By the end of this activity, participants will be able to:

Describe the principle of DTMF signaling.

Map digits to their corresponding frequency pairs.

Write MATLAB code to generate composite tones.

Visualize DTMF signals in time and frequency domains.

Connect DSP concepts with practical communication systems.

DTMF is a core signaling technique in telecommunication systems, particularly in interactive voice response (IVR) and digital dialing. Each keypad digit corresponds to a pair of frequencies—one from a low-frequency group and one from a high-frequency group. By combining them, information can be transmitted reliably over telephone lines.

This experiment bridges theoretical signal processing with real-world applications in telephony and embedded systems.

Activity Description (for students)

In telecommunication systems, each key on a telephone keypad corresponds to a unique pair of sinusoidal frequencies. This technique is known as Dual Tone Multi-Frequency (DTMF) signaling and is used for dialing numbers and controlling interactive systems.

In this activity, you will:

Write MATLAB code to generate a DTMF tone for any chosen digit (0–9, *, #).

Visualize the waveform of the generated tone.

Perform a spectral analysis (FFT) to verify the frequency components.

Extend your code to produce tones for a sequence of digits, simulating a phone number.

By completing this experiment, you will gain hands-on experience in sinusoidal signal generation, superposition, and frequency analysis — key concepts in Digital Signal Processing.

eaching Method (Instructor Guide)

Introduction (20 mins) – Brief lecture on DTMF, keypad frequency mapping, and applications.

Implementation (60 mins) – Students use the provided MATLAB template, filling in the missing logic and extending it for sequences.

Reflection (20 mins) – Group discussion on results, challenges, and possible extensions (e.g., detection).

Encourage students to test multiple digits.

Have them listen to the tones with sound() for real-world connection.

Stress why these exact frequencies were chosen (non-overlapping, easily detectable).

Completed MATLAB code (functional, commented).

Plots of waveform and FFT for at least two digits.

Short written Discussion (~200 words) describing:

How DTMF works.

What their plots show.

One practical application of DTMF.

Evaluation Criteria:

Code correctness and execution (40%).

Accuracy of plots and analysis (30%).

Quality of written explanation (20%).

Oral responses if viva is included (10%).

Web Resources

MathWorks Documentation – FFT (Fast Fourier Transform)
https://www.mathworks.com/help/matlab/ref/fft.html

Explains the FFT function in MATLAB, which is used in this activity for frequency analysis of DTMF tones.

MathWorks Example – Dual-Tone Multi-Frequency (DTMF) Signals
https://www.mathworks.com/help/signal/ug/dual-tone-multi-frequency-dtmf-signals.html

An official MathWorks example that demonstrates how to generate and analyze DTMF signals, closely related to the activity.

ITU-T Recommendation Q.23
https://www.itu.int/rec/T-REC-Q.23/en

International Telecommunication Union (ITU) standard document describing the exact frequencies used for DTMF signaling. Relevant for understanding the frequency mapping used in the experiment.

Print Resources

Proakis, J. G., & Manolakis, D. G. (2007). Digital Signal Processing: Principles, Algorithms, and Applications (4th ed.). Pearson.
A widely used DSP textbook that covers sinusoidal signal generation, Fourier analysis, and their applications in communication systems.

Oppenheim, A. V., & Schafer, R. W. (2009). Discrete-Time Signal Processing (3rd ed.). Pearson.
A classic text providing the theoretical foundation for digital signal processing, including frequency-domain analysis relevant to DTMF.