On Thu, Oct 27, 2005 at 10:20:06PM +0000, carmen wrote: > > There really is no such thing as a 'noise filter' - noise usually > > occupies the whole band, so a noise filter would remove all of the > > signal ! > > there is definitely such thing as a noise filter. i know i had a > couple dozen VSTs in my 'filter/noise' folder on windows...
A filter in the usual (audio) sense is a linear process that has a frequency dependent gain. It will have the same effect on any signal, be it noise or discrete tones. In that sense, a 'noise filter', supposedly acting only on a continuous spectrum (noise) does not exist. There may of course exists things called 'noise filter', and having the effect of reducing unwanted noise in some application. These could be just linear filters attenuating what their designer considered to be the unwanted part of the spectrum. But that is application specific - one (wo)man's noise is another one's signal. If you are recording an interview at the seaside, the waves are noise. If I'm trying to capture the sound of sea, people's voices are 'noise'. There is no filter that can know what either of us wants. So we will both use some filter adapted to our needs, and call that a 'noise filter'. There are also processors that analyse a signal and remove those parts of the spectrum that have little energy, or the parts that correlate with a reference signal. The removed part would be noise in many cases. But these are not filters in the usual sense. >From my experience on the short waves, it's not noise in the normal sense (i.e. a continuous hiss) that makes reading Morse difficult - we can quite easily 'remove' that mentally. It's other forms of interference, and filtering can help to reduce these. You could indeed make some processor that would regenerate a Morse signal (and also decode it on the fly). But you would have a difficult time trying to outperform human hearing in that application. For example, our hearing mechanisms are not limited by the usual 'uncertainty principle' that limits the product of resolution in time and in frequency, they can do much better. -- FA