audio interface for telephone

[pooh2006]

hi all,

this is my first post here and i hope that i found the right place, i want to design a simple cheap interface between the computer's soundcard and the normal telephone through the rj-11 connection, i have googled alot for about a week, and i found alot of doc(s) talking about this issue like home-made chat cords and the modem's hybrid circuits for audio separation (coz of the ordinary duplex 2w system) or phone-recording circuits, but my conclusion that the pure separation for voice should be achieved using 600:600 audio transformer with secondary coil c.t and also it's hard to get a balanced circuit with it, so in my design i may not require this separation (if the clear-separation will be hard & also the transformers must be used !!) and i hope to make the interface for both spkr & mic without using any isolation transformers neither normal nor c.t by using any equivalent methods (i dunno, plz help ??) even if the quality will be less, and about the loop current i red that using normal 9v battery will be bad and providing it (about 20-30 mA) with current-source circuit will be better (is that true ??), the last thing is about dc-blocking capacitors and filters to be inserted before the sound card's jacks for safety and removal of noise (what values of capacitors & resistances should i use ??), thanks.

ibrahim shahin,
computer and electrical engineering - ASU

 

[audioguru]

All telephone external audio circuits use a transformer because it has perfect balance and isolation.

The transformer or electronic circuit must be a "hybrid" type if you want to transmit with a microphone at the same time you receive with an amplified speaker. The Hybrid's impedance match must be adjusted to match the impedance of the telephone line and the gains must be kept very low to avoid acoustical feedback howling.
Usually a speakerphone uses gain switching or added DSP echo cancellation. Then the volume can be louder.

 

[Leftyretro]

Well you state that you want to interface to a sound card but you don't state if you want to be able to both input and output to and from the sound card to the phone line. If you need two way functions then you will need a hybrid circuit to isolate talk from listen. Also while a capacitor will block the DC component you still have to be able to isolate your interface circuit from the sound card when you are in a 'off-hook' condition or a 90V 20hz AC ringing voltage will be sent to your sound card at some time and most likely damage it. Designing and building such an interface is not as simple as you might think. It would probably be better to modify some existing telephone type circuit that already interfaces with a separate earphone and microphone, then all you would have to deal with is matching the level requirements of the sound card in and out levels. I bet E-bay would cough up cheap devices that you could adapt to.

Lefty

 

[ljcox]

I posted a circuit for this some time ago. I made it for my daughter who wanted to record phone conferences.

I used the case and PCB of an old answering machine. I removed a few parts from around the input transformer and added the necessary components so as to use the transformer.

See https://www.electro-tech-online.com...-to-line-level.26988/?highlight=phone+monitor

 

[pooh2006]

some comments

thanks all

All telephone external audio circuits use a transformer because it has perfect balance and isolation.

but there is no any equivalent ??

The transformer or electronic circuit must be a "hybrid" type if you want to transmit with a microphone at the same time you receive with an amplified speaker.

no, i think that if i send both of the voices (in\out) to the mic with a 0% separation so it'll not be a hybrid circuit, and i attached the circuit that does that for me when tested but with medium quality, but i don't know how did the author get this schematic which even not the resistive hybrid bridge

**broken link removed**

Usually a speakerphone uses gain switching or added DSP echo cancellation. Then the volume can be louder.

what does dsp cancellation refer to ??

you don't state if you want to be able to both input and output to and from the sound card to the phone line. If you need two way functions then you will need a hybrid circuit to isolate talk from listen.

i mean that i can playback the sound through the speaker and i may have the sound received from the mic where it is mixed with the sound added from the telephone, so i can separate it on the computer using a suitable software (i will record both of the spkr & mic then do a subtraction), coz i need't a complex circuit for this separation using the hardware (so i can eliminate the usage of transformers also).

Also while a capacitor will block the DC component you still have to be able to isolate your interface circuit from the sound card when you are in a 'off-hook' condition or a 90V 20hz AC ringing voltage will be sent to your sound card at some time and most likely damage it.

i will connect the phone to the computer only !!, aka a pure voip connection.

Designing and building such an interface is not as simple as you might think. It would probably be better to modify some existing telephone type circuit that already interfaces with a separate earphone and microphone.

Lefty

can u recommend any simple one to me ??

 

[ljcox]

i mean that i can playback the sound through the speaker and i may have the sound received from the mic where it is mixed with the sound added from the telephone, so i can separate it on the computer using a suitable software (i will record both of the spkr & mic then do a subtraction), coz i need't a complex circuit for this separation using the hardware (so i can eliminate the usage of transformers also).

NO!!!

The transformer is essential for 2 reasons:-

1. For electrical safety. This is a legal requirement in Australia and probably most other countries as it prevents the possibility of a linesman being electrocuted due to a computer fault applying the mains voltage to the line. The transformer must have adequate insulation between primary and secondary. That is why I used one from the telephone answering machine.

2. There would be noise induced - particularly mains hum. The phone line is balanced in order to minimise induced common mode noise.

The computer input is unbalanced, ie. one side is connected to the computer ground.

 

[pooh2006]

NO!!!

The transformer is essential for 2 reasons:-

1. For electrical safety. This is a legal requirement in Australia and probably most other countries as it prevents the possibility of a linesman being electrocuted due to a computer fault applying the mains voltage to the line. The transformer must have adequate insulation between primary and secondary. That is why I used one from the telephone answering machine.

2. There would be noise induced - particularly mains hum. The phone line is balanced in order to minimise induced common mode noise.

The computer input is unbalanced, ie. one side is connected to the computer ground.

so what is the simplest circuit that i can use (with only one normal 600:600 transformer) to make an interface to the computer's spkr & mic jacks ???


A Question For All,
if i found the c.t transformer that i hardly searched for it, is the basic circuit that combine only a single resistance (100->400 ohm) with the transformer from its c.t sec. coil to the ground will be enough for a well isolated hybrid circuit after adding dc-blocking capacitors, or i should use a modified version ???

 

[mvs sarma]

Hai Pooh2006,
Can you try the attached schematic and see
It did not indicate the power supply connections perhaps. device is like LM358 (dual amp). this element is generally used in Modems.

 

[pooh2006]

Hai Pooh2006,
Can you try the attached schematic and see
It did not indicate the power supply connections perhaps. device is like LM358 (dual amp). this element is generally used in Modems.

i found this circuit before on **broken link removed** through google, and i think it have somethin' wrong coz after analyzing the op-amp subtraction circuit i found that it subtracts the hole mixed signal from the transmitter signal (or may be i'm wrong !), and also i'll require a dual op-amp ic beside the transformer in circuit which is not good for my desired circuit, anyway thanks.

 

[stevez]

You could do some research on "phone patch" - equipment used to connect radio equipment to telephone lines. While probably obsolete the information might help with your situation.

 

[audioguru]

A hybrid is required to subtract the received signal from the transmitted signal:
1) So that the distant end doesn't hear his own voice loud and delayed.
2) In case the distant end has the same circuit which would cause acoustical feedback howling.

 

[pooh2006]

A hybrid is required to subtract the received signal from the transmitted signal

but in my circuit i'll not require this subtraction niether using c.t transformer nor op-amp with normal transformer, i'll do the subtraction on the computer using a software (after recording both of spkr & mic signals as i mentioned before) so my requirment is to design a simple interface that send the sound without amplification from the pc to the phone line through spkr jack and receive the mixed signal (containing the phone user's speech) on the pc from mic jack, without any subtraction on the hardware level, and the circuit that i had posted above do that already for me but i'm asking about better & more standard circuits for that, i hope that audioguru can now understand my issue & can provide me with his experience in designing the circuit scheme, and about using an audio transformer for safety i could use a normal one but not for creating a hybrid (only a 2-way audio interface).

 

[audioguru]

A properly adjusted hybrid or DSP acoustical echo canceller allows full duplex loud speech on a 2-wire telephone line. Or gain switching must be used (one side is muted when the other side is louder).

 

[pooh2006]

A properly adjusted hybrid or DSP acoustical echo canceller allows full duplex loud speech on a 2-wire telephone line. Or gain switching must be used (one side is muted when the other side is louder).

i think that you didn't understand me in the last reply !!, i'm asking about handling the duplex (mixed signal) through the mic without any separation !!, any help in this direction ??

 

[audioguru]

I don't know what you want.
A microphone on a table in a room and an amplified lodspeaker will have acoustical feedback howling when connected to a phone line without a properly adjusted hybrid circuit. Even speakerphones have a hybrid but need to also have a DSP echo canceller or use gain switching to avoid howling.

The microphone in a room picks up the sound and echoes of the distant voice. You don't want to send those sounds to the other end.

 

[pooh2006]

I don't know what you want.
A microphone on a table in a room and an amplified lodspeaker will have acoustical feedback howling when connected to a phone line.

The microphone in a room picks up the sound and echoes of the distant voice. You don't want to send those sounds to the other end.

i think that u totally misunderstood the situation, this is the explanation diagram for what i hope to develop, and sorry for annoying you if my explanation was not obvious enough.

**broken link removed**

 

[audioguru]

Since you are not using the public telephone system and are not using a speakerphone then why don't you re-wire the phone so it has one pair of wires to transmit and a second pair of wires to receive?

 

[pooh2006]

Since you are not using the public telephone system and are not using a speakerphone then why don't you re-wire the phone so it has one pair of wires to transmit and a second pair of wires to receive?

thx, what i need to do is creating a universal circuit that simulate the re-wiring so it could be used with any telephone (with rj-11 connection), not only one telephone !, then what will be the reasonable scheme for this circuit ??.

 

[audioguru]

A telephone has separate wiring internally for transmit and receive then uses a hybrid circuit to combine them into a single pair of wires.
Your circuit must do the opposite. It must use a hybrid circuit to convert the single pair into separate transmit and receive for your sound card. It also must power the phone.

 

[pooh2006]

Your circuit must do the opposite. It must use a hybrid circuit to convert the single pair into separate transmit and receive for your sound card. It also must power the phone.

but somethin' like this circuit (it's not my design) can be called a hybrid circuit (as it simulate the transformer ??), because i tested it and i have got a good quality (but for sure the mic handle a mixed signal) with a loop current for about 15mA (the resistance of phone == 500ohm).
**broken link removed**