Hi Shahzad,

I had written this description actually 2 days back, but couldn't post it.

I still feel like posting it, so here it is as below:


Man you are moving way ahead of what you need to do before asking too much.

Please visit <http://www.empanorama.net> and click on Telephone in the index.

You will find loads of circuits and application notes there.

Please note following:


A BASIC TELEPHONE:


A basic telephone set is connected to the telephone exchange thru a line. The exchange keeps
the line at around 60 volts when the hand set is kept on the cradle (ON HOOK CONDITION).

When you pick up the handset (OFF HOOK) then the line voltage drops to around 7 volts, and
sensing this voltage drop the exchange puts a tone on the line which you can hear.

When the phone is ON hook the line is open ended. When the phone is OFF hook the line is
connected to a trhansformer which is 1:1 on both sides and is of 600 ohms impedence.

This transformer drops the line voltage, and on dial tone you can transmit DTMF for the
excahnge to connect you to heavens.......

BASIC AUDIO INTERFACE WITH LINE:

You don't need a high tech interface basically just to put only the "audio" on line.
(high tech interface like with TEA..... chips is required where data corruption is feared due to
echo and other reasons, and so modem chips are used there ....)


So this line transforemer which drops the line voltage, can also transmit / receive audio.


To only transmit DTMF you connect one leg of transformer (circuit side ) to ground and the other
leg to the DTMF output from a dialer chip and that's all.

If you also have to send asome other audio like from ISD chip then you mix the ISD chip output
with the DTMF with a suitable audio mixer circuit and connect the common audio output to the
transformer.


DTMF GENERATION:


You seem to be not yet familiar with any real DTMF tranmitter, receiver or transceiver.

Go to <http://www.mitel.com> now called Zarlink.

See the dta sheets of MT8870 (receiver) and MT8880CE (transceiver)

You only initilaize the DTMF chip with micro and then whatever binary number (0000 to 1111)
YOU GIVE TO THE CHIP IT WILL CONVERT THAT INTO THE CORRESPONDING DTMF.

These details are all given in the data sheets. If you have gone thru one yet, you would know.


Untill you have gathered sufficient info from data sheets.

Regards

Babar