Telephone call recording

Hi, Jeff

Thank for you prompt reply and you given solution.

But i just want to do some think like below: 

Because the way out of telephone has 4 signals line like recommended above and i can not make the connetion from my board's Ground to the Ground of telephone so when i use LM386 to make my board I got acoustic noise. I am trying to find solution to filter the noise.

Hi Minh,

The phone signals according to your diagram are differential meaning a ground connection should not be necessary. Are you finding the noise when measuring the signal differentially ((Mic+) - (Mic-))?

Also can you share what kind of noise you are seeing?

Thank you,
Jeff McPherson

Hi, Jeff

Actually i want to convert differential input signal to single-ended signal something like below:

But when i do it with LM386 i think i got gruond loop. i got echo when i make a phone call and talking. I found this one

https://e2e.ti.com/support/audio-group/audio/f/audio-forum/1203785/opa1633-differential-input-to-single-end-output/4548170#4548170

but i do not understand do i have to make a connection between phone's ground and my board's ground? Could you give me some OP_AMP that can solve my problem?

Thank you.

Hi Minh, 

Could you provide some more information on what the input signals are going into the op amp (voltage range, freq)? If you have an oscilloscope image, that would be helpful as well. What is the load directly connected to the output? 

Did you connect the feedback loop for the op amp? Based on the schematic, there is no feedback path, but it is difficult to tell what part you are using as well. You mention you are wanting 

Minh Tran35 said:

but i do not understand do i have to make a connection between phone's ground and my board's ground?

Could you clarify how the board layout for each of these components are and each section is connected to each other? 

Thank you!
Best Regards,
Ashley

Hi Ashley,

My purpose is record the telephone call. So I have to make a board then extracting the telephone call analog signal to the PC to record the telephone call. the telephone signal is 5V and my board uses 5V also. I want to make something like this.

As i know the telephone output is differential output and there is no ground at the output of the telephone. So i am trying to find OP-AMP can convert telephone output differential signal to single-ended signal  to connect to PC without using telephone's ground. Or  telephone output differential signal to  differential signal to connect to PC without using telephone's ground.

Thank you.

Hello Minh,

I suggest using audio isolation transformer(s) because the computer ground shouldn't be connected to the phone line. The headset connector should not have its own isolation from main phone line. Mostly because it doesn't need to do that. The speaker and mic are floating transducers. The speaker line may have enough mic signal that you don't need the mic wires. So one transformer might be enough.      

Hi Michallick,

Thank for you suggestion but i dont understand clearly you mean. Do you mean something like this :

Ron Michallick said:

The speaker line may have enough mic signal that you don't need the mic wires. So one transformer might be enough.   

Could you a little bit more clearly explain for me, please?

Could you give me some audio isolation transformer for solution?

Thank you!

Mihn,

As I remember from my telephone days, the mic signal is coupled to the ear piece so you can hear your voice to prevent people from talking too loud.

For audio transformer, you want to match impedances with the speaker and mic. Also add add a series capacitor to block any DC current from DC voltage. I don't have any specific transformers in mind.  

If you plan to make more than a few boards, then transformers will be very expensive. You may be able to do this with capacitors in series with all four wires. This will block both common mode and difference DC voltages. It may or may not solve your coupled noise problem. It will be much less expensive.

Hi Ron,

My purpose is recording seprately the voice from speaker and from mic. But i got mix voice. How can i seprate mic signal and speaker signal? 

Thank you!

Hi Minh, 

Could you clarify what you mean by "mix voice"? Based on your diagram it looks like you have the speaker and mic signals separated. Is it an issue with the audio signals coupling? 

Regards,
Ashley

Mihn,

Some reduction could be implemented, but it would be difficult to to get a lot of separation.

The mic signal will need to be added or subtracted (same thing) with just right amount of gain and phase from/to the mixed signal.

It's a challenge that I wouldn't accept. Your own business should have experts in telephony that are far more knowledgeable than I am. 

Hi Ron,

Many Thank for your replying.

Hi Ashley,

As Ron said: 

Ron Michallick said:

the mic signal is coupled to the ear piece so you can hear your voice to prevent people from talking too loud.

But i want to record Mic and Speaker seprately. I try to find solution to do this.