[RTTY] 200 hz filter on RTTY

Jeff Blaine AC0C keepwalking188 at yahoo.com
Tue Jun 1 11:05:06 PDT 2010


Gentlemen,

Anyone who knows my call, knows that I **love** crystal filters.  But I am 
confused specifically in the case of RTTY.

MMTTY - as well as any other decoding software - all incorporate DSP 
filtering.  To the extent that the rig is not pressed hard enough to 
generate IMD products, adjustment of the DSP filtering should limit the AF 
bandwidth "heard" by the program to a level perfectly adequate to the task.

With respect to the prior comments of Chen, I believe they can be 
paraphrased as the detection threshold depends on recovering the power 
contained in the sidebands.  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...

There must be something I am overlooking.

73/jeff/ac0c


--------------------------------------------------
From: "Tom Osborne" <w7why at verizon.net>
Sent: Tuesday, June 01, 2010 11:48 AM
To: "RTTY" <rtty at contesting.com>
Subject: Re: [RTTY] 200 hz filter on RTTY

> I use 250, with an Inrad filter, all the time for RTTY.  Any wider than 
> that
> is too wide for me.  Not sure about narrower as I can't get less than 250
> cycles here.  The number 2 radio here has a 500 cycle filter and that is 
> WAY
> too wide. for contesting. 73
> Tom W7WHY
>
>
> Don, AA5AU, wrote:
>> I think I remember someone, maybe Chen, saying a 200 Hz
>> filter would not work on RTTY or something to that effect.
>> Maybe I don't have the story just right, but it appears to be
>> working great.  Too tight for contesting, but works when required.
>
> A passband narrower than about 280 Hz removes some of a 45 baud signal.
> OTOH, I have used a 200 Hz DSP filter a lot in contests and pileups 
> because
> removing off-frequency signals is a bigger benefit than the little bit 
> lost
> with the too-narrow filter.  This requires precise tuning but helps 
> separate
> signals.  Even 150 Hz works pretty good, though I never go that narrow.
>
> 73,
> Ed
> --------------
> Ed Muns - W0YK
> www.w0yk.com
>
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