dialing first digit....

[vijay.k]

hi friends,


When a telephone is made off hook, a dial tone is obtained in tel. line. 8)
If a key is pressed, DTMF tone of that digit is generated in the line (where the dial tone was already present).
Even in the presense of dial tone in the line, DTMF tone gets recognised properly by the exchange.(Then dial tone is stopped by the exchange).
So, the exchange sends the dial tone to the subscriber and at the same time also receives DTMF tone(of first digit) from the same subscriber through a single tel. line.....
how is it possible to detect DTMF tone(of first digit)in the presence of a dial tone in the line?
Or, what is happening internally in a tel. line while dialing the first digit?

Is it possible to stop the dial tone in the subscriber line without on hook?

Can any one, clear my doubt please?


vijay.k

 

[Sebi]

The DTMF frequencies have very low tolerance ( divided from 3,579545Mhz quartz) These special fequencies (alwys two) recognized by DTMF decoder (it have also a same quartz) and not disturbed it the dial tone.

 

[JohnBrown]

Also, some telephone circuits use what I seem to remember is called a duplexor circuit, sort of cancels out the "local" audio signal to concentrate on the remote one.

 

[vijay.k]

problem again....

Thank you sebi, for ur clarifications....
i'm trying to interface a DTMF dialer (UM(91214) with tel. line..
When i send the DTMF tone (of first digit) into tel. line, the existing dial tone gets mixed up with the DTMF tone and the dial tone does'nt stop in the tel. line..
i think that the exchange could not recognise it as address signalling....
So what can i do to achieve dialing numbers?
Please help me in this regard.....

VIJAY.K

 

[vijay.k]

problem again....

Thank u John brown for ur reply...

Can u give the interfacing of full duplexor circuit to sancel out the local audio signal?

VIJAY.K

 

[ljcox]

Telephones contain a hybrid that separates the send and receive directions of transmission. In its most basic form, (there may be electronic hybrids available now days) it is essentially a centre tapped transformer with a balance network whose impedance is approximately equal to the line impedance. It forms a Wheatstone bridge so that the transmitted signal received by the receiver (ie. the earpiece) is minimised. This minimises "sidetone". Sidetone is hearing your own voice in the receiver when speaking.

The hybrid also minimises the transmission of the received signal to the transmitter (ie. the microphone)

Do an internet search for hybrid transformer. I believe that hybrids are also used in radio transceivers. But they may have a different name.

Dial tone in Australia is 425 Hz (modulated by 25 Hz) and is thus lower in frequency than the DTMF dialling signals. At the excanhge, there is a filter that passes the DTMF frequencies to the DTMF receiver but not the Dial Tone.

 

[ljcox]

After my post this morning, I spoke to a friend who is a DTMF expert. He said that your problem is most likely that the voltage output from your DTMF sender is either too high (and thus overloads the DTMF receiver at the exchange) or too low (and is thus below the minimum threshold of the DTMF receiver at the exchange).

The output level (of the combined signals, not each frequency) of the sender should be between -8 and -10 dBm as measured across a 600 Ohm resistor. The level meter will respond to the average level of the combined signals.

If you do not have a level meter, a multimeter will do. The average voltage should be between 250 and 350 mV as measured across a 600 Ohm resistor. An analogue mulitmeter will give you the average voltage and I presume that a digital one will also.

In my previous post, I forgot to answer your question about how to break Dial Tone. The only ways this can be done is by dialling either a DTMF or a Decadic digit.

Len