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SYNOPSIS
Dual-tone Multi-Frequency (DTMF) signaling is the basis for voice communications control and is widely used worldwide in modern telephony to dial numbers and configure switchboards. It is also used in systems such as in voice mail, electronic mail and telephone banking.
The detector part of early DTMF systems consisted of analog implemented bandpass filter banks, which were tuned to the eight standard frequencies. As analog lines as well as many other analog systems were converted to digital, researchers became interested in digital DTMF detectors. Digital implementation has many advantages over analog implementation such as accuracy, stability, re-programmability, and chip count; that is, instead of using several analog chips for detecting multi-channel DTMF tones, only a digital signal processor (DSP) chip is used for all channels.
The DTMF coding is based on two tones used to generate a digit. Two of eight tones can be combined so as to generate sixteen different DTMF digits. The DTMF decoding is based on the discrete Fourier transform (DFT). Using this algorithm, 12 DTMF receiver can be implemented on 4 X 3 frequency Goertzel algorithm function blocks. Then, the output of Goertzel algorithm can be decoded into a digit.
The present project performs twist computations only when the signal has become stable, thus achieving more accuracy and reliability. The DTMF detector receives a set of digital samples of a received signal and calculates energy values for each of the set of different uncorrelated frequencies.
A DTMF signal consists of two superimposed sinusoidal waveforms whose frequencies are chosen from a set of seven standardized frequencies. The frequencies are so chosen to prevent any harmonics being incorrectly detected as a valid signal. The frequency pairs chosen are as shown below.

Figure 1
For example, by pressing the 1 button on the touch-tone telephone key pad , a signal consisting of a 697 Hz sinusoid and a 1209 Hz sinusoid is generated. A DTMF detector attempts to detect these frequencies in the presence of noise, and determines which button is pressed. This detection is done using the Goertzel Algorithm. The difficulty of DTMF tone detection is due to the standards which must be satisfied when these signals are detected.

Here we use a standard telephone instrument; the line-in of this instrument is tapped and given to the interface circuit which acts as an interface between telephone line and DSK6416 kit.
Here DSK6416 kit is used as a decoder which decodes the DTMF signal and displays the value of the corresponding key pressed on the LED.