| Jeff McAdams on 14 Dec 2004 18:25:29 -0000 |
|
Doug Crompton wrote:
> Well I still don't understand the technicallity of balanced and unbalanced
> on POTS. It is usually thought of as nominally 600 ohms balanced. 48 volts
> DC fed +/- to two wires through two 500 ohm coils (chokes) and the AC
> (audio) signal applied through two equal capacitors to those two lines at
> the CO This was before we had opamps or even solid state but the principle
> is the same today.
> At the RX end so long as you do not reference anything to ground equal
> currents flow. The AC signal at any given point on the line would be
> balanced. If you connect any resistance or capacitance unequaly to either
> side of the line you upset the balance and induce hum or noise.
Ah...ok...you jogged my brain. We're both right...kinda. :)
A POTS phone line isn't balanced in the traditional way, ie, the two tx
pins and two rx pins. It is, sort of, balanced, though, you're right,
now that I go back and think about it.
The signal is measured (essentially) from ground, but its the CO's
ground, not local ground to the phone. Basically, when you're talking
on a phone (old school, no local power provided sort of phone), the
signal comes from the CO down the wire and back up on the other, with no
connection at all to local ground, so essentially the signal is measured
against "ground", but the ground comes down the line too, its not
locally grounded, so, similarly to the traditional balanced sort of
thing, any induced current is also induced in the "ground", so you get
the same sort of benefits of a more typical type of balanced signal.
Thanks for pushing that point...I hate to be wrong when I'm describing
technical stuff, so I'm glad you pushed it so that we could clarify it.
I also apologize as I suspect some of my usage of terminology is
slightly off here as I haven't had a physics class since college, which
is longer ago than I care to admit. ;) I'm a little rusty on some of
the electrical terminology except through just general usage, where I
tend to be a bit sloppy in usage.
--
Jeff McAdams
"They that can give up essential liberty to obtain a
little temporary safety deserve neither liberty nor safety."
-- Benjamin Franklin
Attachment:
signature.asc ___________________________________________________________________________ Philadelphia Linux Users Group -- http://www.phillylinux.org Announcements - http://lists.phillylinux.org/mailman/listinfo/plug-announce General Discussion -- http://lists.phillylinux.org/mailman/listinfo/plug
|
|