[TenTec] Effect of roofing filter selection on Orion II NB

[Lin Davis]

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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