What is DTMF?

Dual-Tone Multi-Frequency or DTMF is a signaling system used for dialing telephone numbers using a numeric keypad, instead of using the spinning dial on old pulse-dialing telephones. DTMF was popularized under the trademark name Touch-Tone and is almost universally used in telephone instruments worldwide.

The DTMF keypad is arranged in a 4 x 4 grid. Each row of keys is assigned a particular low frequency and column of keys is assigned a particular high frequency. Every key is located at the intersection of a row and a column. Therefore when a key is pressed, it produces two tones, one representing the column in which the key is located, and the other representing the row. This gave rise to the name Dual-Tone Multi-Frequency for this kind of dialing.

The frequency table for the rows and columns of keys is shown below:

	1209 Hz	1336 Hz	1477 Hz	1633 Hz
697 Hz	1	2	3	A
770 Hz	4	5	6	B
852 Hz	7	8	9	C
941 Hz	*	0	#	D

For instance the "8" key is located in the second column and the third row of the keypad. The frequency assigned to this column is 1336 Hz. The "8" key also belongs to the third row. The frequency assigned to this row is 852 Hz. Hence when this key is pressed, both frequencies are triggered resulting in a dual tone beep. Each pair of tones assigned is unique to each key. These signals are transmitted to the switching network which immediately recognizes the keys' position and determines the telephone number being dialed. The alphabets in the keypad were later discarded in many countries.

The frequencies for the rows and columns have been precisely selected such that noise and harmonics introduced in the signal during transmission do not make the frequency ambiguous. None of the original column and row frequencies is a multiple of another, and the difference or the sum of the dual tones of any key do not match any of the original frequencies. This helps the switching network to determine exactly which keys the user had pressed even when a lot of noise exists in the line. The ease of use and the accuracy of the system firmly established its usage in telephone instruments worldwide.