 300-350 Hz on the typical ham
receiver
> works. Less does not.
To paraphrase the other Bill (Clinton), that depends on what you mean by
"works."
If you have built and adjusted demodulators before, you know that they don't
suddenly shut down. They simply degrade.
By how much do they degrade? Again, I recommend that you look *very, very*
carefully (be sure to look at the scales, not just the shape of the curve)
at Figure 2.2 here of what cascading an extra filter does to RTTY error
rates:
http://www.w7ay.net/site/Technical/RTTY%20Transmit%20Filters/index.html
Notice that the error doubles from an ideal 300 Hz filter to an ideal 210 Hz
filter.
Doubling error is never a good thing (especially if you are providing cell
phone service), but look at the scale again.
The ISI from cascading an extra filter (e.g., the filter in your superhet)
adds an extra 1 error per 250 characters for the 300 Hz filter, and adding 1
error per 100 characters for an ideal 200 Hz filter.
An extra 1 error per couple of seconds may bother guys like me, but probably
won't cause a contester to worry -- most exchanges are fewer than 100
characters long.
I venture to add that a contester can probably tolerate an extra error rate
of even 1 error per 50 characters -- in which case, an ideal 180 Hz filter
will work.
So, any practical filter that can pass 180 Hz to 200 Hz worth of clean
passband will probably work more than sufficiently for contesting. If you
want to dig out the weak DXpedition, you will need every little bit of
reduced error as you can, since SNR is not in your favor to start with, and
that is where you will need to widen the bandwidth.
A filter is flat and has less than 1 ms of group delay for 180 Hz is
probably good enough for contesting.
Us modem designers are trying to squeeze every drop of blood from our modems
(I know that Dave W1HKJ, Stefan DO2SMF and David G3YYD are constantly trying
eek out an extra percent or two fewer errors with their modems). So we
absolutely care about 2x type errors (in the end that will benefit
everybody. But that 2x of errors from ISI is not going to bother most
contesters.
73
Chen, W7AY
------------------------------
Message: 2
Date: Sun, 25 Aug 2013 17:38:32 -0700
From: "Tom Osborne" <w7why at frontier.com>
To: "RTTY" <rtty at contesting.com>
Subject: Re: [RTTY] 300hz or 500hz IF filter?
Message-ID: <981DFEB97E6F4F4091CD283526814FD4 at Tom>
Have to disagree, Bill.
I guess it might boil down to what exact width is my Inrad 250 cycle filter?
It might be more - it might be less than 250. But, I do know that for RTTY
and also CW, the 250 cycle filter works much better to reduce QRM than the
500 cycle filter in my other radio.
After using the 250 cycle filter, when I switch to the other radio with the
500 cycle filter, it is like there is no filter at all.
Don't know about all the formulas, this times that, etc, but in real life,
250 cycles works! 73
Tom W7WHY
On 8/25/2013 2:20 PM, Bill Turner wrote:
> Long experience has shown that around 300-350 Hz on the typical ham
> receiver works. Less does not.
------------------------------
Message: 3
Date: Sun, 25 Aug 2013 21:16:22 -0400
From: "Joe Subich, W4TV" <lists at subich.com>
To: rtty at contesting.com
Subject: Re: [RTTY] 300hz or 500hz IF filter?
Message-ID: <521AAC66.8000706 at subich.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> An extra 1 error per couple of seconds may bother guys like me, but
> probably won't cause a contester to worry -- most exchanges are fewer
> than 100 characters long.
>
> I venture to add that a contester can probably tolerate an extra
> error rate of even 1 error per 50 characters -- in which case, an
> ideal 180 Hz filter will work.
However, since that error is not predictable and certainly does *not*
occur after 50 "good" characters, one is taking a big chance that the
error changes ("busts") a call. Just a few busted calls (or busted
exchanges) - with the potential penalties and loss of multipliers -
can be a disaster for a contester.
Chen, it would help if we knew how much group delay can be tolerated
in the filters. How much unpredictable delay is there in "the ether"
and how much might be tolerated in a sharp sided filter before the
the demodulation function falls "over the cliff"? In addition, how
much differential gain ("non-flat" passband) can be tolerated -
particularly when the very narrow filter is slightly mis-tuned or
asymmetrical? What does the built-in "selective fading" do to the
demodulation function, particularly if the software lacks effective
ATC?
I don't think your demodulation bandwidth functions consider either
of those real world error contributions from a "too narrow" roofing
filter.
73,
... Joe, W4TV
On 8/25/2013 7:17 PM, Kok Chen wrote:
>
> On Aug 25, 2013, at 2:20 PM, Bill Turner wrote:
>
>> Long experience has shown that around 300-350 Hz on the typical ham
>> receiver works. Less does not.
>
> To paraphrase the other Bill (Clinton), that depends on what you mean
> by "works."
>
> If you have built and adjusted demodulators before, you know that
> they don't suddenly shut down. They simply degrade.
>
> By how much do they degrade? Again, I recommend that you look
> *very, very* carefully (be sure to look at the scales, not just the
> shape of the curve) at Figure 2.2 here of what cascading an extra
> filter does to RTTY error rates:
>
> http://www.w7ay.net/site/Technical/RTTY%20Transmit%20Filters/index.html
>
> Notice that the error doubles from an ideal 300 Hz filter to an
> ideal 210 Hz filter.
>
> Doubling error is never a good thing (especially if you are providing
> cell phone service), but look at the scale again.
>
> The ISI from cascading an extra filter (e.g., the filter in your
> superhet) adds an extra 1 error per 250 characters for the 300 Hz
> filter, and adding 1 error per 100 characters for an ideal 200 Hz
> filter.
>
> An extra 1 error per couple of seconds may bother guys like me, but
> probably won't cause a contester to worry -- most exchanges are fewer
> than 100 characters long.
>
> I venture to add that a contester can probably tolerate an extra
> error rate of even 1 error per 50 characters -- in which case, an
> ideal 180 Hz filter will work.
>
> So, any practical filter that can pass 180 Hz to 200 Hz worth of
> clean passband will probably work more than sufficiently for
> contesting. If you want to dig out the weak DXpedition, you will
> need every little bit of reduced error as you can, since SNR is not
> in your favor to start with, and that is where you will need to widen
> the bandwidth.
>
> A filter is flat and has less than 1 ms of group delay for 180 Hz is
> probably good enough for contesting.
>
> Us modem designers are trying to squeeze every drop of blood from our
> modems (I know that Dave W1HKJ, Stefan DO2SMF and David G3YYD are
> constantly trying eek out an extra percent or two fewer errors with
> their modems). So we absolutely care about 2x type errors (in the
> end that will benefit everybody. But that 2x of errors from ISI is
> not going to bother most contesters.
>
> 73 Chen, W7AY
>
>
>
>
>
>
>
>
>
> _______________________________________________ RTTY mailing list
> RTTY at contesting.com
> http://lists.contesting.com/mailman/listinfo/rtty
>
------------------------------
Message: 4
Date: Mon, 26 Aug 2013 10:51:27 +0900
From: "Jeff Blaine" <keepwalking188 at yahoo.com>
To: "Joe Subich, W4TV" <lists at subich.com>, <rtty at contesting.com>
Subject: Re: [RTTY] 300hz or 500hz IF filter?
Message-ID: <5C22C0E8C0954A42A78598E0E6528BB1 at w520>
Content-Type: text/plain; format=flowed; charset="iso-8859-1";
reply-type=response
That busted call observation is a pretty good point, Joe. Especially for a
contest where the copy is not necessarily great to begin with (like the CQ
DX RTTY, etc).
73/jeff/ac0c
www.ac0c.com
alpha-charlie-zero-charlie
-----Original Message-----
From: Joe Subich, W4TV
Sent: Monday, August 26, 2013 10:16 AM
To: rtty at contesting.com
Subject: Re: [RTTY] 300hz or 500hz IF filter?
> An extra 1 error per couple of seconds may bother guys like me, but
> probably won't cause a contester to worry -- most exchanges are fewer
> than 100 characters long.
>
> I venture to add that a contester can probably tolerate an extra
> error rate of even 1 error per 50 characters -- in which case, an
> ideal 180 Hz filter will work.
However, since that error is not predictable and certainly does *not*
occur after 50 "good" characters, one is taking a big chance that the
error changes ("busts") a call. Just a few busted calls (or busted
exchanges) - with the potential penalties and loss of multipliers -
can be a disaster for a contester.
Chen, it would help if we knew how much group delay can be tolerated
in the filters. How much unpredictable delay is there in "the ether"
and how much might be tolerated in a sharp sided filter before the
the demodulation function falls "over the cliff"? In addition, how
much differential gain ("non-flat" passband) can be tolerated -
particularly when the very narrow filter is slightly mis-tuned or
asymmetrical? What does the built-in "selective fading" do to the
demodulation function, particularly if the software lacks effective
ATC?
I don't think your demodulation bandwidth functions consider either
of those real world error contributions from a "too narrow" roofing
filter.
73,
... Joe, W4TV
On 8/25/2013 7:17 PM, Kok Chen wrote:
>
> On Aug 25, 2013, at 2:20 PM, Bill Turner wrote:
>
>> Long experience has shown that around 300-350 Hz on the typical ham
>> receiver works. Less does not.
>
> To paraphrase the other Bill (Clinton), that depends on what you mean
> by "works."
>
> If you have built and adjusted demodulators before, you know that
> they don't suddenly shut down. They simply degrade.
>
> By how much do they degrade? Again, I recommend that you look
> *very, very* carefully (be sure to look at the scales, not just the
> shape of the curve) at Figure 2.2 here of what cascading an extra
> filter does to RTTY error rates:
>
> http://www.w7ay.net/site/Technical/RTTY%20Transmit%20Filters/index.html
>
> Notice that the error doubles from an ideal 300 Hz filter to an
> ideal 210 Hz filter.
>
> Doubling error is never a good thing (especially if you are providing
> cell phone service), but look at the scale again.
>
> The ISI from cascading an extra filter (e.g., the filter in your
> superhet) adds an extra 1 error per 250 characters for the 300 Hz
> filter, and adding 1 error per 100 characters for an ideal 200 Hz
> filter.
>
> An extra 1 error per couple of seconds may bother guys like me, but
> probably won't cause a contester to worry -- most exchanges are fewer
> than 100 characters long.
>
> I venture to add that a contester can probably tolerate an extra
> error rate of even 1 error per 50 characters -- in which case, an
> ideal 180 Hz filter will work.
>
> So, any practical filter that can pass 180 Hz to 200 Hz worth of
> clean passband will probably work more than sufficiently for
> contesting. If you want to dig out the weak DXpedition, you will
> need every little bit of reduced error as you can, since SNR is not
> in your favor to start with, and that is where you will need to widen
> the bandwidth.
>
> A filter is flat and has less than 1 ms of group delay for 180 Hz is
> probably good enough for contesting.
>
> Us modem designers are trying to squeeze every drop of blood from our
> modems (I know that Dave W1HKJ, Stefan DO2SMF and David G3YYD are
> constantly trying eek out an extra percent or two fewer errors with
> their modems). So we absolutely care about 2x type errors (in the
> end that will benefit everybody. But that 2x of errors from ISI is
> not going to bother most contesters.
>
> 73 Chen, W7AY
>
>
>
>
>
>
>
>
>
> _______________________________________________ RTTY mailing list
> RTTY at contesting.com
> http://lists.contesting.com/mailman/listinfo/rtty
>
_______________________________________________
RTTY mailing list
RTTY at contesting.com
http://lists.contesting.com/mailman/listinfo/rtty
------------------------------
Message: 5
Date: Sun, 25 Aug 2013 20:08:04 -0700
From: Kok Chen <chen at mac.com>
To: RTTY Reflector <rtty at contesting.com>
Cc: "Joe Subich, W4TV" <lists at subich.com>
Subject: Re: [RTTY] 300hz or 500hz IF filter?
Message-ID: <76864808-6CFD-4D7C-875A-E49379E120C9 at mac.com>
Content-Type: text/plain; charset=us-ascii
On Aug 25, 2013, at 6:16 PM, Joe Subich, W4TV wrote:
> Chen, it would help if we knew how much group delay can be tolerated
> in the filters.
Joe, you already have seen those. Perhaps, you just don't realize it.
Just go take a look at the flutter curves at VE3NEA's site. You can then
relate the path delays back to filter group delays.
http://www.dxatlas.com/rttycompare/
Do you notice that some modems asymptotes out (i.e., error rate stops
improving as SNR improves)? That is the effect of ISI caused by multipath
(relative delays between Mark and Space bits).
The irony is that MMTTY does not asymptote like TrueTTY and MixW.
The explanation is simple -- MMTTY does not use any filter that is close to
either a Raised Cosine or a Matched Filter. It is wide as a barn door in
DSP terms. So basically, while MMTTY filters have terrible SNR
characteristics (as shown in the AWGN curves), it also does not ring as much
because it is so wide (but not infinite HI HI).
Remember what I wrote about Raised Cosine and ringing? While MMTTY does not
place exact zeros at the mid-bit locations, the ringing is low, so that
while the ISI may be non-zero, it is also much lower than multipath through
a Raised Cosine.
How much multipath is in VE3NEA's plots? Alex had used the CCIR Flutter
Curve (the HF model comes from the CCIR 520 document that was the precursor
of the ITU 1487 document that has much more comprehensive HF Channel
models).
Those few curves in VE3NEA tell a whole lot of story -- if you know how to
read them.
The delay between two propagation rays in the CCIR Flutter curve is only 0.5
ms (with a Doppler spreading of 10 Hz though; so that also required a wider
filter to pass the fluttered signal). And look at how destructive that
profile is already to demodulating RTTY -- it pretty much limited TrueTTY to
about about 1 error out of 200, and limited the performance of MixW to one
error in 50 characters *no matter how loud the station is* over the noise
floor!!
MMTTY on the other hand is capable of producing 1 error out of 1000 if you
give it a strong signal with good enough SNR.
I agree, Joe, that the proper thing to do is to substitute a model of a
crystal filter in place of a model of the HF channel (it is really quite
simple to do). I probably will never do it though, since I have moved away
from using superhets, and have no need to use any crystal filter at all in
my receiver chain. Perhaps we can persuade Andy K0SM to do it?
73
Chen, W7AY
------------------------------
Message: 6
Date: Sun, 25 Aug 2013 23:10:52 -0700
From: Bill Turner <dezrat1242 at yahoo.com>
To: RTTY Reflector <rtty at contesting.com>
Subject: Re: [RTTY] 300hz or 500hz IF filter?
Message-ID: <1srl191n245hae9ouru5aqrkcge6krm8se at 4ax.com>
Content-Type: text/plain; charset=us-ascii
ORIGINAL MESSAGE: (may be snipped)
On Sun, 25 Aug 2013 17:38:32 -0700, Tom wrote:
>I guess it might boil down to what exact width is my Inrad 250 cycle
filter?
>It might be more - it might be less than 250. But, I do know that for RTTY
>and also CW, the 250 cycle filter works much better to reduce QRM than the
>500 cycle filter in my other radio.
REPLY:
As I said before and will say again, don't take the "250 Hz" label on your
filter as absolute truth. Whether it works or not depends on the steepness
of the skirts. My IC-756Pro3 had a "250 Hz" filter that printed perfectly.
My IC-7600 also has a "250 Hz" filter that does not print worth a darn. What
is the difference? The steepness of the skirts. The 'Pro3 had relatively
wide skirts, the '7600 skirts are steeper. When I widened the '7600 out to
300 Hz, the print was fine again.
The correct bandwidth for your receiver is one, that if you widen it out,
you see no improvement.
73, Bill W6WRT
------------------------------
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