In the DttSP software, the sdr core for PowerSDR (Flex Radio) and others, we have a block LMS approach to automatic noise reduction. It does a really good job of doing noise reduction and automatic notch (your choice) and it definitely does not introduce nasty sounding distortion. It is much more stable than standard LMS based automatic notch filtering. When I developed the algorithm, I was surprised that others had not done it. I then learned of some subtleties which do need mitigation but were pretty straightforward. You adjust the filter once per block rather than once per sample, so you must be careful not to introduce discontinues (distortion) at the block boundaries. Once handled, this is the only way this should be done.
That code is available for svn download (visit the Flex Radio web site or the dttsp-linux yahoo group).
Bob N4HY
Tony Langdon wrote:
At 12:04 PM 4/12/2007, Roger Kolakowski wrote:
And the winning answer was....?
That one's got me curious too. I'm particularly fussy about DSP noise reduction, and there are very few that I actually like. The one I use is made by Michels Engineering in Germany, which provides an excellent degree of noise reduction and doesn't distort the audio as severely as many others. For me, that distortion interferes with my ability to pull a signal out of the mud, and most of the DSP noise reductions work best for me in the "off" position, as far as readability goes. The Michaels one, OTOH, has no impact on readability at worst, and often a modest improvement, as well as excellent noise reduction. On very weak FM signals (ideal for SO-50 :) ), the readability is dramatically improved, and the DSP can almost act as an FM voice activated mute.
73 de VK3JED http://vkradio.com
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