[RTTY] 300hz or 500hz IF filter?

[Bostjan Voncina - S55O]

hello!
Thanks for the theoretical explanation of why using a theoretical
280Hz filter, takin in account the 6db would be optimal.

In noisy and crouded bands i use double 250 filters in my ft1k mark v
and i have the 300 filter in my sub vfo. In ctest environment i find
that 300 hz is too wide and lets the qrm from other eu stations in the
"picture" so it disrtups my recieving. So the 2x 250 does a better
job, but i have to use the rit button a lot.

My 5 cents that come from actual operating.

Bostjan - Ian s55o (s51a)

2013/8/23, Kok Chen <chen at mac.com>:
>
> On Aug 22, 2013, at 7:34 PM, Joe Subich, W4TV wrote:
>
>> Absolutely incorrect as 250 Hz does not account for the necessary
>> modulation sidebands or for the discontinuity (additional bandwidth)
>> generated by the 1.5 bit stop.  Due of the half bit, the necessary
>> bandwidth for 170 Hz shift RTTY approaches 170 + (2 * 90.9 * 1.2) or
>> slightly over 370 Hz as the shortest element is now 11 ms.
>
> Joe,
>
> Kai is correct.
>
> Even in May 1964,  in his "Filters For RTTY" article in the RTTY Bulletin
> Vic Poor (SK) had mentioned that data filters has nothing to do with keying
> sidebands.
>
> I had mentioned this in my RTTY Demodulator article, quoting Poor:
>
>> As emphatically stated by Mr. Poor, the proper filter has nothing to do
>> with being "wide enough to let the third (or fifth or umpteenth) harmonic
>> pass." For any design bandwidth, the optimal filter has to meet his two
>> conditions.
>
> (from
> http://www.w7ay.net/site/Technical/RTTY%20Demodulators/Contents/filters.html).
>
> The two conditions that Poor mentioned were of course the first two Nyquist
> Criteria (up until today, Nyquist's third criteria is seldom used).
>
> As long as there is no ISI, you can make the filters so narrow as to pass
> the fundamentals of a single keying sideband, as discovered by Harry Nyquist
> in 1928.
>
> For OOK keying, the Raised Cosine is the narrowest filter that has no
> intersymbol interference (ISI).  For a steam RTTY signal, it falls to zero
> (zilch, nada) at 45 Hz away from the carrier.  So, an optimal RTTY filter
> has zero response outside of 170 Hz + 91 Hz = 261 Hz.
>
> The equivalent noise bandwidth of the Raised Cosines are narrow than that,
> giving the narrowest filter's equivalent noise bandwidth that is about 216
> Hz (the number that Kai cited).
>
> For optimal SNR, the Matched Filter is quite a bit wider and about 0.1 dB
> better than the Raised Cosine, which is the narrowest filter that has no
> ISI.
>
> 73
> Chen, W7AY
>
>
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