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 systems.
Yet, there is always a possibility that a random sound will be on a 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 a Low DTMF frequency group.
Basic Principle | How Are DTMF Tones Generated
- 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
| Frequency | 1209 Hz | 1336 Hz | 1477 Hz |
| 697 Hz | 1 | 2 | 3 |
| 770 Hz | 4 | 5 | 6 |
| 852 Hz | 7 | 8 | 9 |
| 941 Hz | * | 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 |
Conclusion
DTMF signals can be transmitted over the radio to switch ON or switch OFF home appliances, flashlights, motors, cameras, warning systems, irrigation systems, and so on. So, knowing how are DTMF tones generated can surely be an interesting bit of info. These encoded data can be stored and processed in a microcontroller to perform different tasks.
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