Getting matching transformer from telephone

Sigh. You are just being an ass. The point was that we fed the same level we would have on a dedicated pair, in spite of the DC on the pair. The equipment was designed well enough that it didn't saturate the transformers on either end. It was within Telco specs for level and frequency response, as well.

The point is that no one in their right mind expect a phone line to be THAT quiet. OTOH, you might. I recall you bragging about recording lots of phone calls in a C.O. at one time. What did that have to do with PSTN?

Then explain the difference between feeding a phone line to a computer for the OP and feeding a phone line to a computer for talk radio.

Because I have no way of knowing the level they actually supplied to the Army on that line. Long distance calls were all loud and clear, while the net feed sucked.

yawn. You sound like the typical T&R guy who doesn't give a shit when your feed is down.

Of course you don't. The circuits performed the same function, but at a higher level of performance. Something well beyond what was used in telco. Telco doesn't use redundant systems and combine the outputs to improve reliability. But then, people rarely were foolish enough to depend on the telco to provide mission critical communications if anything else was available.

Yawn.

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Reply to
Michael A. Terrell
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1) I've never recorded a telephone call. 2) I've never worked in a C.O. 3) I know exactly how quiet a telephone line might be.

Except that isn't the case.

Are you sure? (Remember all those whooshing noises on circuits that went through White Alice? That was a combiner, mixing baseband signals from redundant systems.)

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Floyd L. Davidson 
Ukpeagvik (Barrow, Alaska)              floyd@apaflo.com
Reply to
Floyd L. Davidson

Untrue. A non-optimally loaded audio transformer will not have a flat frequency response. Nor is it about power transfer.

Graham

Reply to
Eeyore

I never heard any noise like that on Alice. In fact, it was the cleanest long distance calls I ever made, till fiber optics entered the picture. Local calls that required the operator to find a number went to Fairbanks. The road to was over 100 miles, so it would be something near that for the alice microwave trunk. It was quiet. A hell of a lot quieter than the lead cable inter CO trunk lines i used right after I got out of the service. A call to another city that was less than ten miles away was so bad you had to scream to be heard. Some of that went away when Middletown got an early ESS to replace the '20s mechanical scrap heap they had been repairing with parts from scrapped C.Os.

Tell me, Floyd. Could you balance those White Alice systems to .01 dB and keep them that way?

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Reply to
Michael A. Terrell

: : :Ross Herbert wrote: : :> Eeyore wrote: :> :Ross Herbert wrote: :> : :> :> Since your PC is mains powered and it may not have the required isolation :> :> between the mains side and the sound card input you can do your own thing :> :> using an approved 600:600 transformer with 3kV isolation rating to interface :> the :> :> telephone line to the sound card input. :> : :> :He DOES NOT need a 600 ohm transformer since the input impedance of the sound :> :card is not 600 ohms ! :>

:> Since the application is merely detecting signal "voltage" it hardly matters :> that the secondary impedance of the transformer is 600 ohms and the input :> impedance of the sound card is more like 10Kohms. The only reason one tries to :> match impedances is where one needs to maximise "power transfer" and that :> doesn't apply in this case. : :Untrue. A non-optimally loaded audio transformer will not have a flat frequency :response. Nor is it about power transfer. : :Graham

With regard to a POTS line the VF bandwidth is some 300 - 3400Hz - hardly hi-fi

- so optimal flat frequency response is not an issue.

The fact that the secondary impedance of the 600 ohm transformer does not match the input impedance of the sound card is totally unimportant in this application.

Reply to
Ross Herbert

frequency

So your attitude is "it's so bad it doesn't matter messing it up even more".

A proper 600:600 transformer will be more expensive than a 10k:10k one too.

Graham

Reply to
Eeyore

:> With regard to a POTS line the VF bandwidth is some 300 - 3400Hz - hardly hi-fi :> - so optimal flat frequency response is not an issue. :>

:> The fact that the secondary impedance of the 600 ohm transformer does not match :> the input impedance of the sound card is totally unimportant in this :> application. : :So your attitude is "it's so bad it doesn't matter messing it up even more".

Not at all. Any practical measurement of the degradation of a 300-3400Hz signal using the 600 ohm transformer would be insignificant. If your assumption that audio degradation when using a 600:600 transformer was valid then why is there no available audio transformer, with a 3.5Kv isolation rating, to match a 600 ohm telephone line to a high impedance (eg. 10K ohm) input? The answer has to be, "it's not necessary".

: :A proper 600:600 transformer will be more expensive than a 10k:10k one too. :

That may be true but how many readily available 10K:10K audio transformers have a 3.5Kv isolation rating?

In order to conform to the required isolation rating of 3.5Kv between the mains powered PC and the telephone line, the only transformer which is readily available, and complies, is an approved 600:600 transformer.

Reply to
Ross Herbert

I believe this was required IIRC for only brief use. He could avoid using it during thunderstorms too.

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Probably in stock at Newark

Graham

Reply to
Eeyore

have

during

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Data sheet implies may be suitable for telephone coupling.

Graham

Reply to
Eeyore

during

Why? That takes the fun out of it!

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"

Reply to
Jamie

One signal lead is connected to positive the other signal lead is connected to negative. Much the same way as telephones are powered. For screening to work it must not be carrying an electrical current.

Reply to
Mark Evans

In the context presented, that is generally true... but I'd like to point out that the shielding on multipair telephone cables is 1) grounded, 2) commonly conducts current, and 3) the noise reduction provided is

*enhanced* by the current!

Both ends of the cable are well grounded and any induced current in the shield will be greater than in any individual cable pair. The current in the shield induces an opposite current into the cable pair, thus reducing the total noise signal in each pair.

--
Floyd L. Davidson 
Ukpeagvik (Barrow, Alaska)              floyd@apaflo.com
Reply to
Floyd L. Davidson

Do you manage to get grounds in that soil over there, seemed to think it was more permafrost;)..

Those pix on the Floyd website look rather, well "cool" :)...

--
Tony Sayer
Reply to
tony sayer

Try driving a 60 foot ground rod through permafrost some time.

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There are two kinds of people on this earth: The crazy, and the insane. The first sign of insanity is denying that you're crazy.

Reply to
Michael A. Terrell

The OPs problem can be solved by using the circuit from p62 of Elektor magazine March 2006, "Telephone evesdropper". You should be able to obtain a copy via their website:

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Reply to
Stuart

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