Getting matching transformer from telephone

Salmon Egg wrote:


Twisted pair cable as used for telecoms has a nominal 100-110 ohm inpedance. See the ADSL specs.
600 ohms is an irrelevant historical nonsense from the days when they used telegraph wires for phone circuits.
Graham
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

For some cables. Not for others. It might, for example, be as high as a couple thousand Ohms too.

Not at all true. 600 Ohms is somewhat of a compromise, between the low impedance of an unloaded cable pair and the higher impedance if loading coils are used.
Telegraph wires were typically a #11 rusty wire, not a twisted pair.
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

600 ohms was the theoretical impedance of an infinite length of GPO telephone cable. In the early days of broadcast radio (1930s), when program was sent to the transmitters by GPO cable, assuming a line impedance of 600 ohms and matching to it, was found to give the best overall results. When you're sending program 600 miles from London to a transmitter in Scotland, it matters!

In the UK these cables went underground from studio, via as many exchanges and repeater stations as necessary, to transmitter and never overhead.
--
Stuart Winsor

For Barn dances and folk evenings in the Coventry and Warwickshire area
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

No it is not. (What is "GPO" ???)
Are you aware that inter-office trunking always used 900 Ohms, not 600?

I hate to tell you, but you don't just feed 600 miles of cable with a signal and expect anything to be at the end...

Let me repeat that, just so you'll understand: "Telegraph wires were typically a #11 rusty wire, not a twisted pair." Telephone cable is twisted pair, and sometimes it has been used for teletype service, which might even have been called "telegraph". But when there were wires that were "telegraph wires", they weren't twisted pair.
(Years ago I worked on telegraph systems...)
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Which has nothing to do with the information I am giving here.
I have come across your attitude in other groups where americans don't seem to understand that their way of doing things isn't universal throughout the rest of the world. Also an inability to read other people's posts and understand what is being said.
For example, you seem to have entirely missed:

^^^^^^^^^^^^^^^^^^^^^
Yes, we did send program feeds from London to Burghead in Scotland via landline.
Yes, although different impedences are *now* used under different circumstances, the cables *were* /all/ 600 ohms twisted pair. In more recent times we've had rep-coils on the incoming lines to match to 150 ohms and 75 ohms but by and large all *main* feeds are now by digital systems.
GPO=General Post Office, the organisation in the UK originally responsible for all telecommunications in the UK.
--
Stuart Winsor

For Barn dances and folk evenings in the Coventry and Warwickshire area
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

It has everything to do with the information we are discussing.
You don't know the difference between telegraph wire and telephone wire, just for starters.

If that was in reference to the previous paragraph of yours, then you should have put the information there. In that case I would have jumped all over your claim that it makes a difference for the 600 mile distance mentioned when in fact it is not 600 miles *per* *section*.
You also apparently don't know what "repeater stations" are when dealing with analog carrier systems.

You sent them that distance via FDM carrier systems, not via landlines, even in the 1930's.

Twisted pair cables are not 600 Ohms.
Open wire might be though... ;-)

By and large? And 75 Ohms??? (Please don't try buzz words...)

Thank you. (I'm familiar with BT, which evolved from the General Post Office.)
Your problem is that you just don't actually understand the telephone system, at all.
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Floyd L. Davidson wrote:

Repeater Stations were not limited to FDM carrier systems. "Repeater" refers primarily to amplification. Plenty baseband amplified circuits as well (2 wire and 4 wire).

Correct. I've worked on circuit equalisation at Burghead end. It was baseband audio - not carrier.

Very rarely - most carrier channels were 300-3400Hz. The noise and distortion figures came nowhere near the requirements for broadcast audio. ISTR there were attempts to use carrier, but with all sorts of problems, not the least being that the CCITT carrier frequency plans (Groups-Supergroups-Hypergroups) are based on 4KHz channel spacing - not 10KHz. Single Sideband translations are also problematic with music (broadcast) circuits, as the frequency stability requirements are MUCH higher than telephone-quality speech. Master oscillators DID go off frequency ....

WRONG!!!!!
The characteristic impedance of a transmission line(Zo)varies by frequency. At audio the influence of series resistance and parallel conductance outweighs the ratio of inductive to capacitive reactance, and the Zo rises as the frequency falls. A twisted pair will be typically 600 ohms at 800Hz. At RF the reactances become much more significant than the conductance/resistance, and the Zo will level out at the figure usually quoted as Characteristic Impedance. The same twisted pair will be about 140 ohms at RF.

See above.

"rep-coils"???? WTF ??
Known as "Transformers" in ever repeater station I ever worked in .....

Been there. Done it.
John
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

It was the term we normally used when referring to the transformer that, by and large, separated our (BBC) wiring from that of the GPO/BT. GPO/BT technicians that I talked to seemed fully conversant with the term.
Depending on the size and layout of the station, it might be at the bottom of a bay or in a seperate "Line termination room"
Plenty of other transformers about at amplifier inputs/outputs.
--
Stuart Winsor

For Barn dances and folk evenings in the Coventry and Warwickshire area
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Your description is quite correct. *Every* cable pair would have been terminated in a repeat coil of some kind, for a number of reasons. The primary reason is longitudinal balance, next would be DC isolation and/or impedance matching.
An interesting history on that too, as pre-WWII that type of device was typically far larger than necessary, but with efforts to save iron the WWII era designs were all smaller and lighter. After WWII the design criteria changed to saving space to allow smaller overall size of equipment.
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

But 600 miles of "baseband amplified circuits" at VF frequencies will leave you with nothing but noise.
It appears that you worked at the customer premise location and saw only the drop equipment.

And you have no idea what was in between.

Yes, most were. But not all. It was relatively easy to use a double wide channel, or use a groupband device that took up 48 KHz.

So you think an equalized audio channel over cable would, after 600 miles????

Even by the 1930's they were using phase locked oscillators (for example L carrier systems used a 64 KHz pilot to for frequency synchronization).
I worked for about 20 years on carrier systems designed in the 1930's, and never saw a single instance of a system going off frequency.

I have a book here that is definitive. It is the 1938 edition of the Bell System publication "Principals of Electricty Applied to Telephone and Telegraph Work".
A fold out chart (page unnumbered) between pages 192 and 193 shows cable characteristics for just about every common cable used at the time. The highest characteristic impedance shown (at 1000 Hz) for a non loaded cable is 19 gauge NLS at 470.1 Ohms.
There is not one single cable shown as +/- 10% of 600 Ohms.
On the other hand, on page 190 there is a chart showing open wire characteristics, and more than a third of the configurations shown have an impedance within 10% of 600 Ohms.

Do see above! :-)

Repeat coils. The common designation on the device itself was "Rep. Coil".

You've never worked in the telecommunication industy?

See above for why buzz words won't get it in this conversation.
75 Ohm impedances are virtually *only* used for unbalance coaxial circuits at baseband levels for carrier systems. It is *never* used for audio, and is never used with twisted pair cables that extend past the end of rack a unit is mounted in.

Obviously not.
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I now have some more information on this.
To quote:
"Studio Engineering for Sound Broadcasting" Illife 1955. Chapter 7 "Programme circuits on Post Office Lines" G Stannard, Bsc A.M.I.E.E - Lines department.
p142. "The distribution of BBC Light Programme to Burghead at the time of writing contains 693 miles of 16mH/1.136m. and the estimated delay distortion relative to 1kc/s is: 50 c/s 50m.sec 100 c/s 9.5m.sec 7 kc/s 7.3 m.sec"
16mH/1.136m describes a loaded line and m, in this case, would be miles. Elsewhere a table gives the following information for this type of line:
Weight of conductor 40lb/mile, Approx characteristic impedance 490 ohms, Cut-off frequency 9.3kc/s, maximum useable frequency 7.44kc/s.
The book further goes on to discuss carrier circuits
p131 (Same chapter)
"In 1938 the Post Office began a big expansion in their communication network by laying 12-channel carrier cables. These are low capacitance cables specifically designed for the transmission of frequencies up to 60 kc/s and subsequently up to 120kc/s"
Two schemes are discussed for using carrier circuit lines but they are described (p155) as inferior to circuits obtained by more conventional methods.
--
Stuart Winsor

For Barn dances and folk evenings in the Coventry and Warwickshire area
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Stuart wrote:

Thanks Stuart - this fits exactly with my memories of the time. It was a while back though ...... The BBC now has its own digital audio distribution systems, and no longer requires the widespread use of BT programme circuits.
To reiterate for those who have generated all the heat and the fabulously inaccurate rantings - The programme circuits were BASEBAND AUDIO all the way - amplified en route. As Stuart quotes - carrier is possible, but provides worse noise and distortion standards.
Carrier was not generally used for main UK programme distribution (sorry Floyd - you may be right about American practice, but not UK).
John
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Now that was the right, properly worded response to Floyd. Good job.
I wonder where this places the retarded donkey's assertions, if he ever actually made any, other than his rantings. He did claim to have a fairly deep knowledge... that he had a schematic.... somewhere.
It would be nice to see your document posted up in alt.binaries.schematics.electronic, if, that is, you have access to it.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

It's a 200 page book and I'm afraid I don't really have the time. Copyright might also be an issue. One or two pages I suppose I could manage but which ones? :-)
My current project is going through hundreds of old electronics magazines and scanning to PDF, stuff that I find of interest.
--
Stuart Winsor

For Barn dances and folk evenings in the Coventry and Warwickshire area
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Gosh, Stuart, why??
I do the exact opposite - if I get an *interesting* PDF, then I print it and store the hard copy.
I just cannot cope with trying to read complex documents on a screen, and the pink highlighter on the screen does not scroll with the page, either :-))
--
Russell
http://www.russell-hafter-holidays.co.uk
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 04 Jan 2009 20:54:04 +0000 (GMT), Russell Hafter News

Indicative of piss poor grasp of modern computer technology, not to mention quite lame document handling capacity. You don't know how to add a highlight to a doc on screen? Perhaps you need a mini NASCAR scrolling bug screen you can scroll with your document, dipshit?
Bwuahahahahahah!
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

It is difficult to writer complex documents for publication. Most, but not all, postings are not written well. I am guilty of contributing a few myself. It is not the medium; it is the writer.
Bill
--
Private Profit; Public Poop! Avoid collateral windfall!

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Hi Russell,

Space.
I need to make more space for books and new magazines so I need to get rid of some old magazines. I'm currently going through my copies of Elektor which date back to UK issue 1. I don't have them all mind you, there was a sizable period when I didn't subscribe, before I took out a new subscription to see what it was like, and I'm not currently a subscriber - it's too obsessed with micro-controllers.
So far I've gone through about 100 magazines and used up just over 40M of hard drive space.

There was a time when I used to photocopy something that really caught my eye and I'd store in files by content (especially if it was a magazine I didn't own!) but that is also too wasteful of space - not to say paper.
--
Stuart Winsor

For Barn dances and folk evenings in the Coventry and Warwickshire area
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
John Livingston wrote:

RIGHT !
Do the damn sums.
Graham
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

But not it seems with practical aspects. If you have you might know about the following
What is a reversible line and how does it work? What was the "test match" effect?

I have a moderate understanding of telephone systems but a much greater one of the use of program lines and why they were 600 ohms.
--
Stuart Winsor

For Barn dances and folk evenings in the Coventry and Warwickshire area
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.