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#2
November 24th 06, 02:11 PM
posted to rec.radio.amateur.homebrew
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CW to FM Remodulator?
"Mike Monett" wrote in message
... Andy writes: Jeff, I haven't tried this method, but one rule of thumb I have always believed in is: "No matter how much you shift, limit, amplify or divide noise, it still ends up as noise" The only effective way I have ever found is to narrow the bandwidth around the signal until the signal starts to get degraded. If done digitally, it can be done by digital processing, but that changes only the technique, not the principle... Actually, digital processing CAN change the principle. FIR filters and similar digital filters do provide a way to reduce the bandwidth digitally, and as you point out, reducing the bandwidth reduces the noise. This helps the same way a crystal filter helps, except perhaps giving a little more flexibility. However, many modern radios have digital noise reduction which is quite a different animal. With digital noise reduction, the incoming signal is analyzed to identify noise components and differentiate them from signal components. The noise is then subtracted from the signal. While this isn't perfect, it can result in quite a substantial reduction in noise without reducing bandwidth. The combination of bandwidth reduction and digital noise reduction can greatly improve readability. I agree that other typical analog techniques don't really affect things all that much, but I'm not convinced that the same techniques that are used for noise reduction digitally couldn't be duplicated with analog components; I've just never seen it done, and without some considerable creativity on the part of the designer it will be quite complex. One analog behavior I have noticed that helps, at least with CW. For passive balanced mixers, there is a diode threshold voltage required for the signal to be detected. If the gain is managed so that the noise level is very close to this threshold, the signal to noise ratio seems to be improved (although I have not personally validated this analytically). Of course, if the signal is at the noise level this doesn't help, and if the signal is barely above the noise level the adjustment is too critical to be a great help, but where the signal has enough headroom, it can pretty dramatically improve the pleasure of listening to a weak signal. ... |
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#3
November 7th 06, 05:33 AM
posted to rec.radio.amateur.homebrew
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CW to FM Remodulator?
) writes:
I have been looking into gizmos that improve CW copy. Most are audio tone detectors that ignore short impulse noise bursts and then regenerate the CW with a keyed tone oscillator. There are several of these designs around and they are all well and good, but I stumbled across something different and was wondering if any of you have had personal experience with it? An October 1971 article in Ham Radio magazine (pg 17) titled "high-performance CW processor for communications receivers", "Frequency modulating the telegraphy signals in your receiver provides an interesting and profitable addition to conventional receiver design". I finally dug out the article. I haven't a clue to its worth, but I don't recall that sort of scenario coming up in other places (while the one about good filtering and using a detected voltage to key an audio oscillator came up a number of times), which may mean nobody found it useful, or nobody else could be bothered replicating the circuitry. What you want to do is check a few issues later, to see if there were any letters related to it in the "Comments" section. It's interesting that the November 1971 issue of Ham Radio had an article entitled "Weak Signal Reception in CW Receivers", which used nothing cutting edge but was a summation of various things one could do to improve reception. Go back a few years, and you'd see an article or two about "under the noise" CW reception, which of course amounted to PLLs driving some indicator, but at the time were pretty out of the ordinary since IC phase locked loops hadn't arrived. I suspect to evaluate this, one really needs to dig through the magazines and look at all the schemes. Ham Radio seemed to have a fair number in the first decade or so. Something about that article you reference reminds me of something in an article about a Hallicrafter's diversity receiver, I forget the issue but it likely was in one of the annual October (or was it November?) receiver issues. About '74 or '75, someone named Hilbert had some scheme that involved active audio filters, but there was more to it than I can think of at the moment. (I seem to recall there was some "stereo" effect, in that different signals were fed to each ear, which in itself may be worth pursuing. Use one of those schemes with the detectors to key the audio oscillator, but also include some of the signal from the receiver output, so you get the noise and the actual signal in it.) Wait, I think it must have been "Hildreth", who also wrote this article you reference. In which case, you can look up what he did later; did he see some fault in this system, or did he just realize it was easier to implement something at audio? Someone mentioned in this thread something that hinted at Coherent CW, which sync'd up the time and frequency at both ends to allow for good filters and fairly deep in the noise CW reception. If you know when and where to look, then it's easier to gather whether there's a signal there or not. By looking at the various schemes people have come up with, one can get a better idea of each one's worth better than looking at each one by itself. Some of the schemes likely panned out to be duds. Others required too much circuitry, at least at the time of the articles, so nobody wanted to replicate them. And then likely they've been forgotten, because otherwise more recent technology advances make the past easier (look at how phasing SSB returned to some level of popularity when ICs and digital audio came along). Others, like Coherent CW had the disadvantage that they were a whole system, rather than a processor, so you needed matching stations at both ends in order for it all to work. You can at least look over the cumulative index of Ham Radio magazine, since someone has put it (or at least some version of it) online at http://webhome.idirect.com/~griffith/hrindex.htm Ham Radio seemed to be the place to look for that sort of out of the ordinary schemes. The idea is to sample the last IF of a receiver after as much IF filtering as you can muster, and then using this as the RF input to a FM modulator. The RF/IF is modulated at the audio frequency you like to hear while copying CW. The next step is to frequency multiply the FM modulated signal to increase the bandwidth and up the modulation index. The following step is to treat it like any normal FM receiver IF and run it through a limiter stripping off any amplitude information. The last step is to put the signal into a normal FM discriminator to recover the modulating tone you used. What this is supposed to do is reduce or eliminate QRN (not QRM) from the CW signal making a "quiet" background to copy the CW. It gets the on/off of the keying, but yes it limits the signals. So widely varying signals will be at about the same level (though that may not always be a feature), and any QRN will be limited too. In thinking about it, I'm not so sure it's all that distant from the schemes that detect the CW and use that to key an audio oscillator. The bulk of the circuitry is not there to improve the CW reception, but to get that needed FM signal, with the incoming CW signal as the "carrier". Again, the more I think about it the more I think his later audio based schemes may implement a similar concept. Michael VE2BVW |
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#4
November 7th 06, 12:07 PM
posted to rec.radio.amateur.homebrew
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CW to FM Remodulator?
About tone detection in noise with biaural hearing:
In an ancient magazine article (If I recall correctly it was "dubus") there was a scientific reference mentioned. The signal must be delayed AND a difference in power level feeding it to the ears. They mentioned it is possible to get 3dB improvement with this methode. Of course, detecting "submarine" is a good search for Google I think. They do worn stereo head phones. Signal detection in general is how the processing is done: - online or offline. Offline gives the added benefit of knowing all the signal in advance. - bit-speed needed (Here the brain is bounded to limits) - power level achievable (over background) You can't beat the modern detection systems doing DSP algorithms. - Henry |
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