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.
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.
aioe.org, Goggle Groups, and Web TV users must request to be white
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 ?
Not irrelavant when we were discussing the history of why 600 ohms became
the standard impedance for audio distribution.
Modern phones don't use carbon mics either yet you claim
Perhaps you'd prefer the inside of the ubiquitous BT 746 which was
introduced in 1967 and was around for a very long time - we didn't replace
ours till well into the 90s
This is a 1978 version.
In 1985 it was fitted with the new style plug and became the 8746 but
still had the "Induction coil"
However, regardless of what you know or don't know, this thread was
started by someone who wanted to know whether he could re-use the
transformers in telephones for another purpose so clearly he has some!
For Barn dances and folk evenings in the Coventry and Warwickshire area
** That is entirely false.
What the OP *actually asked* was this:
" If I strip down some landline phones I 've got here, then will there
be a matching transformer in each one? "
Chances are high there are no iron core transformers ( hybrid or other) in
his phones at all - cos they are too modern.
Secondly, his purpose requires a transformer with high voltage safety
isolation from the phone line - which the common 600:600 ohms phone line
types all have and others do not.
Thirdly, his purpose requires a transformer with very high CMRR - cos that
is why he is getting humming noises at the moment when making recordings
with his passive ( ie non-transformer ) adaptor device.
Have you ever done this ??
I can assure you it ain't as simple as you suspect to get a hum free and
distortion free result.
Not the least of the problems is that portable voice recorders have high
sensitivity mic inputs with automatic gain control circuits - which cannot
be defeated by the user. This results in annoying gain pumping and regular
bursts of overload distortion PLUS increasing background noise & hum
whenever there is a pause in the conversation.
The trick is to use just the right amount of resistive divider attenuation
before and after the 1:1 transformer to eliminate this - plus cap couple
the input side to stop pulling the line low.
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