--- Steve Underwood <[EMAIL PROTECTED]> wrote:????The lowest note on the piano is A0=55 Hz. I don't know a single person who cannot hear A0. Most people can hear down to 30 Hz, a lot can hear down to 20 Hz and some can even hear as low as 10 Hz.
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.
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.
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?
It's alsmost assuredly power line stuff - very narrow peaks (0.1 Hz or less). Harmonics derive from non-linear bits in the signal path - rusty/corroded connections, salt in contact with wires, weird magnetic induction effects, etc. Even if rather badly distorted, the energy at the higher peaks won't be too large, and anything above the third or fourth harmonic will be negligible.
I would expect the power spectrum of a
"hum" to have multiple peaks.I love these discussions. Anyone who has actually tried to realize real filter designs should be having a real laugh right about now. While it is be possible to filter the dominant 60 Hz fundamental component (where 80-90 percent of the energy should reside), and even, with some effort, to knock off the most energetic harmonics, why bother. The far end will still hear the hum. On the other hand, if you actually load the phone line to shunt the common mode portion of the 60 Hz signal, you will badly mess up line balance (echo anyone). If you shunt the differential mode portion, unless you really use sophisticated circuitry, you will kill the ringing voltage - as was pointed out, getting even 2 poles of rejection with passive components at 60 Hz requires some pretty hefty stuff (90 Hz is not even an octave away), and active solutions are a real joy, considering that all of the easily designed topologies use op-amps with +/- 15 volt supplies (remember that 90V ringing voltage?) - and it still won't be bi-directional like passive designs - which means that you will need two hybrids with a filter on each 2-wire leg between them. I could go on (and on). Ah, the joys of filter design. This kind of approach is a bandaid for something that is really someone else's (i.e., the phone company's) problem. I echo (sic) the advice of several other posters: go after the phone provider.
Incidentally, my brother works for Verizion, and they have fond names for squirrels and other critters that attack aerial lines - it keeps them in work after all. When on line duty, he spent most of his time tracking down line faults just like the one(s) described here. Verizon was (is?) heavily invested in line quality and considered it a priority to resolve customer complaints. I find it amazing that your provider is so recalcitrant, when in fact it is their problem.
Oh, by the way, it has been my experience that, sometimes, when woodpeckers attack houses, it is because there may be food (i.e. insects - like carpenter ants or termites) to be had, in addition to perhaps just being on the hunt for a place to live. If there are a lot of smaller holes, especially if they seem to attack only localized areas of the house, then perhaps you should consult an exterminator as well. I know that this last bit is a somewhat off topic, but hell, it does involve a possible phone call or two, and probably one using * to boot!
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