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Soundcard/POTS interface question

J

Jon Danniken

Jan 1, 1970
0
Hello,

I have built one of the basic isolated POTS interface circuits, using it
record a call (into the mic jack of my soundcard), or using it to play an
audio file into the line (from the output of my soundcard).

Currently, I just plug into either the microphone jack or the output jack,
but I would like a way to not have to keep plugging and unplugging every
time I change mode from play to record.

So, would there be anything inherently wrong with connecting *both* the
microphone input and the output together at the same point in the circuit?
I would naturally mute the output while recording, to avoid feedback.

Will this work, or is there a more elegant solution to isolate the two
points on the soundcard?

Thanks,

Jon
 
A

Andrew Holme

Jan 1, 1970
0
Jon said:
Hello,

I have built one of the basic isolated POTS interface circuits, using it
record a call (into the mic jack of my soundcard), or using it to play an
audio file into the line (from the output of my soundcard).

Currently, I just plug into either the microphone jack or the output jack,
but I would like a way to not have to keep plugging and unplugging every
time I change mode from play to record.

So, would there be anything inherently wrong with connecting *both* the
microphone input and the output together at the same point in the circuit?
I would naturally mute the output while recording, to avoid feedback.

Will this work, or is there a more elegant solution to isolate the two
points on the soundcard?

Thanks,

Jon

Well, if you're literally moving the jack plug from one socket to the
other, without changing anything else, it might work - the levels
should be similar. There's a slight risk of clipping due to
over-loading, but it shouldn't cause permanent damage; however, what
about when AC ringing voltage is present on the line? That could blow
the soundcard. I take it your POTS interface contains an isolating
transformer? What voltage do you see on the jack plug when the 'phone
is ringing? Have you looked at it on a 'scope? Have you already had
it plugged into *both* sockets at different times, when the 'phone has
rung?
 
M

martin griffith

Jan 1, 1970
0
Hello,

I have built one of the basic isolated POTS interface circuits, using it
record a call (into the mic jack of my soundcard), or using it to play an
audio file into the line (from the output of my soundcard).

Currently, I just plug into either the microphone jack or the output jack,
but I would like a way to not have to keep plugging and unplugging every
time I change mode from play to record.

So, would there be anything inherently wrong with connecting *both* the
microphone input and the output together at the same point in the circuit?
I would naturally mute the output while recording, to avoid feedback.

Will this work, or is there a more elegant solution to isolate the two
points on the soundcard?

Thanks,

Jon
One "correct" way of doing this is with a hybrid transformer

http://www.lundahl.se/pdfs/datash/6702.pdf

These transformers are expensive, but you can use cheaper OEP ones

http://www.oep.co.uk/audio_transformers_high_performance/audio_transformers_high_performance.html




martin
 
A

Anthony Fremont

Jan 1, 1970
0
"Jon Danniken" wrote
Hello,

I have built one of the basic isolated POTS interface circuits, using it
record a call (into the mic jack of my soundcard), or using it to play an
audio file into the line (from the output of my soundcard).

Currently, I just plug into either the microphone jack or the output jack,
but I would like a way to not have to keep plugging and unplugging every
time I change mode from play to record.

So, would there be anything inherently wrong with connecting *both* the
microphone input and the output together at the same point in the circuit?
I would naturally mute the output while recording, to avoid feedback.

Will this work, or is there a more elegant solution to isolate the two
points on the soundcard?

Here's a bunch of useful information about telephone line interfacing.
http://www.tkk.fi/Misc/Electronics/circuits/teleinterface.html
 
J

Jon Danniken

Jan 1, 1970
0
Andrew Holme said:
Well, if you're literally moving the jack plug from one socket to the
other, without changing anything else, it might work - the levels
should be similar. There's a slight risk of clipping due to
over-loading, but it shouldn't cause permanent damage; however, what
about when AC ringing voltage is present on the line? That could blow
the soundcard. I take it your POTS interface contains an isolating
transformer?

Yes, indeed. The one I have built (breadboarded) is based on this model;
there are zeners on the output to limit the back end voltage:

http://www.rane.com/note150.html
What voltage do you see on the jack plug when the 'phone
is ringing? Have you looked at it on a 'scope?

Don't know, and my scope is AWOL presently.
Have you already had it plugged into *both* sockets at different times, when the 'phone has
rung?

No, currently I just plug it into one or the other.

Jon
 
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