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.
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.
I'm intruiged! Go on - tell us!
This should be real interesting...
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.
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.
I'm waiting to see your guesses to *both* questions :-)
Hint: both answers relay on the fact that the lines are straight analogue from end to end.
They weren't using something that resembles a single pair of Cat 5 back then.
As I said, it's HISTORICAL only and of no practical value.
Graham
General Post Office.
Which is why it's 100 ohms or so just like Cat 5 etc.
Graham
Quite so !
Absolutely not. About 100 ohms.
Via 'Post Office Telecommunications'.
He has made that evidently apparent.
Graham
RIGHT !
Do the damn sums.
Graham
Correct. It is in fact the classic 'characteristic impedance'.
Only on long circuits which were treated differently.
Only ever seen that in a fax machine I helped develop for Xerox (RXEG) and the hybrid part was done with differential amps, NOT the transformer. MUCH cheaper.
I have never seen a transformer in ANY phone including the carbon mic type which also 'draws power from the line' to power the mic as you say.
How do the clowns think they determine ON and OFF HOOK ?
Graham
True.
But that is NOT the 'characteristic impedance' and will vary depending whether you're 100m from the exchange / C.O. or 5km !
I appreciate your other information but it sure isn't the way we do things here in the UK.
Graham
I think he was having a laugh !
USA and back via either underground cable or satellite can (did) do some weird stuff, but even that has gone now with IP telephony which is becoming near universal now. I can make certain calls to the USA from the UK for LESS than the cost of certain types of UK call.
In fact a US call for me from the UK to a landline OR CELLPHONE costs 7.5c to connect and 1.5c / minute.
Graham
You really are stupid.
No, you build monstrosities, then brag about them.
I have had severe echo on as little as ten miles of underground telephone cable on an old mechanical exchange. The phone company had run new lines out to rural areas, then connected some downtown businesses to the trunk a block from the C.O. We had one of the first digital alarm monitoring services, and had trouble decoding some numbers because of the echo. Ohio Bell refused to correct their problem stating it would be done on schedule in six years so we had to move the business to another town that had recently undergone a full rebuild. The line ran about ten miles from the C.O. to the end.
I have to agree.
Plus think of the advantage in reduction of methane emissions !
Graham
Oh dear, you need educating.
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