"Andy Evans" wrote in message
...
I want high quality sound from the telephone for making long distance
coaching calls. So I want to use a binaural headset and microphone.

By "binaural" I hope you just mean a headset with two earpieces, you won't
get stereo!

I
could get a seperate headset like the Plantronics ones, but it seems
to me that I could just link the telephone to my Mac computer and
listen and record (I'd like to record too) etc from the computer,
using the headset and microphone I use for Skype already.

Do I need an interface, or is the line in and line out to the
telephone just normal 2 volts type line level? In other words I could
make a DIY interconnect with just phono plugs for the computer line
in/
line out sockets?

From an audio point of view the telephone line just carries baseband audio
of around minus ten to zero dBU, nominally 600ohm. Though in practice modern
twisted-pair telephone cable is closer to 100 ohm the terminating impedance
of telephones doesn't take account of this, with analogue telephones you can
get away with such mismatching!

However the telephone line also has DC on it. This is sourced from a -50V
supply at the exchange, and is current limited to around 40mA by the
line-feed at the exchange. In order to seize and hold the connection you
need to allow at least 25mA to flow. This can be achieved either with a
specially designed audio transformer, one winding across the line, the other
providing isolated audio, or you can use a transistor connected as a
constant-current sink, which is what you will find inside a modern
telephone. In that case a polarity-protection bridge is required as the
polarity of the line may change. You will still need some sort of audio
balun as the telephone line is balanced, and the audio ports of your Mac
aren't. Finally if you want two-way communication you require a hybrid,
which is a device for minimising the amount of sidetone (your own voice
coming back to you via your headset). This was traditionally done with
transformers, but is now more likely to be done with op-amps.

Oh, and when the line is ringing there will be around 75V rms at 25Hz
superimposed on the DC. The isolation of your "hook-switch" needs to be able
to cope with that.

All help welcome for this. I can solder connections and am handy with
sound equipment.

If you feel confident about it you can modify an old (but electronic)
telephone. Disconnect the microphone and earpiece and connect the wires that
used to go to them to your computer via a couple of cheap 1:1 audio
transformers. Otherwise suitable interface units are available from the
likes of Canford audio.

David.