Dual Tone Multi-Frequency (DTMF) Technology and Its Applications

In olden days, our cell phone system was operated by physically within a telephone exchange switch room. The telephone callers can lift the call and helps by providing the instructions to the operator for connecting their end line. This technology offers an ultimate solution to the telephone companies for automatically switching two lines. This technique is very useful in changing the analog to a digital signal with the help of a DTMF decoder, which is frequently used in mobile communications to identify the series of DTMF tones from the usual mobile keypad. This article discusses an overview of DTMF (Dual Tone Multi-Frequency), working, and its applications.

What is Dual Tone Multi-Frequency?

The DTMF (dual tone multi-frequency) is one kind of signaling system intended for identifying a push button on a DTMF keypad. A key can be formed by combining two sine wave tones. These tones are the frequencies of rows as well as column which are represented on the keypad of the DTMF. It activates the long-distance signaling to the entered key through the frequency of voice.

DTMF Keypad
DTMF Keypad

In this keypad, if we push any input key on your handset, then immediately it gives two tones of particular frequencies, the first tone is a high-frequency one, as well as the second tone, is a low-frequency one. The following tabular form demonstrates different signals that you transmit whenever you push your mobile keypad.

Key Row Frequencies Column Frequencies
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 Decoder Circuit using IC M8870

This DTMF decoder circuit recognizes the phone tone from the phone line and then decodes the pressed key on the keypad of the telephone. This circuit can be built with a decoder IC MT8870DE for the recognition of DTMF indications. The decoder IC decodes the DTMF input to five digital outputs. This IC uses a technique of digital counting for deciding the tones frequencies, as well as to confirm that they communicate to normal frequencies of DTMF.

DTMF based Decoder Circuit
DTMF based Decoder Circuit

The tone of the DTMF is a type of one way communication among the user and the telephone exchange switch room. The entire communication includes the touch tone inventor as well as a tone decoder. The bits which are decoded can be associated with a microprocessor or a computer for future application.

The required components of this circuit mainly include M8870 decoder IC, Resistors- 70kΩ, 100kΩ, and 390kΩ, two capacitors- 0.1µF, and crystal oscillator– 3.579545MHz.


Dual Tone Multi-Frequency Decoder Circuit Working

The operation of Dual Tone Multi-Frequency is when a caller produces a call tone that includes two frequencies. It is conveyed through the communication media or telephone line. The telephone exchange switch room uses a DTMF decoder to decode the frequencies of the caller into digital code. The digital codes are the address of the subscriber at the destination. Finally, it will be examined and progressed with a computer which changes caller to the subscriber at the end.

The applications of DTMF keypads almost include in mobile phones and landlines. Therefore this keypad is used for recognizing the dialed number by the caller in the telephone exchange rooms.

The DTMF decoder differentiates the tones of DTMF & generates the binary series equal to a key pushed in a keypad of DTMF. The above circuit can be built with a DTMF decoder IC namely M8870 to decode the keypad tone of a cell phone.

The signals from the DTMF can be tapped straight using microphone pin in the cell phone. The microphone consists of two wires namely red wire and green wire. Here the red wire is an input of the DTMF circuit. The microphone wire signals can be processed with the decoder IC to generate a binary series as a parallel o/p such as Q1, Q2, Q3, & Q4.

The Decoded Output for Row and Column Frequencies

Key Row Frequencies Column Frequencies Q1 Q2 Q3 Q4
1 697 1209 0 0 0 1
2 697 1336 0 0 1 0
3 697 1477 0 0 1 1
4 770 1209 0 1 0 0
5 770 1336 0 1 0 1
6 770 1477 0 1 1 0
7 852 1209 0 1 1 1
8 852 1336 1 0 0 0
9 852 1477 1 0 0 1
0 941 1336 1 0 1 0
* 941 1209 1 0 1 1
# 941 1477 1 1 0 0

The decoder IC M8870 includes an operational amplifier. The signals from the microphone pin are connected to an inverting i/p of an inbuilt operational amplifier in decoder IC through a resistor as well as a capacitor.

The op-amp’s non-inverting terminal is fed to pin-4, and the voltage at pin-4 is Vcc/2. The output of an inbuilt op-amp in the decoder IC is pin-3, the response signal is given by linking the pin3 to inverting input of pin-2 using a 270kΩ resistor.

The operational amplifier is passed through a filter network like pre-filter, low group filter, and high group filters. These filters use switched capacitors dividing DTMF tones into high and low group signals.

Further processing segments in the IC are code detector as well as frequency detector circuits. The frequency which is filtered will be passed via these detectors. Finally, the four-digit output binary code will be latched at the output of M8870 IC.

Advantages of Dual Tone Multi-Frequency

The advantages of DTMF include the following.

  • By using this we can get a quick response
  • It is not expensive to construct.
  • High reliability and fast efficient
  • By using a single key we control six devices.
  • By using this one can control the home appliances wirelessly
  • The power consumption will be reduced and power efficiency will be increased.

Dual Tone Multi-Frequency Applications

The applications of DTMF mainly involve the following.

  • The DTMF is used to identify the dialed numbers in the telephone switching centers
  • These are used to operate the remote transmitters in the terrestrial stations
  • DTMF is applicable in IVR systems, home automation, call centers, security systems, as well as industrial applications

Please refer the following links for DTMF applications

Thus, this is all about Dual Tone Multi-Frequency technology, working, and its applications. From the above information finally, we can conclude that the frequencies of DTMF tones are very beneficial as they permit telephones for indicating which digit is being pushed by the operator. These are very useful as well as less expensive for executing compare with the earlier signaling technique utilized by rotary dialed telephones. What are the disadvantages of DTMF?