"Don Pearce" wrote in message
news:4a55f0af.1492558562@localhost...
On Tue, 30 Jun 2009 11:52:24 +0100, "Serge Auckland"
wrote:


"Don Pearce" wrote in message
news:4a54e8a0.1490496031@localhost...
On Tue, 30 Jun 2009 11:18:44 +0100, "David Looser"
wrote:

"Eeyore" wrote in message
...


Even POTS is balanced after a fashion.


More than "after a fashion". POTS is very well balanced, and of course
balancing was invented by and for the telephone industry.

David.


Pots is both balanced and unbalanced - that is how it manages full
duplex over a single pair.

d

I've always been fascinated by telephone hybrids that separate out the
send
and return audio. I can understand how modern digital hybrids work,
buggered
if I can understand how the original hybrid transformers work though.

S.

I wonder if I can explain without pictures - probably not. The
transformer has a single winding for the line (and yes, it really is
balanced both ways). Facing the handset is a centre tapped winding.
with exactly the same impedance load to ground from all three points.
One of these windings will be for the microphone, the other for the
earpiece. A signal arriving along the line goes to both the earpiece
and the microphone - doesn't matter about the one hitting the mic, as
long as the earpiece gets some too. So there is no discrimination in
the incoming direction.

The clever bit is when you talk into the microphone. Obviously the
transformer sends some of it along the line, but an equal signal will
appear at the centre tap (equal because all the resistive loads are
the same. At the same time, the second half of the winding generates
the identical voltage, but this time out of phase with the centre tap.
So the net voltage at this end of the winding is the vector sum of two
identical, opposite phase signals. Ie zero. This goes to the earphone.

Did that make any sense?

d

It does thanks. I'll draw myself a circuit diagram, and compare it with
published texts. Doing it in DSP seems a lot less bother...and with damn
near perfect cancellation, pity they didn't have DSP in the 20s and 30s.

S.
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