[RTTY] 200 hz filter on RTTY

[Jeff Blaine AC0C]

Joe,

You are right.  I think we discussed the shape factor of the K3 250 hz 8 
pole design earlier on the board few months back.  I have the same memory as 
you.

The shape factor (if you want to use that term) on the MMTTY BPF can be set 
by the number of taps selected in the software.

I need to sit down with it and play around to get some hard numbers.  But 
anyone else can do it as well - there is a simulate button in MMTTY which 
will show you what the passband and transition band shape will look like for 
any of the BPF filter configurations.

So even for a guy without tight crystal filters and no rig based DSP, they 
can use MMTTY's BPF feature to provide the similar functionality.

The selling point, if you want to use that, for rig based filtering is one 
of overload prevention - similar to Chen's caviet on sound card clipping. 
As long as the signal heading into the MMTTY DSP is clean, even if it's 4 
Khz wide, then some adjustment of the MMTTY BPF settings should provide copy 
to the same extent as rig based DSP or crystal fitlering.

But there is the rub - in a contest environment, especially if K3LR is your 
neighbor, then rig overload is a big issue.  And no matter how good is the 
DSP on the rig or the MMTTY, if a bunch of IMD products are bouncing around 
on your passband, we got trouble...

73/jeff/ac0c


--------------------------------------------------
From: "Joe Subich, W4TV" <lists at subich.com>
Sent: Tuesday, June 01, 2010 2:43 PM
To: <rtty at contesting.com>; "DONALD A HILL" <aa5au at bellsouth.net>
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.
>
> Most of the INRAD "250 Hz" filters are actually 325-275 Hz wide at
> the - 6dB points as the "250 Hz" is based on cascading a pair of
> filters in the IF (i.e., Yaesu or Kenwood style).
>
> As documented by Chen, a 325 - 350 Hz "roofing filter" is a very
> good design for 45.45 baud 170 Hz shift RTTY.  It provides good
> selectivity while preserving most of the signal energy in the
> keying sidebands.
>
> When talking about RTTY bandwidth, care needs to be taken in
> defining terms ... even a 200 Hz crystal filter like the one
> produced by Elecraft (a true 200 Hz, 5-pole design) can be
> useful if the skirts eliminate QRM more rapidly than they
> cut signal energy in the keying sidebands.
>
> When talking about DSP filters, the "bandwidth" is more critical
> as many DSP filters have much steeper skirts than those in
> practical crystal filters.  DSP filters can exhibit skirt
> selectivity comparable to the 10 dB/100 Hz slopes in the
> Elecraft implementation.  Such sharp skirts would make a true
> 200 Hz DSP filter a poor choice for RTTY.  On the other hand,
> if the skirts were "softer" like those in some Icom implementations
> (or Yaesu's "soft" filters), a 250 Hz DSP filter might work well
> in heavy QRM.
>
> 73,
>
>    ... Joe, W4TV
>
> On 6/1/2010 12:48 PM, Tom Osborne wrote:
>> 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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