[CM] telephone noise spectrum

[Bill Schottstaedt]

I was goofing around lately trying to clean up a
recorded telephone conversation (see sndscm.html,
the last section), and was wondering if anyone
knows the source/characteristics of the loud
background tone -- it's not 60 cycle hum harmonics
as far as I can tell; sounds like a high-voltage
tranformer.  I tried to notch it out, but I'm
not really having great success so far.

I tried to check the net, and wasted a lot of time...