On Aug 22, 2013, at 3:01 PM, Jay WS7I wrote:
> Nah, If Chen chimes in it will be 2.8 Khz filter and "real" software!!!!
Any hardware filter should only be narrow enough to keep the sound card from
clipping. Think of the crystal or audio filter as being a "roofing" filter for
the sound card, and that will be the right way to think about what crystal
filter to use.
This allows the software filters of good software to be optimal.
If you are using Windows, both 2Tone and fldigi already have optimal filters,
and anything narrower will only make copy worse.
This is why it is super critical to set up the sound card properly: it should
never clip on the largest signal and the sound card's own noise floor should be
at least 10 dB above the noise floor of the receiver -- for pretty much all
cases (except perhaps on 10m on a crummy antenna), the noise floor from the
receiver is determined by sky noise.
If you cannot set the sound card gain to satisfy both the noise floor and
clipping criteria, then you need a sound card that has a better dynamic range.
There are audio sound cards out there that have 115 dB of dynamic range -- more
than the dynamic range of your superhet receiver. This is why I never, ever
recommend cheap sound cards; you end up having to ride the gain of the receiver
manually to get the best performance.
Take a look at Fig 2.2 here
http://www.w7ay.net/site/Technical/RTTY%20Transmit%20Filters/index.html
The article is written for a transmit filter, but applies to any additional
filtering that is placed in between the FSK generator and the receive
demodulation filter. I.e., it applies to receive crystal filters also. Notice
that the filter I used to produce that curve has no group delay -- with a
crystal filter, you will need an even wider filter.
With something like 2Tone, you pretty much have optimal decoding with weak
signals AND better QRM protection than possible from any crystal filter. As
long as the criteria I gave above are satisfied.
(Since MMTTY is now open sourced, presumably some smart guy can now also
implement real RTTY filters for it.)
73
Chen, W7AY
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