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 ----- No virus found in this message. Checked by AVG - www.avg.com Version: 2012.0.2242 / Virus Database: 3162/5830 - Release Date: 05/16/13 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
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