David's analysis is correct. Go to his last sentence - that is the
Noise Blanker (NB) and not Noise Reduction (NR). NB punches a hole in
the signal in response to impulse noise. It will cause signal
distortion, getting worse with more aggressive settings.
Noise Reduction (NR) depends on correlation techniques. It has to
determine what is a signal, and then build a filter around that.
Once the filter is built, then the signal audio should be the same as
normal, but the noise content will be reduced. Yes, NR will cause
distortion of the signal, but the idea is to be able to maintain
communications rather than simply to get rid of the noise.
The K3/K3S/KX3/KX2 NR has several settings to allow you to customize the
noise reduction to the particular noise you are encountering. There is
no "one size fits all" for all noise sources.
I have "repaired" several K2s equipped with the KDSP2 which were sent to
me for "distorted audio" and the "cure" was to simply turn off NR in the
KDSP2. Yes, you should expect some audio distortion when using NR.
73,
Don W3FPR
On 1/12/2019 11:51 AM, David Woolley wrote:
Do you have a reference for an algorithm that will do this?
Noise reduction is difficult because any effective noise reduction has
to recognize what is signal, and ideally what is the part of the signal
that matters to the human. The hearing aid industry has been trying to
do this for years, with limited success.
I think the sort of noise reduction we are talking about here
essentially tries to decide which frequencies matter and which don't and
eliminate the latter. However, it has to do this when what it is trying
to identify as signal is hidden by noise.
In practice, I think what these systems achieve is increased user
comfort, rather than recovering signal from noise, as humans are
probably still a lot better at extracting signal from noise than
algorithms, but they get tired in doing so.
One consequence of selective filtering will be a reduction in total
audio power. I'd expect the total loudness to go down. I guess you
could then renormalise, and increase the signal power to bring the total
power up to the same level. However, most of are old enough to have a
lot of high frequency hearing loss, so one may find that correction
needed depends on specific hearing loss of the user and the original
spectrum of the noise; one needs to renormalise the power as waited by
the hearing sensitivity curve of the user. I imagine you would need, at
least, a parameter to determine the degree of renormalisation.
Also, the more aggressive you make this sort of noise suppression, the
more likely it is to have false positives, and suppress important
frequencies. Also, the more aggressive you make it, the more you will
get distortion as the result of modifying filter parameters on the fly.
Ultimately, though, the sort of noise that these systems are trying to
remove is random in nature, so you can never be completely sure what is
signal and what is noise.
(Hearing aids have a particularly difficult problem in that they are
dealing with cocktail party noise, where the noise is the summation of
lots of things that would, individually, be signals.)
(The ultimate noise reduction system would be one that recognized the
speech and regenerated it, complete with characteristics of the original
speaker. However, doing that really well can only be done by looking
ahead several seconds, to be able to interpret meaning from what
follows, as well as what precedes.)
As a caution, I believe the K3 has two different noise handling
strategies: the one I am talking about here, and one designed to deal
with impulse noise, where you simply cut out a short section of signal
around the noise pulse.
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