![]() Notice in the above figure that computational complexity of taking a Discrete Fourier Transform (DFT) is high due to $N$ frequencies involved. Clearly, the two spikes at $\pm 770$ Hz and $\pm 1477$ Hz indicate the presence of the two tones corresponding to key 6. ![]() The spectrum of this signal can be viewed through the FFT command and drawn in the figure below. Pl.savefig('dtmf-tone.png', bbox_inches='tight') Keytones =, coltones] for j in range(3)] for i in range(4)]įig, axs = pl.subplots(3, figsize=(figWidth, figHeight))įig.suptitle("DTMF Signal for Key 6", fontsize=14) If you work in Maltab/Octave, you can also read about how to generate a sinusoid and view its spectrum. Python code to generate the above signals is given below. For example, pressing the number 6 generates a sum of two sinusoids, one at 770 Hz and the other at 1477 Hz. ![]() The last column showing letters A through D are missing in the above figure because they are rarely used.Įach key press generates two tones, or sinusoids: one from the low frequency group (row) and the other from a high frequency group (column). In addition, there are 4 tones in a high group of frequencies (corresponding to each column in the keypad above). In DTMF, there are 4 tones in a low group of frequencies (corresponding to each row of the keypad above). Let us start with a keypad as shown in the figure below. Examples are customers navigating a telephone menu of a company, responding to certain questions by pressing the buttons on the dial pad and entering credit card or account details for a phone transaction. Their significance decayed with time due to separate control channels in cellular transmissions but their association with the keypad has kept them alive for many applications today. DTMF Signal Generationĭual Tone Multi-Frequency (DTMF) signals were designed to send the control signals (e.g., the destination phone number or a particular number) within the voice band. A control signaling mechanism for automatic routing was the answer. People need to connect to a lot of people beyond the capacity of this kind of system. Whenever you picked up the phone, you told the operator where to connect and they manually relayed your call on a central switchboard. Instead, a telephone operator used to sit on the other side of the line. ![]() In the early days of telephone, you could not call anyone directly.
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