A couple of people have privately asked me some questions about my NoiseNuller application, but they are good questions so I'm going to repeat my answers here.
The first question has to do with the character of the noise that NoiseNuller can handle. Most noise reduction techniques make assumptions about the profile of the noise, and usually give some options or adjustments for different noise profiles. If the actual noise fits one of the available profiles, the noise reduction works. If not, it doesn't.
The NoiseNuller application doesn't make any assumptions about the character of the noise. It analyzes the noise over the course of an initial few seconds using FFT techniques, statistically generates a phase profile of that noise for each FFT bin (i.e., frequency component), and subtracts the noise bin by bin from whatever follows. It doesn't even care much about the amplitude of each frequency component since it makes its decisions only based upon phase and how often each FFT bin shows up during the analysis period. It is therefore theoretically applicable to almost any noise profile, although noise that varies a lot in spectrum over time can reduce the ultimate SNR because if the noise covers enough territory the application ends up at least partly reducing too many of the bins in amplitude when it reconstitutes the FFT series back to normal audio. That's why CW works better than voice ... because CW occupies fewer FFT bins to start with, and even if the application tries to reduce them it reduces FAR more FFT bins that are associated with the noise. It's literally a statistical filter.
The second question I've received has to do about latency. Today I made a setup where I used my KX2 to periodically transmit some single dits into a dummy load near my K3, and I set up a microphone near the K3 speaker to capture the resulting audio from the K3. The Line Out of the K3 was fed into the Line In of the computer sound card, and the computer was running my NoiseNuller application. I used a video screen capture application (OBS Studio) to simultaneously record both the microphone near the K3 speaker and the output of the NoiseNuller application. I then extracted the audio from the video recording and inspected it in the audio application Audacity using an expanded time scale. The latency between what was recorded by the microphone and what was coming out of the NoiseNuller application for each dit appeared to be about 43 milliseconds. I can send that audio file to anyone who might want to try to determine the latency themself, or to determine if that amount of latency would be a problem for them.
However, I don't think that 43 msec latency should be a problem even in a contest. Our brains can distinguish 15 msec but we cannot physically react anywhere near that fast. There are websites where you can test your hand reaction time by pressing a mouse button in response to a visual prompt, and if anyone can beat even 200 msec I'd be amazed. I don't see how that kind of test is much different than pressing a paddle lever in response to the end of a CW transmission.
Here are three online reaction tests: https://humanbenchmark.com/tests/reactiontime https://www.reaction-time-test.io/ https://www.mathsisfun.com/games/reaction-time.html As always, comments (pro or con) and suggestions are welcome. 73, Dave AB7E ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
