By Kiran Vetteth

Have you ever wondered how you recharge/top-up your cellphone currency through a call line?  How is it that when you call a helpline you can seamlessly navigate through the menu by the mere push of buttons on your keypad? How does the called party/server receive all the digits you pressed accurately? When at times the environment around you causes interference to the call line!

Well the concept used here is known as Dual Tone Multi Frequency, abbreviated as DTMF. It was developed by Western Electric and first used by the Bell System, later on DTMF was standardized by ITU-T Recommendation.

Now in every basic telecommunication line, there are 2 channels offered to the subscriber. Apart from the voice channel, which carries audio frequency signals ranging from 300Hz-4000Hz there is also a Dual Tone Multi Frequency Channel having a frequency range of 1906Hz-2574Hz*.

Signaling through the voice channel is done by varying parameters such as pitch, intensity, duration and voice quality during a speech.Signaling through the DTMF channel is interestingly different.

DTMF channel is not used to transmit audio signals but, a mixture of two pure tone (pure sine wave) sounds.

What does that mean?

Now in your mobile/landline keypad, each number (including ‘#’ and ‘*’) is associated with unique frequency, this frequency is the sum of a high frequency and a low frequency component, i.e. mixture of two pure tone, hence Dual Tone Multi Frequency.

To be clearer, when a call channel is established, if you press a key on the keypad, you and the called party hear a distinct sound, this is the dual tone which is associated with that number pressed and is passed through the DTMF channel to the called party. Similarly, all 10 numbers (0-9), including ‘#’ (pound/octothorpe-“hash” or “gate” in UK) and ‘*’ (asterisk or star) has a unique dual tone associated with it.

The 2 individual pure tones which constitute the dual tone for each number is as shown below, wherein the corresponding rows give the low frequency tone and the columns, the high frequency tone.

*A B C D was removed from standard keypads and is now used in military for priority call.

If you press 7, your phone sends a signal having a frequency of 852Hz+1209Hz=2061Hz and the receiver receives the same.

The main advantage of such a concept is that the dual tone frequency is relatively higher than that of the average environmental noise, hence the DTMF channel only transmits the dual tone and not the relatively low noise frequencies. Also the receiving station can be automated. i.e. the receiving device can be designed (using pass filters) to receive and decode the dual tone and programmed to carry out specific functions corresponding to the number pressed. An example of such automated systems are your service providers IVRS (Interactive voice Response Station) which is used in customer helpline.

I hope this was informative and precise. Follow through on this article is welcome.

DTMF concept can be used in microcontrollers to automate a wide range of gadgets and appliances using a cell phone. For further details check out the projects tab. 

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