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basics of dtmf
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                                                      DTMF BASICS

 

DTMF is a generic communication term for touch tone (a Registered Trademark of AT&T). The tones produced when dialing on the keypad on the phone could be used to represent the digits, and a separate tone is used for each digit. However, there is always a chance that a random sound will be on the same frequency which will trip up the system. It was suggested that if two tones were used to represent a digit, the likelihood of a false signal occurring is ruled out. This is the basis of using dual tone in DTMF communication.

DTMF dialing uses a keypad with 12/16 buttons. Each key pressed on the phone generates two tones of specific frequencies, so a voice or a random signal cannot imitate the tones. One tone is generated from a high frequency group of tones and the other from low frequency group.

The frequencies generated on pressing different phone keys are shown in the Table 1.

Each row and column of the keypad corresponds to a certain tone and creates a specific frequency. Each button lies at the intersection of the two tones as shown in Table 2.

When a button is pressed, both the row and column tones are generated by the telephone instrument. These two tones will be unique and different from tones of other keys. So, whenever we say that there is a low and high frequency associated with a button, it is actually the sum of two waves is transmitted. This fundamental principle can be extended to various applications.

 

DTMF signals can be transmitted over a radio to switch on or switch off home appliances, flash lights, motors, cameras, warning systems, irrigation systems and so on. These encoded data can be stored in a microcontroller and can be transmitted serially to another system for processing.

                                 

                                 Table 1 – Frequencies generated on Key presses

 

      Button

 

Low Frequency(Hz)

 

High Frequency(Hz)

 

         1                                                       

           697

           1209

         2

           697

           1336

         3

           697

           1477

         4

           770

           1209

         5

           770

           1336

         6

           770

           1477

         7

           852

           1209

         8

           852

           1336

         9

           852

           1477

         0

           941

           1209

       *

           941

           1336

       #

           941

           1477

 

                             Table 2 – Row and Column Frequency Correspondence

                                

       1

    2

   3

       697

     4

    5

   6

       770

     7

    8

   9

       852

     *

    0

   #

       941

   1209

  1336

  1477

Frequency    (Hz)