Oops... pretty bad with my cut and pasting...
The sentence:
"This behavior increases the passband attenuation noise during pauses in speech
or between CW elements."
should have been:
The filter increases the passband attenuation quickly during pauses in speech
and between CW elements, quieting the noise during those intervals.
Lin
Lin Davis wrote:
> If I may be so bold as to offer another way in which S+N/N may be improved,
> albeit with distortion; that is by using non-linear gain.
>
> The NR, probably still being some form of Doug Smith's "adaptive predictor
> with
> leaky LMS" design, will attenuate periodic waveforms less than random ones,
> even
> in the narrowest of passbands. This behavior increases the passband
> attenuation
> noise during pauses in speech or between CW elements. When the
> periodic/coherent
> signal returns, the gain is restored/increased. Thus both the noise and the
> signal are heard. In effect, the noise and desired signal are modulated with
> the
> desired signal envelope. This could very well bring about the S+N/N
> improvement
> Grant has experienced.
>
> This isn't the only thing at work, since the NR filter is frequency savvy, so
> it
> can attenuate the parts of the passband spectrum that carry random signals,
> and
> pass those that show periodicity; hence, build a filter around the signal, as
> others have observed. More than that, the stronger the periodic signal, the
> less
> it is attenuated. Utterances of speech (Grant, help me out with the correct
> nomenclature!) contains many frequency components simultaneously, each at a
> different amplitude. Since this type of filter attenuates weaker components
> more
> that stronger ones, the voice signal becomes distorted, but our brains are
> able
> to recover the info, for the most part.
>
> The thing is, how best do we use the NR to bring the desired effect about? I
> found that reducing the rx gain, (by rf gain, adding attenuation, or by AGC
> threshold) with no signal present until the noise is now just audible (with
> NR
> on), gives the best improvement to the desired signal. If there is a better
> way
> to set it for best performance, please let us know!
>
> 73,
> Lin
> WB1AIW
>
>
> Grant Youngman wrote:
>
>
>>
>>
>>
>>> Grant, I don't believe the 331A is True RMS.
>>> I cannot find anything on Google to confirm that the
>>>331A isn't True RMS, but that's what I recall.
>>
>>
>>You're correct. I had to dig around and find my spec sheet. Live and learn
>>:-)
>>
>>It is RMS, calibrated to a sine wave.
>>
>>
>>
>>> Let's just think about your conclusion.
>>>8-10 dB S/N improvement over the 100 Hz DSP BW implies a
>>>10-15 Hz filter bandwidth
>>
>>
>>I understand that, and appreciate it. The fact remains, there does appear
>>to be further improvement in SNNR with NR engaged, even at narrow
>>bandwidths.
>>
>>Perhaps someone with better lab resources will poke around at this stuff,
>>too. In the meantime, I'm going to go shopping for a 339A or something
>>similar :-)
>>
>>Grant/NQ5T
>>
>>
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>>
>
>
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>
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