DTMF is an abbreviation of Dual ToneMultiple Frequency. When you press a button in the telephone set keypad, aconnection is made that generates a resultant signal of two tones at the same time. These two tones are takenfrom a row frequency and a column frequency. Theresultant frequency signal is called “Dual ToneMultiple Frequency“. These tones are identical and unique.
Telephone keypad with frequency
DTMF signaling uses two tones to represent each key on the touch pad. There are 12 distinct tones. When any key is pressed the tone of the column and the tone of the row are generated. As an example, pressing the ‘5’ button generates the tones 770 Hz and 1336 Hz. In this example, use the number 10 to represent the ‘*’ key and 11 to represent the ‘#’ key.
What are DTMF Frequencies:
ANSI standards state that DTMF tones used in telephony must meet the following criteria:
- The DTMF duration must be at least 50ms.
- The inter-digit interval (silence) between tones must be at least 45ms.
- The receiver should receive the DTMF digits with a signal strength of at least -25dBm and no more than 0dBm.
- The energy strength of the high-group frequency must be -8 dB to +4 dB relative to the energy strength of the low-group frequency as measured at the receiver.
When the level between the two frequencies is not equal it is said to have a twist. Most modern equipment is fairly tolerant of unbalanced levels between tones.
| Event | Low frequency | High frequency |
|---|---|---|
| Busy signal (US) | 480 Hz | 620 Hz |
| Ringback tone (US) | 440 Hz | 480 Hz |
| Dial tone (US) | 350 Hz | 440 Hz |
Chips Used to Encode & Decode DTMF
- TP5089 DTMF (TOUCH-TONE) Generator
- DTMF decoder MT8870DE
