Dual Tone Multiple Frequency DTMF Decoding and applications

(Last Updated On: March 20, 2021)

Dual Tone Multiple Frequency DTMF:

Dual tone multi frequency which is also known as DTMF. In my previous article ”DTMF Dual tone Multiple Frequency, MT8870E DTMF Decoder” I explained the practical use of the DTMF technology and I used the MT8870E DTMF Decoder with and with the Arduino for controlling electrical devices. I explained the maximum basic things, but I mainly focused on the hardware side. So, that’s why I decided to write another article to explain the theory.  Dual tone multi frequency is part of digital signal processing and we are going to derive or device a mechanism or our new device which is used in digital telephony so the topic for today in the application of Digital telephony is the DTMF using Geortzals’s algorithm. So I will be using Dual tone multi frequency or we will be using Geortzals’s algorithm to detect DTMF signals in digital telephony. So let us first understand what do you mean by DTMF for DTMF stands for dual tone multiple frequencies.


Multiple frequency mechanism we will be using and why we will use that also we will see very shortly but will be using words s algorithm to detect this multi tone order tone multiple frequencies. Now in digital telephone or in any telephone probably you have a dialing or you have a dialer.

Dual tone multi frequency as we have already stated is dual-tone multi-frequency is a digit transport method for telecommunications and it uses a unique combination of two different tones which is why it’s called dual-tone multi-frequency part of why it’s called multi frequency as well is because over here we have the low group which is known as the low frequencies and over here we have the high group which has the high frequencies. When the high frequency 1209 is combined with 697 that represents the number one when 1209 is combined with 770 hertz. It represents the digit numbers.

Before Dual tone multi frequency was a thing we had old-school rotary phones these old-school rotary phones had a an electrical current that would be opened and closed.

Dual Tone Multiple Frequency

So when you spun this wheel around on the rotary phone it would open and close the electrical circuit. So if you wanted to dial a nine you would turn the wheel all the way to the nine and then as the wheel would spin it would open and close the electrical current nine times letting the other side know that a nine was done. There were some problems with that and  Dual tone multi frequency came along to kind of help with alleviating the problems that would occur with the old-school method of doing it.

in the previous era or during the very early stages of the technology of telephone we know that there was a mechanical dial which when we will we have to put our finger into a number and then rotate it the number of rotation that goes the numbers are detected accordingly so that was a completely mechanical process. Now using the electronics switching mechanism we have developed our electronically switching mechanism or electronic switches. So how does this electronic switches understand when we press a switch it is number one or when we press a switch it is number two for example when i dial zero to two how does electronically we should understand that I have pressed zero to two. So this dual tone multiple frequency is being used in the dialer mechanism to understand how to analyze the numerical values. Sometimes we also gave a Liberty you use no numerical values also but you use some special keys also for example we use some special key called star or we use hash. So this type of special keys are also there on our keypad. So how does the keypad or how does the telephone understands at which button has been pressed on which number has been pressed on the keypad. So we have developed of a mechanism or an intelligent system which we call as do to a multiple frequency in which we have this arrangement. Now you can see there are some vertical green lines and horizontal red lines and next to each line I have written a frequency. So what we did is based on the numbers that we are going to put or based on the numbers that was been put on the telephone normally was 0 to 9 and this 2 special keys so we have 1 2 3 4 5 6 7 8 9 10 11 12. So 12 total combination of or toil total keys were there with me and we have decided that they will set up different frequency for particular key for example if I press number one, one frequency will be generated if I press a number two another frequency will be generated but what we have used in the normal keypad is in generating one frequency. We develop a call of a matrix of frequency in which there are some columns and there are some rows for the roles we have low frequencies for the columns we have high frequency. So whenever a quick key is being pressed that time we will get a low frequency generated along with a high frequency generated. So if suppose I pressed zero to two number so I associated with zero. Now with respect to zero associated with zero the frequency that will be getting generated in the row that is a low frequency row is nine four one Hertz whereas the column frequency is 1336 Hertz.

Dual Tone Multiple Frequency



So whenever I generate or whenever I press a number to tone frequency or two tones are generated or multiple frequencies are generated one frequency will be higher and one frequency will be lower. So when I press zero for example we will press zero when I press zero that time I am going to get frequencies as 941 Hertz and 1336 Hertz. When I press to the temp which two frequencies 697 hertz and the column over here is again 1336 hertz and again. When I press two it is again 697 Hertz and 1336 Hertz so in a given interval when you press ok for a given span of time a continuous tone is generated for a given period of time and that continuous tone have two frequencies one is a lower frequency and another is higher frequency at the detector. My job is to pull out this lower frequency component and higher frequency component correctly so that I can understand that what number was so because we are using two frequencies it is called as dual tone because tone is associated because these are two sine waves. So that’s why we are saying it has dual tone multiple frequency this is a meaning of Dual tone multi frequency. Now let us understand how this algorithm help us in understanding these frequencies. So what we basically do is in this algorithm what I am going to do is we are getting a linear PCM input basically the wise signals are pulse code modulation. When they are transmitted over the line we speak analog Li or we speak analog signal that speech signals are in the range of 0 to roughly around 4 kilohertz but actual number is 3.4 kilohertz but we are taking up to 4 kilo Hertz and then we are sampling it at a rate of 8 kilo Hertz that is to satisfy the Nyquist criteria 1 3 sample it at 8 kilohertz then we send it to a coder and then coder will assign a bits per sample. So the approximate rate comes to be 64 kbps for 1 voice Channel. so what happens is whenever I press a tone okay with this two frequencies what is going to happen the highest frequency will be considered and then it will get it will be getting into and then we are going to sample it by the highest frequency is it that we cannot sample it by 8 kilo Hertz okay so what is happening is now I am getting number tones.


Dual tone multi frequency DTMF Decoding:

Now as we know that DTMF stands for dual-tone multi-frequency and those are the tones you hear when you make a phone call on your touch-tone phone. Now this little module uses the MT 8870 DTMF decoder chip which you can see here it’s made by Mitel which is a canadian telecom Company.

Dual Tone Multiple Frequency

You can see the header pins if we start at the very right there’s VCC that’s +5 volts and then we have ground then we have input that’s where we input our Dual tone multi frequency audio tones then we have two steering pins one is active low one is active high and they go active when there’s a valid DTMF tones present then we have the four data lines q4, q3, q2 and q1. So it puts a binary a 4-bit binary data output corresponding to each button on the keypad.

Dual Tone Multiple Frequency

So we are using a 12 button keypad which has rows and columns which has the rows of the low group frequencies and the columns of the high group frequencies and when you press a button will have a high group in a low group tone being present and then it will output a Dual tone multi frequency binary output from Q 1 to Q 4.

So we will  power up this module and We will have a look at its operation  I have my DTMF decoder powered up and It is connected to a Dual tone multi frequency DTMF encoder but you can see here it’s a 12 button encoder. Now you could get 16 button encoders which have ABCD buttons and I actually have a D button wired in on the side of the encoder so when I press a button on the keypad. I will get an indication on my Dual tone multi frequency decoder. So we have 0 to 9 and we have a star or asterisks and we have an octave for where sometimes called a number sign pound or hash tag so every one of these buttons has a binary code now when I press any of the buttons the very left LED is our our steering LED which is our data valid LED which means there’s a valid DTMF tones being entered. So if I press a button and hold it you can see the very left LED comes on as long as I’m holding the button indicating a bell at DTMF tone and the rest of the LEDs the four will give our binary output.


Applications of DTMF Dual tone multi frequency:

There is so many application of Dual tone multi frequency DTMF technology home automation system, cell phone controlled vehicles, auto emergency call used for security purposes tools used and advantages of Dual tone multi frequency fft fast flurry a transform is the mathematical tool that can be used to calculate the frequency component in the given signal. Dtmf technology was first introduced in a telephone system in 1963 but nowadays it is used in different field of life it reduces the waiting time response time and increases efficiency. The dialer is now connected with the receiver without the involvement of the third-party person telephone exchange reducing the theft rate burglary and used dtmf in security places military banks etc also use this technology in home automation systems further improvement is obviously required automatic calling to security institute by watching burglary or guns or any inappropriate scene in camera using artificial intelligence develop a smartphone application and interface the robotic vehicles to control it with the phone self-driving vehicles contains dtmf decoder to performs different tasks.

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About the Author: Engr Fahad

My name is Shahzada Fahad and I am an Electrical Engineer. I have been doing Job in UAE as a site engineer in an Electrical Construction Company. Currently, I am running my own YouTube channel "Electronic Clinic", and managing this Website. My Hobbies are * Watching Movies * Music * Martial Arts * Photography * Travelling * Make Sketches and so on...

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