On Tue, 30 Jun 2009 12:55:25 +0100, "David Looser"
wrote:

"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.


Yup - I was just trying to describe the simplest implementation. The
more complex ones tend to hurt my head a bit - I can't see how they
work just by looking at them, I have to draw little arrows all over
them to see how the phases add and subtract at the various ports.

d

In message , David Looser
writes
"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?

As we Brits call them 'Phono' connectors, it may be that nobody has ever
bothered to measure their Zo. However, they certainly work fine at RF
and, especially with fully-screened types of connectors, are sometimes
used right up to VHF and even UHF - certainly in applications where
matching is not really important.
--
Ian

"Ian Jackson" wrote in message
...

As we Brits call them 'Phono' connectors, it may be that nobody has ever
bothered to measure their Zo. However, they certainly work fine at RF and,
especially with fully-screened types of connectors, are sometimes used
right up to VHF and even UHF - certainly in applications where matching is
not really important.
--

Indeed, they can often be found connecting aerial leads to UHF TV tuners. In
that context I wonder how well they compare with the common Belling_Lee
type?, or the 'F' come to that.

Nobody is pretending that the phono, aka RCA, connector is the greatest RF
connector of all time. It is clearly out-performed by "real" RF connectors
such as the BNC and N types, but it is plenty good enough for baseband video
and even non-critical RF use.

David.

In article , Ian Jackson
wrote:
In message , David Looser
writes
"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?

As we Brits call them 'Phono' connectors, it may be that nobody has ever
bothered to measure their Zo. However, they certainly work fine at RF
and, especially with fully-screened types of connectors, are sometimes
used right up to VHF and even UHF - certainly in applications where
matching is not really important.

Since they have quite a small 'length' for signals below about a GHz it
probably makes more sense to regard them as a lumped network in this
context. I'd assume they have an effective Zc that varies with frequency,
and from one make to another. Thus making an attempt to assign a Zc value
to them as a class rather pointless, I'd suspect. :-)

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

On Tue, 30 Jun 2009 14:14:03 +0100, "David Looser"
wrote:

"Ian Jackson" wrote in message
...

As we Brits call them 'Phono' connectors, it may be that nobody has ever
bothered to measure their Zo. However, they certainly work fine at RF and,
especially with fully-screened types of connectors, are sometimes used
right up to VHF and even UHF - certainly in applications where matching is
not really important.
--

Indeed, they can often be found connecting aerial leads to UHF TV tuners. In
that context I wonder how well they compare with the common Belling_Lee
type?, or the 'F' come to that.

Nobody is pretending that the phono, aka RCA, connector is the greatest RF
connector of all time. It is clearly out-performed by "real" RF connectors
such as the BNC and N types, but it is plenty good enough for baseband video
and even non-critical RF use.

David.


Both are identical in that the problem isn't inside the connector, but
in the way you have to split and tail the coax to join to the socket.
That is a huge RF discontinuity even at UHF.

d

In article , David Looser
wrote:
"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.

i'm having problems working out both the examples given. :-)

I'd assumed that they simply used a network that could detect the relative
signs of I and V, and thus couple appropriately in accord with the
direction of the signal power flow. Is that in essence what each example is
doing?

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

On Tue, 30 Jun 2009 14:18:08 +0100, Jim Lesurf
wrote:

In article , David Looser
wrote:
"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.

i'm having problems working out both the examples given. :-)

I'd assumed that they simply used a network that could detect the relative
signs of I and V, and thus couple appropriately in accord with the
direction of the signal power flow. Is that in essence what each example is
doing?


Pretty much yes. In the case of my description it is just a question
of where you inject signal into a transformer, and the resulting
phases across the various windings.

d

In article ,
tony sayer wrote:
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....

Years ago, I used to set up all the calls to Swap Shop. After being
approved by production, they'd phone back and pass the call onto the sound
department. The BBC line balancing unit converted the GPO two wire to four
wire but needed manual balancing. You'd set the incoming level 'what did
you have for breakfast' and then send tone down the line and balance for
minimum return. I reckon they worked rather better than the modern auto
units. Used to be fun - only 5 lines outgoing lines used - so you had to
be quick getting things set up for going on air. Biggest problem was
explaining they'd have to turn the sound down on their TV.

--
*How can I miss you if you won't go away?

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

On Tue, 30 Jun 2009 15:28:46 +0100, "Dave Plowman (News)"
wrote:

In article ,
tony sayer wrote:
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....

Years ago, I used to set up all the calls to Swap Shop. After being
approved by production, they'd phone back and pass the call onto the sound
department. The BBC line balancing unit converted the GPO two wire to four
wire but needed manual balancing. You'd set the incoming level 'what did
you have for breakfast' and then send tone down the line and balance for
minimum return. I reckon they worked rather better than the modern auto
units. Used to be fun - only 5 lines outgoing lines used - so you had to
be quick getting things set up for going on air. Biggest problem was
explaining they'd have to turn the sound down on their TV.

That's a problem that never went away. Still plagues every phone-in to
this day. They have all their mates round to hear, and the idea of
turning the sound off just as the only important bit happens is pretty
much a non-starter.

d

In article 4a5e2346.1505509656@localhost,
Don Pearce wrote:
Biggest problem was
explaining they'd have to turn the sound down on their TV.

That's a problem that never went away. Still plagues every phone-in to
this day. They have all their mates round to hear, and the idea of
turning the sound off just as the only important bit happens is pretty
much a non-starter.

Doing this manual line balancing I could hear if they had the TV on in the
room with the phone. Something no auto unit could. ;-)

--
*The man who fell into an upholstery machine is fully recovered*

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