Telephone line audio interface to PC

[Country Loon]

I want my PC to interface to the telephone line and was wondering about the
problems. I know how to interface the audio via an ac coupled 1:1 isolation
transformer and a diode limiter to prevent the ringer ac voltage from
damaging the sound card. How do I do the off the hook connection and the
audio output from the PC feeds where ? How do I accept a call etc

Thanks

Tom

 

[Rich Grise]

I want my PC to interface to the telephone line and was wondering about
the problems. I know how to interface the audio via an ac coupled 1:1
isolation transformer and a diode limiter to prevent the ringer ac voltage
from damaging the sound card. How do I do the off the hook connection and
the audio output from the PC feeds where ? How do I accept a call etc

Go buy a "voicemodem".

Have Fun!
Rich

 

[Mac]

I want my PC to interface to the telephone line and was wondering about the
problems. I know how to interface the audio via an ac coupled 1:1 isolation
transformer and a diode limiter to prevent the ringer ac voltage from
damaging the sound card. How do I do the off the hook connection and the
audio output from the PC feeds where ? How do I accept a call etc

Thanks

Tom

If you are in the US, there is approximately 50 VDC across the two lines
that make up your telephone. If you draw a certain amount of DC current
from that 50 VDC, it will bring down the Voltage, and it will cause the
central office to consider your telephone "off the hook."

You can do this by switching in a resistor of the correct value, I guess.
Or applying a dead short, but that might make it hard to pick up the audio.

I don't know much about telephones. Hopefully someone will fill in more of
the details.

I wonder, would you need to worry about high power line transients and
lighting and so on? It would be a shame if your computer was completely
destroyed by a lighting strike miles away from your house.

And it would be even more of a shame if your house burned down
altogether.

--Mac

 

[Country Loon]

Go buy a "voicemodem".

I have heard the quality is not as good.

Tom

 

[Country Loon]

If you are in the US, there is approximately 50 VDC across the two lines
that make up your telephone. If you draw a certain amount of DC current
from that 50 VDC, it will bring down the Voltage, and it will cause the
central office to consider your telephone "off the hook."

You can do this by switching in a resistor of the correct value, I guess.
Or applying a dead short, but that might make it hard to pick up the audio.

I don't know much about telephones. Hopefully someone will fill in more of
the details.

I wonder, would you need to worry about high power line transients and
lighting and so on? It would be a shame if your computer was completely
destroyed by a lighting strike miles away from your house.

And it would be even more of a shame if your house burned down
altogether.

--Mac

I found this
http://www.colinfahey.com/2003may_phone_interface/2003may_phone_interface.ht
m
http://www.hut.fi/Misc/Electronics/circuits/teleinterface.html


and other page that explained about the surge voltage protection.

Tom