[RTTY] 300hz or 500hz IF filter?

[Kok Chen]

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