connecting telephone wire to USB and 3.5mm

[jimkess]

I want to connect and route telephone wire either to USB or 3.5mm jack by switching using microcontroller when call comes in. How can I do that?

 

[FvM]

"Connecting" telephone wire to USB or 3.5 mm jack is not suitable, and not compatible with safety rules. To "route" the telephone voice signal to low voltage systems, galvanic isolation (e.g. by audio transformer) is mandatory. Transmitting voice signals over USB requires an audio codec.

 

[jimkess]

there must some IC that can do these? any idea?

 

[betwixt]

As FvM said, you need a USB audio codec, there are many to choose from. However, they do not just connect to the phone line, you still need an isolating transformer and some isolated loading circuit to seize the line. The phone lines can have over 100V on them, more than enough to permanently damage anything directly connected and you also risk pulling the phone line voltage down to ground and triggering a fault detector back at the Telco.

Brian.

 

[FvM]

A specification would be helpful.

voice input? output? ring detection? loop current/off-hook detection? pulse or tone dialing? clip?

 

[jimkess]

what is the typical voltages and current type and magnitude in a typical phone microphone and speaker? that is the specification all i can say i have. The bridge, protection unit and the speech circuit are in place so that there is no worry about the high voltage from the line to damage the electronics circuit. So at the end we have audio signal from the line and microphone input to be fed into the line. From my guess, this is signal is good to be fed to the 3.5mm headset or via usb to headset.

I searched USB implementation with microcontroller. I think this https://www.obdev.at/products/vusb/index.html is useful. Any project link of implementing audio transfer from speech n/w to the headset via USB using microcontroller would be what I am looking for.

 

[betwixt]

It isn't only the high voltages you have to worry about, a normal telephone has no exposed conductive parts so it can safely be used while the voltages inside are quite high above ground potential. If you connect something to the microphone or speaker that provides a conductive path to ground you WILL have problems. Both wires on a telephone line are 'live', although one will have more voltage than the other when measured to ground. Under certain circumstances the voltage on the wires may be reversed at the Telco end and both may carry in excess of 100V when the ringing signal is applied. Essentially, a normal phone 'floats' between the two line wires and if you add path to another voltage or ground, you upset the balance between them. Connecting to USB or any other interface that itself has ground connection will do just that so your only option is an isolation transformer.

Typical voltages from a telephone microphone will be less than 100mV and the voltage to the speaker will be less than about 500mV. As I warn though, those are the voltages across the microphone and speaker pins but they may both be at high voltage with respect to ground.#

Brian.

 

[jimkess]

thanks brian, i have noted your concerns about the analog part.

do you know about some microcontroller to usb implementation(codec)? what micro would you consider for my application? a micro with telephone interface and USB, 3.5mm would be great.

 

[betwixt]

My personal choice would be a PIC device but thats because I use a lot of them and I know their hardware and firmware quite well, you could use almost any micro with enough pins and storage.

Nobody makes a micro with a USB codec, telephone interface and audio amplifier built in, that would be a silly waste of resources.

Before going further, you need a specification to work to. For example:

1. does your gadget have to make outgoing calls and if so, for what line characteristics and what dialing method, these vary from country to country.
2. What should it do when an incoming call arrives, should it answer automatically?
3. What impedances and line levels are needed at yout 3.5mm socket?
4. What speed does the USB interface run at and is it a client or host?
5. What is the USB interface actually going to carry?
6. How is it to be powered, considering the safety problems and legal requirements of your telco?


Brian.

 

[jimkess]

1, 2 are not required. impedance and line level i have to check.

about usb, the usb should be connected to computer and computer sent music to my headset. another role of usb is to connect telephone with headset(which has call accept/deny, mute buttons). the speed is what I don't understand, what should be the usb speed for listening music and audio. isn't there a general usb speed for everything. i know there are classes of usb but which one to use? what is requirement for 3.5mm jack for audio and music? like DAC resolution when the audio voice is to be sent from headset mic to the tel line?

 

[FvM]

Presume that standard full speed USB (12 Mbps) will work well for audio. Low speed (1.5 Mpbs) as supported by the VUSB project mentioned in post #6 will most likely not provide sufficient throughput.

 

[jimkess]

i noticed that about vusb, that, it has some speed limit. FTDI is what i am thinking of but it is complicated it seems, but would go for it. any other solution like using vusb? what kind of usb class is used for connecting headphone to computer via USB? i think it is called CDC.