[Asterisk-Users] Woodpeckers

[Steve Underwood]

Chris Albertson wrote:

>--- Steve Underwood <steveu at coppice.org> wrote:
>
>  
>
>>A power spectrum plot will tell him he has a 60Hz hum. I think he 
>>already knows that. I think he can definitely consider solutions
>>without 
>>following your suggestion. :-)
>>    
>>
>
>No,  It's not a "60Hz hum".  Yes, 60Hz is getting into the line
>but the existing filters are removing the 60hz.  What he hears
>is most likely 120Hz, 240Hz or something else or most likely
>a combination of various multiples for 60hz.  
>  
>
The existing filters are removing a lot of 60Hz, but filters aren't 
perfect. Of course he is getting a combination of various multiples. 
He's getting all the odd harmonics, decaying gently in amplitude across 
the band.

>I'd bet that the tiny speaker inside a telephone handset can not
>even reproduce a 60Hz tome.  Yes you can hear a hum but it's
>the overtomes of 60 that you hear.  Many people can not even hear
>down to 60Hz, some can but not everyone.  
>  
>
The speaker will be rather insensitive at 60Hz, but it will produce some 
output there. More significantly, a speaker exiting below its natural 
resonance will be outputing energy at is resonant frequency, and at the 
odd harmonics of the excitation. You need to filter away the residual 
60Hz excitation to stop this happening.

>If you were to design a filter wouldn't it be nice to know some
>thing about the noise?  Is there a big peak at 360?  how broad is
>that paek 5hz or 20hz?  I would expect the power spectrum of a
>"hum" to have multiple peaks?
>  
>
So were exactly would be causing this 5Hz to 20Hz of spectral spread in 
a signal from the power grid? I know exactly what is coming down his 
line. After the A/D converter's high pass filter I have a pretty good 
idea what is left too. As long as the power has none been through nasty 
non-linear components, like voltage regulators or constant voltage 
transformers, the mains voltage waveform doesn't have too much harmonic 
content. His energy will be concentrated in the low few hundred Hz.

Regards,
Steve