Connecting computer soundcard to telephone line

Recording telephone conversations

Sometimes it is useful to record telephone conversation with computer soundcard. To do that you need to buy or build a telephone conversation recording adapter which is is designed to be connected between telephone line and the tape recorder. The basic idea is to plug the adapter to the telephone line and it's output (usually 3.5 mm mono plug) to soundcard microphone input.

After you have made this connection you can use any sound recording application to record telephone conversations to your hard disk. Telephone network samples all normal voice calls at 8 kHz sample rate. You get good results with 8 kHz or 11 kHz sample rated when recording telephone conversations. Using higher sample rate does not give any better sound quality.

NOTE: If the adapter has a tape recoder control output also, do not conenct it to anything. It can't be used with computer soundcards.

What kind of adapter to get

A proper telephone recording adapter will pass the audio signal from the line to the output and at the same time provide electricla isolation from telephone line. The isolation is very important part of the adapter, it guarantess problem free operation and system safety.

What not to get

Do not use recording adapters which does not provide the isolation. Using them in combination with the sound card will get you into noise problems, cause that the system is against telephone equipment regulations and can damage your soundcard.

According the feedback on Usenet newsgroups and the e-mails I have received (about dozen incidents) my recommendation is: Do not use the telephone recording adapters sold by Radio Shack with your computer soundcard! According the symptoms reports I have got they do not provide necessary isolation between telephone line and can't be used. According the problem reports I have got, nobody has been able to successfylly get them to work when plugged to soundcard.

Possible problems and how to solve them

Problem: Very low sound level and lots of humming in sound

Problem: You have a wrong type of telephone recording adapter. It is not suitable to be used with sound cards.

Solution: Buy a better recording adapter and use it.

Problem: No sound at all gets recorded

Problem: You have made the connections wrong, your adapter is broken, you have selected wrong recording source or set recording level to zero.

Solution: First check that all connections done in the right way. Then check the soundcard mixer program setting that you have selected the recording source to be microphone input and that you have some decent recording level set there (put the level settings initially to center position if you know nothing better).

Problem: Sound level recorded is very low

Problem: Too low recording gain

Solution: Increase the recording volume with your soundcard sound mixer controlling program.

Problem: Sound level recorded is too high

Problem: Too high recording gain

Solution: Decrese the recording volume with your soundcard sound mixer controlling program.

Problem: Sound recorded is distorted

Problem: Too high recording gain

Solution: Decrese the recording volume with your soundcard sound mixer controlling program. If a very low volume does not hep, try to connect the adapter to line level input and change the mixer setting sight for this.

Playing sounds from soundcard to telephone line

If you want to connect your sound output to a telephone line to play sounds from your computer to the line you need a telphone line audio adapter which supports this. Basically this kind of adapters consists of audio isolation transformer and few extra components. Most telephone recording adapters are constructed this way and can work more or less well in other direction also.

The basic idea is to connect the adapter sound input to the soundcard line level or speaker level sound output. Then connect the other end of the adapter to telephone line. Then make a telephone call and test your soundcard output volume settings so that the other end of the line get the sound at a pleasing volume.

Possible problems and how to solve them

Problem: Sound level heard on telephone line is too low

Problem: You have your soundcard output level (volume) set too low or you are using an adapter which is not suitable for sound playback to line.

Solution: Try if increasing output volume on soundcard hels. If this does not get acceptable results, try to get another adapter which suits better for this.

Problem: Sound level heard is too high

Problem: Too high soundcard output volume

Solution: Decrese the output volume with your soundcard sound mixer controlling program.

Problem: Sound volume heard on other end is high and distorted

Problem: Too high soundcard output volume

Solution: Decrese the output volume with your soundcard sound mixer controlling program.

Problem: Sound volume on other other end is very low and distorted

Problem: Your adapater you are trying to use for this is is not suitable for this kind of application.

Solution: Get a more suitable adapter


Tomi Engdahl <Tomi.Engdahl@iki.fi>