Re: using Nyquist filters for RTTY. One complication I didn't mention is the fact that, because angle modulation (FM, FSK, PM, PSK) is fundamentally non-linear, the pre-detection and post-detection filters are fundamentally different.
With linear modulation types (AM, on-off keying, QPSK, QAM), the spectrum of the pre-detection filter will appear the same at baseband. For example, if the IF filter has a 0.3-alpha Nyquist bandpass response, the resulting post-detection response at baseband will also be a 0.3-alpha Nyquist lowpass response. But that's not true with angle modulation. Using a Nyquist pre-detection filter does not result in a Nyquist response after detection. That is why most FSK and MSK systems use non-Nyquist filters (typically Gaussian). To use a Nyquist filter with FSK you would have to use a pre-detection filter much wider than the baud rate (as is done now for RTTY) and then add a Nyquist post-detection filter that would further tighten up the frequency response without causing additional inter-symbol interference. There may be a good reason why this is not usually done. When I get time maybe I'll do some Mathcad simulations to see if it looks like this could work. > Some RTTY decoders take advantage of the tone separation and actually > employ a pair of detectors, one for each tone, and use the outputs > differentially to get some protection from selective fading. I think it is not just for selective fading but also for interference that occurs primarily on one of the two tones. It might be possible to use separate Nyquist filters and detectors for the two tones, treating each one as if it were a separate on/off-keyed signal. If the transmitted signal is not filtered too heavily (i.e. jumps quickly from one tone to the other) it seems like that should work. Al N1AL On Mon, 2008-05-19 at 07:00, Lyle Johnson wrote: > >> Using a excessively narrow filter on RTTY is a balancing act. > >> > >> If the bandwidth is too narrow the tones tend to become "smeared" > >> - the transitions become less easy to detect (in other words, > >> the "eye" begins to close). However, narrowing the bandwidth > >> also reduces the amount of QRM/QRN making it through the filter > >> and to the decoder/detector. > > > > How about using a Nyquist filter? > > > > A Nyquist filter is one that results in no inter-symbol interference. > > One of the difficulties with RTTY is that the tone spacings are so wide. > For a 45 baud FSK signal, a shift of about 30 Hz would minimize the > occupied bandwidth and have no intersymbol interference. But standard > practice is to use 170 Hz or 200 Hz shift. [ 200 Hz Shift is about right > for 300 baud FSK, which is what we used (and why we used it) for HF > packet some 25 years ago. And optimized it for common 400 Hz to 500 Hz > wide "CW" crystal filters of that era. ] > > Thus, the rise of the dual tone filter (K3) and its work-alikes in > various radios and demodulators, with fairly tight filters around each > tone to pass it along with its significant sidebands. But the overall > IF passband still needs to be wide enough to pass both tones and their > sidebands. I imagine you can observe all of this on your "P1". > > Some RTTY decoders take advantage of the tone separation and actually > employ a pair of detectors, one for each tone, and use the outputs > differentially to get some protection from selective fading. 170 Hz > spacing is probably too close for such protection -- 850 Hz spacing > would be better, but that requires an even wider IF passband... > > Some soundcard-based RTTY programs support really narrow shifts, but > such signals are not often heard at this QTH. Maybe it is because > everyone is listening and no one is calling CQ? > > Enjoy! > > Lyle KK7P _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
