How are DTMF Tones Generated? Basic Principle and use of DTMF in Communication

What is DTMF (Dual Tone Multi Frequency)? DTMF is a common communication term for touch tone phones. The tones formed when dialing on the keypad (DTMF digits or DTMF number) on the phone can be used to characterize the digits, and some different tone is used for each digit. DTMF signaling is mainly used in DTMF phones, telephone switching system.

Yet, there is always a possibility that a random sound will be on the similar frequency which will trip up the DTMF sounds system. It was recommended that if two tones were used to represent a digit, the probability of a false signal happening is ruled out, thus the name ‘Dual Tone’. This is the basis of using dual tone in DTMF communication. DTMF dialing uses a keypad with 12 or 16 buttons. Each key pressed on the keypad generates two tones of particular frequencies, so a voice or a random signal cannot mimic DTMF signaling tones. One tone is generated from a High DTMF frequency group of tones and the other from Low DTMF frequency group.

DTMF+Keypad,+Dual+tone+Multi+frequency+working+pribciple How are DTMF Tones Generated? Basic Principle and use of DTMF in Communication
  • When a button is pressed, both the row and column tones are generated by the telephone or touch tone instrument.
  • These two tones will be distinctive and different from tones of other keys. So there is a low and high frequency associated with a button, it is essentially the sum of two waves is transmitted.
  • This elementary principle can be extended to a range of applications.

Row and Column Frequency Correspondence

The frequencies generated on pressing different phone keys are shown in the table

DTNF Frequency
1209 

1336 

1447 

697 

1 

2 

3 

770 

4 

5 

6 

852 

7 

8 

9 

941 

* 

0 

# 

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.

DTMF Frequencies generated on Key press

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

Button 

Low DTMF frequency (Hz) 

High DTMF 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 

1336 

* 

941 

1209 

# 

941 

1477 

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 and processed in a microcontroller to perform different tasks.

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