Touchtone Phone Code

The touchtone phone is also known as a dual-tone multi-frequency (DTMF) transmitter/receiver. Each keypress on the keypad generates the sum of two audible sinusoidal tones, where the two frequencies uniquely define the key that was pressed, as shown below

The A, B, C, D keys appear on service keypads. The tones are typically implemented with a sampling frequency of 8 kHz.


  1. S. L. Freeny, J. F. Kaiser, and H. S. McDonald, "Some Applications of Digital Signal Processing in Telecommunications", p.1 in Applications of Digital Signal Processing, A.V.Oppenheim, ed., Prentice-Hall, Englewood Cliffs NJ, 1988.
  2. J. R. Boddie, N. Sachs, and J. Tow, "Receiver for TOUCH-TONE Service", Bell Syst. Tech. J, 60, 1573 (1981).
  3. P. Mock, "Add DTMF Generation and Decoding to DSP-microprocessor Designs", EDN, March 21, 1985.
  4. A. Mar, ed., Digital Signal Processing Applications Using the ADSP-2100 Family, Prentice-Hall, Englewood Cliffs NJ, 1990.

Maintained by John Loomis, last updated 6 Nov 1997