View Single Post
  #30   Report Post  
Posted to sci.electronics.repair
RobertMacy RobertMacy is offline
external usenet poster
 
Posts: 732
Default Highly Shielded Audio Cable

On Wed, 12 Mar 2014 23:00:33 -0700, Michael Black wrote:

On Thu, 13 Mar 2014, Michael A. Terrell wrote:


dave wrote:

The Phone Company doesn't use shielded cable for baseband audio.



No, but they use twisted pair and they can still pick up a lot of RF.
I've seen over 5 volts of RF on phone lines at AM radio stations that
were wired with 'station wire' instead of twisted pair. The radio
station audio was louder than either party on the line could talk. The
fix was to rip out everything, run 25 pair twisted cable to localized
terminals and use short runs to the phones. There was still some common
mode RF, but at least the lines were usable since it no longer caused
the volume limiter to go into continuous conduction.

Yes, and even in the days of the no real electronics in the phones,
there were tips in the books about keeping RF out of the phones.

The issue becomes more significant when all the phones are made of
electronics, and there's a lot more that can act ad diodes to detect the
signals.

Michael


AND! the designers usually violate concept of exactly what 'balanced' line
means, then the telephone's own protection system will rectify the AM
signals.

Most radio stations will supply little pigtails to place between your
phone and the line that pretty much drops that RF, for free, as a good
neighbor act.

In one doctor's office sitting by the relatively low powered 10kW AM
towers [Jeff will know where this is, driving south along highway 1 to
your left, just south of Santa Cruz] had between 1V/m upwards to 3V/m and
in some places concentrated to over 7V/m and the station came in louder
than conversations. Now,..extrapolate that to EMP at 20kV to 50kV/m and
you can see why nobody wants missiles AND nuclear capability in the same
hands.