[Elecraft] Ideal RTTY filter.

[Joe Subich, W4TV]

> Anyhow this may confirm what has been said on this reflector. The 350
> Hz (AKA 250 Hz) filter is probably the narrowest practical choice for
> RTTY.

This is a point that can not be emphasized too often.  The "250 Hz"
filter has a very "rounded" top that is being pushed hard to reach
Chen's "270 Hz within a fraction of a dB" criteria.  Further, because
of the significant "rounding," the filter is not completely phase
linear - although it is better than an alternative design that might
be absolutely flat for 350 Hz and transition from passband (flat) to
stopband (skirts) very suddenly over a 20 to 20 Hz (e.g. 300 Hz at
-1 dB and 350 Hz at -6dB).

Except for issues of AGC pumping (blocking) and in band IMD, one would
probably be better served by using a 400, 500 or 1000 Hz "roofing"
filter with a 300 or 350 Hz DSP filter setting.  The difference in
IMD3 between 400 Hz and 1000 Hz roofing filters is very small in the
context of RTTY - the incremental distance from center to cutoff can
not accommodate much more than one additional signal on each skirt and
the chance of two very strong signals or more than two strong signals
having the right frequency relationship within even a 1 KHz (@ -6dB)
bandwidth to create interfering IMD3 is quite small.

With modern DSP based RTTY software, the "goal" should be sufficient
"roofing" filter selectivity to prevent blocking and IMD3 along with
DSP filtering that is "good enough" to reduce the number of signals
to the point that the instantaneous peak does not cause distortion in
the audio stages or overflow the A/D converter in the sound card.
*Beyond that point,* so long as the "sound card" does not have an
excessively high noise floor and/or limited dynamic range (as exhibited
by some "inexpensive" ham products), RTTY performance will be
determined almost entirely by the performance of the DSP "resonators"
and the ability of the demodulator algorithm to deal with multipath,
and selective fading (ATC).

73,

    ... Joe, W4TV


On 5/17/2013 1:07 PM, Brian Alsop wrote:
> This comes from June 2013 QST page 59.
>
> According to W7AY:
>
> The ideal RTTY filter is 280 Hz wide.  Narrowing it further by 60 Hz
> doubles the error rate.
>
> The article references:
> http://www.w7ay.net/site/Technical/RTTY%20Transmit%20Filters/index.html
>
> Which doesn't come out and say the above!  It's talking about transmit
> filters.  W7AY doesn't like uneven power in transmit tones either.
>
> Anyhow this may confirm what has been said on this reflector. The 350 Hz
> (AKA 250 Hz) filter is probably the narrowest practical choice for RTTY.
>
> 73 de Brian/K3KO
>
>
>
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