Interface from telephone to computer - options?
 

I want high quality sound from the telephone for making long distance
coaching calls. So I want to use a binaural headset and microphone. 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?

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

andy

Interface from telephone to computer - options?
 

"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.

Interface from telephone to computer - options?
 

On Nov 20, 10:35 am, "David Looser"
wrote:
"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.

Hi David,

Maplin has this - any use?

http://www.maplin.co.uk/Module.aspx?ModuleNo=30352

Interface from telephone to computer - options?
 

"Andy Evans" wrote

Hi David,

Maplin has this - any use?

http://www.maplin.co.uk/Module.aspx?ModuleNo=30352

Yes it should be. I see it plugs into the handset socket on a phone, rather
than directly into a telephone line, so it looks like my "couple of cheap
audio transformers" but made up in a box with leads and plugs. At the price
it's not worth bothering to make your own.

David.

Interface from telephone to computer - options?
 

"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. 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?

Telephone lines are special. They are typically balanced, ungrounded, and
still called on to pass ringing signals that are like 50-90 volts AC with
significant current behind them.

In other words I could make a DIY interconnect with just
phono plugs for the computer line in/
line out sockets?

Look up telephone interface schematics on the web. You'll typically find a
matching/isolating/balancing transformer and some kind of peak voltage
limiter.

http://www.epanorama.net/circuits/teleinterface.html

Interface from telephone to computer - options?
 

"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. 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?

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

andy

I use Skype for our radio-station phone-ins as it sounds a lot better than
using a hybrid on the analogue system.
I have a good quality sound card and just use the audio line-level in and
out of the Left channel and ignore the other. This of course means that the
telephone call is in mono, but there's no stereo telephony yet.

Skype works very well for speech, but is useless for passing music, really
horrible. It also uses a low sample rate (haven't found out what yet) so
needs a sound card that will handle more than just 44.1 and 48k.

As to recording the telephone conversation, I use an external logger, but
there are ways I'm told, of recording Skype.

S.

Interface from telephone to computer - options?
 

In article , Serge Auckland
wrote:

"Andy Evans" wrote in message

As to recording the telephone conversation, I use an external logger,
but there are ways I'm told, of recording Skype.

I don't know much about this specific area, and what I little do tends to
be for RO/Linux not Mac. But I presume that a Mac can run software similar
to PulseAudio or a stream daemon/server, or something like a 'net radio'
transmission. Indeed, since Mac is now BSD based IIUC you may find some
open software for this.

Slainte,

Jim

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