In article ,
Eeyore wrote:
Indeed. I've got a pretty big installation here with some stuff in one
part of the room - TV, PVR, satellite receiver - and the main audio
stuff at another. All interconnected unbalanced. It's also linked to
sound and vision in the kitchen so I can have the same thing playing
etc while wandering between the two. And no problems whatsoever with
hum. It would be a poor piece of design which has high levels of
nasties on an audio ground.

Try measuring one. Cable screen to mains earth either on AC voltage or
low current. It may surprise you. Mind you, if it's ALL Class II
equipment the return path to ground doesn't exist so you may get away
with it.

I obviously have. The only earthed component is the AV amp. Otherwise I'd
have hum ;-)

--
*There are two kinds of pedestrians... the quick and the dead.

Dave Plowman London SW
To e-mail, change noise into sound.

"tony sayer" wrote in message
...

Urrrgh!..

PL 259 plugs!.. The spawn of Satan;!..

Why? They are large, robust, easy to wire up and have an excellent locking
mechanism, so what's the problem?

David.

"Eeyore" wrote in message
...


You're splitting hairs.

Not at all, I've just shot your argument down in flames. Don't forget that
it's common practice to loop-through video from one piece of equipment to
the next. What do you think that does to the VSWR?

And I'm sure
it has a better VSWR than an RCA phono POS.

You're "sure" are you? What's that, sixth sense?

David.

"Don Pearce" wrote in message
news:4a54e8a0.1490496031@localhost...

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


Pardon? The line is balanced - period. Full duplex merely means that the
speech currents from each end combine in the line.

If you are thinking about anti-sidetone hybrids (which I guess you are from
other posts) then they are not necessary for full duplex, the telephone
industry did without them for decades, and they do not in any respect
involve unbalancing.

David.

On Tue, 30 Jun 2009 12:36:47 +0100, "David Looser"
wrote:

"Don Pearce" wrote in message
news:4a54e8a0.1490496031@localhost...

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


Pardon? The line is balanced - period. Full duplex merely means that the
speech currents from each end combine in the line.

If you are thinking about anti-sidetone hybrids (which I guess you are from
other posts) then they are not necessary for full duplex, the telephone
industry did without them for decades, and they do not in any respect
involve unbalancing.

David.


Sorry about that - started typing before thinking. See my other post
with a handwaving description of a hybrid transformer. That is the
item that actually made the telephone acceptable in normal use.

d

In article 4a55f0af.1492558562@localhost, Don Pearce
scribeth thus
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

Sometimes with a C and variable R in that circuit to cope with line
impedance variations for things like telephone interface units for
broadcast and landline Two wire working for two-way radio applications
'tho the better ones will have -auto- circuitry to cope with that....
--
Tony Sayer

In article , David Looser
scribeth thus
"tony sayer" wrote in message
...

Urrrgh!..

PL 259 plugs!.. The spawn of Satan;!..

Why? They are large, robust, easy to wire up and have an excellent locking
mechanism, so what's the problem?


Horrible things in my opinion far better are BNC and N types..

Very difficult to get crimp ones..

Out of favour in the UK, 'tho still used quite a bit in the USofA..
David.



--
Tony Sayer

"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.
--
http://audiopages.googlepages.com

On Tue, 30 Jun 2009 12:46:17 +0100, "Serge Auckland"
wrote:


"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.


Nah - we wouldn't have got any of those beautifully elegant circuits -
like the telephone hybrid or Baxandall's tone control. That would have
been a real shame,

d

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


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).

snip to save bandwidth

Did that make any sense?


That's only one implementation, and not one that's been used on an standard
UK telephone. Since pre-electronic CB telephones had carbon microphones
powered from the line current it was necessary for the microphone to be
directly in the line as far as DC is concerned. One simple implementation
that was used here in the 200 and 300 series telephones used a 3- winding
transformer. The microphone was wired in series with winding (a) across the
line. In parallel with the microphone was a series combination of a 2
microfarad capacitor, winding (b), winding (c) and the earpiece. Finally a
30 ohm resistor was connected from the junction of windings (b) and (c) to
the junction of the microphone and earpiece.

When the user spoke current was induced in winding (c) which circulated
through the earpiece and 30 ohm resistor and which cancelled out the speech
currents flowing in the whole series circuit.

The later 700 series used a slightly different variation on the same theme.

Higher performance hybrid transformers were used to split the go and return
paths at each end of a 4-wire amplified circuit. Higher performance was
necessary here to stop the whole 4-wire circuit "singing".

David.