Thank you Chen, I am happy to have remembered the summary of your earlier
comments.
I find using an extremely narrow crystal filter to be difficult for the
tuning reason you mentioned. In S&P especially, contest exchanges may not
be very long in duration making tuning with an extremely sharp crystal
filter actually counter productive.
My preference is to use a bit wider filter, about 300-500 hz, for the
crystal side. And then let the demod program DSP "pull" the signal from
that - my experience (limited by this group standards, of course) is that
the MMTTY program when presented with a pretty small window can AFC well in
cooperation with the operator - if the bandwidth from the rig (either rig
crystal or DSP sourced) has both tones within this 300-500 hz passband.
Maybe some day I will upgrade to Mac so I can use Coco. Right?
73/jeff/ac0c
--------------------------------------------------
From: "Kok Chen" <chen@mac.com>
Sent: Tuesday, June 01, 2010 2:47 PM
To: "RTTY Reflector" <rtty@contesting.com>
Cc: "Jeff Blaine AC0C" <keepwalking188@yahoo.com>
Subject: Re: [RTTY] 200 hz filter on RTTY
>
> On Jun 1, 2010, at 11:05 AM, Jeff Blaine AC0C wrote:
>
>> The stronger the signal being heard, the less
>> reliance there is on the power contained in the sidebands. So the 325 hz
>> number discussed earlier on the board some time back is I think inclusive
>> of
>> the first 2 sidebands on either side. Hope I have that right...
>
> Let me see if I can make it a little cleared with the three "filter
> selection" (my excuse is English is not my first language, HI HI):
>
>
> (1) To pass the first keying sideband completely, you need to pass 215 Hz
> worth of perfectly centered RTTY signal to the demodulator.
>
> When the SNR is good, you can get 100% print even if you don't pass the
> higher order terms of the keying sideband. 215 Hz is all the bandwidth
> you need (if you are perfectly centered on the signal) when the signal is
> loud.
>
>
> (2) If the filter does not pass 215 Hz, then the demodulator will throw a
> finite amount of error even when you have extremely good SNR.
>
> In this case, no matter how clean and clear the signal is, you will always
> see some error. Notice you can even copy an RTTY signal, but the error
> rate will be quite high.
>
>
> (3) Being able to include at least the third harmonic of the keying
> sidebands will help improve copy under marginal SNR conditions if there is
> no QRM within this wider passband.
>
> Also, a matched filter will only help if you are right at the threshold of
> the bit error curve. When the SNR is very high, a additional 1 dB to 2 dB
> of sensitivity improvement won't be noticed (no one can tell the
> difference between one error hit in 1000 characters and 2 error hits in
> 1000 characters).
>
> When there is any selective fades or flutter, you will also not notice any
> improvement with a filter that is matched to the keying sidebands of the
> signal. A matched filter can get you a dB or two improvement. Take a
> look again at VE3NEA's curves http://www.dxatlas.com/RttyCompare/ .
>
> Notice in the AWGN curve that 1 dB shift along the SNR axis can move the
> character error rate significantly, by a factor of more than 2 in error
> rate around the detection threshold.
>
> Now look at the Selective Fading case... a 1 dB shift along the SNR axis
> does not improve the character error rate noticeably (unless you are doing
> a long term statistical study).
>
> Also notice in all cases that once SNR is very good (the right hand side
> of the plots), a 1 dB change in SNR again won't make any difference in
> perceived error rate.
>
> In practice...
>
> 215 Hz will be almost impossible to tune quickly without some software aid
> such as a crossed banana. A 215 Hz only helps when there is QRM close by.
> That is why most people don't recommend filters that are narrower than 250
> Hz to 300 Hz.
>
> On the other side of the coin, a matched filter is only helpful when you
> are trying to dig out that really weak signal that you can't seem to copy
> even when bands are stable and there is no QRM. A wider I.F. filter
> together with a matched filter in the software can help to dig out the
> weak DX operating split as long as no one is transmitting close to the DX'
> frequency.
>
> Remember than filter width in all the 3 cases above can be achieved in
> either hardware or in software. As long as the sound card is not
> clipping, there are advantages to using a wider I.F. filter and select the
> filter you need in software.
>
> All that being said... an antenna with good directivity can easily give
> you many dB of better SNR; easily better than any demodulator tweeks that
> you can make. If you have a SteppIR, it is more important to adjust for
> best directivity than for maximum gain (they often do, but don't
> necessarily happen at the same point). With digital mode copying, it is
> all about SNR and "loudness" has nothing directly to do with how well you
> can copy.
>
> When there is fading, diversity reception can also help tremendously with
> digital modes. Just look at the difference in SNR threshold in the VE3NEA
> AWGN plot (no fading) with his flat fading plot (Rayleigh channel with no
> multipath) and his Selective fading plot (Rayleigh channel with two
> paths).
>
> 73
> Chen, W7AY
>
>
>
>
>
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