[RTTY] Crystal filter width preferences for RTTY contesting

[Jeff Blaine AC0C]

Chen,

I met my Chinese wife when I lived in China and wanted to learn Chinese.  So 
from 3rd language standpoint, I consider your English to be really 
excellent.  And I also 100% understand your meaning.  As non-English 
writers, it's quite difficult to notice your writing is of 3rd language 
standpoint.  The fault of understanding is mine.

So let me rephrase the question then because I am hoping we can put a number 
value to the trade-off between width and SNR...  For the case of our 
discussion, I am assuming there is NO software optimization capability - no 
DSP - just a "dumb demodulator" who's performance is better or worse based 
on the quality of the signal fed to it.  I do agree with you in the "anal" 
case of the practical world, but here we can consider the theoritical ideal 
and hopefully I am not driving you crazy with these many follow-ups.

1. Let's assume the same single filter is to serve selectivity purposes in a 
dual-duty mode format, meaning we want this filter to work for our RTTY 
**AND** CW mode for the contest environment.

2. Allowing for the variable of keying sideband widths in either mode - we 
can say - generally - a somewhat narrower filter than 306 hz is better for 
the CW op - and a somewhat wider RTTY filter is better for the RTTY op.

3. Let's also assume that 306 hz is the absolute minimum acceptable width 
for the RTTY application.  And so our filter needs to be 306 or wider.

4. Finally, let's say that this is a single signal case - meaning no other 
keying sidebands are involved - only the pure signal of our target and 
desired RTTY signal.

In that context.  Can we estimate what the relative SNR improvement is by 
expanding the passband to a little bit wider value?  Let me suggest, based 
on the popular comment, that the alternative widths are 400 hz and 500 hz?

I see from the examples on the web site, assuming I am readying the charts 
properly, the difference between a filter of 370 hz (100 hz per side) width 
vs. the 306 hz (3rd harmonic cut point) is around 1.5 db.

What I am wondering is if we can distill the argument to something as simple 
as this:

Width        SNR impact
-------        ---------------
306 hz        0 (baseline)
370 hz        -1.5
400 hz        ?
500 hz        ?

Does that make sense?

73/jeff/ac0c


--------------------------------------------------
From: "Kok Chen" <chen at mac.com>
Sent: Thursday, August 27, 2009 2:49 PM
To: "Jeff Blaine AC0C" <keepwalking188 at yahoo.com>
Cc: "RTTY Reflector" <rtty at contesting.com>
Subject: Re: [RTTY] Crystal filter width preferences for RTTY contesting

>
> On Aug 27, 2009, at 11:31 AM, Jeff Blaine AC0C wrote:
>
>> 1. With a more narrow bandwidth, we have a better ability to  suppress 
>> adjacent strong stations, especially their sideband  amplitude - which 
>> will somewhat increase the SNR of our desired  station.
>
> I think that you may have misunderstood.  I apologize that I write 
> horribly; English is my third language, even though I should not be  using 
> it as an excuse considering the fact that I have been in this  country for 
> longer than the majority of people who were born here, and  have all but 
> forgotten my first two languages! :-).
>
> The point of those plots is to show that a narrower receiver bandwidth 
> will NOT help you suppress the keying sidebands from a close by strong 
> FSK transmitter. Once the strong sidebands encroaches on the weak  signal 
> you are trying to copy, it is too late to do anything.   (Similar 
> situation to SSB signals that are too close to one another,  no amount of 
> receiving filter will get rid of what people complain as  "splatter.")   A 
> narrower I.F. filter will not help in that case, and  yet the narrower 
> I.F. filter will hurt when it comes to copying weak  signals when there is 
> no QRM.
>
> My personal methodology: use a relatively wide I.F. filter --  something 
> that is only narrow enough to (1) keep my sound card from  saturating and 
> (2) keep the AGC from pumping due to very strong close  in signals 
> (something that has not been brought up yet in this  thread).  Then find 
> some software that lets me change the demodulation  bandwidth to match the 
> situation.
>
> I do not tell everyone else that they must, or even should do the same 
> thing.  But I do recommend that they do study the problem carefully 
> before choosing their own solution.
>
> The advice I usually give is to never ever, ever, ever, ever allow the 
> sound card to saturate, not even for a millisecond, while making sure 
> that the weak signal is a good 10 or 20 dB above the sound card's  noise 
> floor.
>
> 73
> Chen, W7AY
>
>