Ideal RTTY filter.
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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:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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> 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:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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In reply to this post by alsopb
Brian K3KO wrote: This comes from June 2013 QST page 59. First of all, Chen's article is about transmit filtering which is not directly translatable to optimal receive filtering. Second, the cascade effect of the K3 crystal filter and DSP filter must be considered in determining the net receive bandwidth. So very different net receiver bandwidths result depending on what DSP bandwidth is used with the engaged crystal filter bandwidth, e.g., KFLA250 which is really a 370 Hz filter. Third, the ideal receive bandwidth for optimal decoding is not the same as the transmit bandwidth for minimum QRM. Depending on the decoder, a receiver bandwidth of around 400 Hz is optimum ... unless there is such a heavy QRM situation that a better overall system trade-off is obtained with narrower, e.g., 250 Hz, net IF bandwidth. A transmit filter of 280 Hz is an optimum trade-off between minimizing QRM to neighboring QSOs and maintaining signal integrity for the intended receiver. Finally, this transmit filter can be implemented in either the radio or the encoder. MMTTY, for example, provides a number of transmit filter bandwidths and the default 48-tap TX bandwidth for AFSK meets Chen's proposal. Ed W0YK 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. ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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Ed et al,
As I said in my posting the receive filter info came from a quote attributed to Chen in the QST article. I pointed out that the link supplied by QST was not for receive. So we either have to accept the quote of Chen on the receive side (additional data exists that Chen has?) or the QST author got it wrong. Nothing at all was said about dual peak filtering which is used by many of us in conjunction with a 400 or "250" filter. It would be nice to someday finally nail this whole RTTY filter issue down. Also it would be nice to find a set of optimum AGC settings for RTTY. I suspect there are parameters or a formulation that would produce less spurious clicks. AGC off is definitely not a practical solution. 73 de Brian/K3KO On 5/18/2013 04:17, Ed Muns wrote: > > > Brian K3KO wrote: > > This comes from June 2013 QST page 59. > First of all, Chen's article is about transmit filtering which is not > directly translatable to optimal receive filtering. Second, the cascade > effect of the K3 crystal filter and DSP filter must be considered in > determining the net receive bandwidth. So very different net receiver > bandwidths result depending on what DSP bandwidth is used with the engaged > crystal filter bandwidth, e.g., KFLA250 which is really a 370 Hz filter. > Third, the ideal receive bandwidth for optimal decoding is not the same as > the transmit bandwidth for minimum QRM. Depending on the decoder, a > receiver bandwidth of around 400 Hz is optimum ... unless there is such a > heavy QRM situation that a better overall system trade-off is obtained with > narrower, e.g., 250 Hz, net IF bandwidth. A transmit filter of 280 Hz is an > optimum trade-off between minimizing QRM to neighboring QSOs and maintaining > signal integrity for the intended receiver. Finally, this transmit filter > can be implemented in either the radio or the encoder. MMTTY, for example, > provides a number of transmit filter bandwidths and the default 48-tap TX > bandwidth for AFSK meets Chen's proposal. > > > Ed W0YK > > > 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. > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > > > ----- > No virus found in this message. > Checked by AVG - www.avg.com > Version: 2012.0.2242 / Virus Database: 3162/5835 - Release Date: 05/18/13 > > ----- No virus found in this message. Checked by AVG - www.avg.com Version: 2012.0.2242 / Virus Database: 3162/5835 - Release Date: 05/18/13 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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Back in ye olde days of RTTY when we used mechanical printers, the
thinking was that the minimum bandwidth required was that sufficient to pass the 3rd keying sideband without too much attenuation or time shift. (This was way before measuring group delay was something that could be done other than in a well equipped lab, but if you looked at the output of the modem detector after the low pass filter and before the slicer, an oscilloscope clearly showed the changes resulting from changing the tone filter and low pass filter bandwidths. 60 WPM Baudot RTTY has a data rate of 22 Hz*, so the 3rd keying sidebands would be ±66 Hz from the tone. With 170 Hz shift, and a single passband filter, the outer (upper and lower) keying sidebands would be at 66 Hz above and below. Hence the target filter bandwidth would be 302 Hz. A more technical explanation is that the tone filters, post-detection low pass filter and slicer work to restore the transmitted waveform and that waveform reconstruction becomes more difficult and less accurate the more keying sidebands are removed. A Fourier analysis of a square wave will show this as you increase the number of terms (harmonics of the keying waveform) in the reconstruction. If you want to tinker a bit, there's an on-line Fourier simulator at http://www.facstaff.bucknell.edu/mastascu/elessonsHTML/Freq/Freq4FourierSeriesSimulators.htm -- select the square wave and run it with 1, 3, 5, 7 etc. harmonics and you will see that passing just the 3rd harmonic yields a not that bad appearing square wave. Where things get a bit more complicated is that these simple rules and the Fourier simulator assume the harmonics are passed without significant time (or phase if you prefer to think of it that way) shift. Depending on the tone filter design, there may well be significant time shift between the tone frequency and the keying sidebands. A filter with uniform time delay, such as a Bessel will be much better in this regard than the same order Chebyshev, for example, but for the same filter order and -3 dB bandwidth, the Bessel will demonstrate much wider skirts. At least in the days when we built filters from 88mH loading coils, there was always tension between designing a filter with a picture perfect square sided response but with gross time distortion and one with a rounded nose and gentle flank selectivity and low time distortion. A Butterworth filter was used in most modem designs of that era as a compromise between time distortion and flank response and also the ability to design and implement the filter. DSP based filters are a huge improvement over those LC filters and permit good time delay performance and skirt selectivity. Jack K8ZOA ... my first piece of RTTY gear was a model 15 page printer acquired surplus in the late 1960's from Michigan Bell Telephone through their ham radio - RTTY program. * 60 WPM Baudo = 368 operations per minute, 7.42 length code = 45.5 baud, or 22.7 Hz. On 5/18/2013 8:18 AM, Brian Alsop wrote: > Ed et al, > > As I said in my posting the receive filter info came from a quote > attributed to Chen in the QST article. I pointed out that the link > supplied by QST was not for receive. > > So we either have to accept the quote of Chen on the receive side > (additional data exists that Chen has?) or the QST author got it wrong. > > Nothing at all was said about dual peak filtering which is used by > many of us in conjunction with a 400 or "250" filter. > > It would be nice to someday finally nail this whole RTTY filter issue > down. Also it would be nice to find a set of optimum AGC settings for > RTTY. I suspect there are parameters or a formulation that would > produce less spurious clicks. AGC off is definitely not a practical > solution. > > 73 de Brian/K3KO > > On 5/18/2013 04:17, Ed Muns wrote: >> >> >> Brian K3KO wrote: >> >> This comes from June 2013 QST page 59. >> First of all, Chen's article is about transmit filtering which is not >> directly translatable to optimal receive filtering. Second, the cascade >> effect of the K3 crystal filter and DSP filter must be considered in >> determining the net receive bandwidth. So very different net receiver >> bandwidths result depending on what DSP bandwidth is used with the >> engaged >> crystal filter bandwidth, e.g., KFLA250 which is really a 370 Hz filter. >> Third, the ideal receive bandwidth for optimal decoding is not the >> same as >> the transmit bandwidth for minimum QRM. Depending on the decoder, a >> receiver bandwidth of around 400 Hz is optimum ... unless there is >> such a >> heavy QRM situation that a better overall system trade-off is >> obtained with >> narrower, e.g., 250 Hz, net IF bandwidth. A transmit filter of 280 >> Hz is an >> optimum trade-off between minimizing QRM to neighboring QSOs and >> maintaining >> signal integrity for the intended receiver. Finally, this transmit >> filter >> can be implemented in either the radio or the encoder. MMTTY, for >> example, >> provides a number of transmit filter bandwidths and the default >> 48-tap TX >> bandwidth for AFSK meets Chen's proposal. >> >> >> Ed W0YK >> >> >> 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. >> >> ______________________________________________________________ >> Elecraft mailing list >> Home: http://mailman.qth.net/mailman/listinfo/elecraft >> Help: http://mailman.qth.net/mmfaq.htm >> Post: mailto:[hidden email] >> >> This list hosted by: http://www.qsl.net >> Please help support this email list: http://www.qsl.net/donate.html >> >> >> ----- >> No virus found in this message. >> Checked by AVG - www.avg.com >> Version: 2012.0.2242 / Virus Database: 3162/5835 - Release Date: >> 05/18/13 >> >> > > > > ----- > No virus found in this message. > Checked by AVG - www.avg.com > Version: 2012.0.2242 / Virus Database: 3162/5835 - Release Date: 05/18/13 > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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In reply to this post by alsopb
> So we either have to accept the quote of Chen on the receive side > (additional data exists that Chen has?) or the QST author got it > wrong. Chen has written extensively on RTTY filtering both on his web site as well as in his postings on the RTTY list. In addition, Chen has provided links to work by others (Hoff, Nyquist, etc.) in the field - one can't take a single link in QST as the sum total of Chen's work. > It would be nice to someday finally nail this whole RTTY filter issue > down. Read *all* of Chen's work and check out some of the bibliographical information as well. The information will make the concepts quite clear (although the mathematics is a whole different matter). One does need to understand that filtering in the receiver and filtering in the demodulator are two *entirely different issues* although they must be considered together along with transmit filtering - because all three "filters" are connected in series and any group delays (filter induced multipath) additive. > Also it would be nice to find a set of optimum AGC settings for > RTTY. I suspect there are parameters or a formulation that would > produce less spurious clicks. "Key clicks" in RTTY are a result of improper FSK (or AFSK) generation and the lack of appropriate transmit filtering/waveshaping as described in Chen's link on transmit filters. AGC has no effect on them. 73, ... Joe, W4TV On 5/18/2013 8:18 AM, Brian Alsop wrote: > Ed et al, > > As I said in my posting the receive filter info came from a quote > attributed to Chen in the QST article. I pointed out that the link > supplied by QST was not for receive. > > So we either have to accept the quote of Chen on the receive side > (additional data exists that Chen has?) or the QST author got it wrong. > > Nothing at all was said about dual peak filtering which is used by many > of us in conjunction with a 400 or "250" filter. > > It would be nice to someday finally nail this whole RTTY filter issue > down. Also it would be nice to find a set of optimum AGC settings for > RTTY. I suspect there are parameters or a formulation that would > produce less spurious clicks. AGC off is definitely not a practical > solution. > > 73 de Brian/K3KO > > On 5/18/2013 04:17, Ed Muns wrote: >> >> >> Brian K3KO wrote: >> >> This comes from June 2013 QST page 59. >> First of all, Chen's article is about transmit filtering which is not >> directly translatable to optimal receive filtering. Second, the cascade >> effect of the K3 crystal filter and DSP filter must be considered in >> determining the net receive bandwidth. So very different net receiver >> bandwidths result depending on what DSP bandwidth is used with the >> engaged >> crystal filter bandwidth, e.g., KFLA250 which is really a 370 Hz filter. >> Third, the ideal receive bandwidth for optimal decoding is not the >> same as >> the transmit bandwidth for minimum QRM. Depending on the decoder, a >> receiver bandwidth of around 400 Hz is optimum ... unless there is such a >> heavy QRM situation that a better overall system trade-off is obtained >> with >> narrower, e.g., 250 Hz, net IF bandwidth. A transmit filter of 280 Hz >> is an >> optimum trade-off between minimizing QRM to neighboring QSOs and >> maintaining >> signal integrity for the intended receiver. Finally, this transmit >> filter >> can be implemented in either the radio or the encoder. MMTTY, for >> example, >> provides a number of transmit filter bandwidths and the default 48-tap TX >> bandwidth for AFSK meets Chen's proposal. >> >> >> Ed W0YK >> >> >> 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. >> >> ______________________________________________________________ >> Elecraft mailing list >> Home: http://mailman.qth.net/mailman/listinfo/elecraft >> Help: http://mailman.qth.net/mmfaq.htm >> Post: mailto:[hidden email] >> >> This list hosted by: http://www.qsl.net >> Please help support this email list: http://www.qsl.net/donate.html >> >> >> ----- >> No virus found in this message. >> Checked by AVG - www.avg.com >> Version: 2012.0.2242 / Virus Database: 3162/5835 - Release Date: 05/18/13 >> >> > > > > ----- > No virus found in this message. > Checked by AVG - www.avg.com > Version: 2012.0.2242 / Virus Database: 3162/5835 - Release Date: 05/18/13 > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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