DTMF Frequencies

The following are the frequencies used for the DTMF (dual-tone, multi-frequency) system, which is also referred to as tone dialling. The signal is encoded as a pair of sinusoidal (sine wave) tones from the table below which are mixed with each other. DTMF is used by most PSTN (public switched telephone networks) systems for number dialling, and is also used for voice-response systems such as telephone banking and sometimes over private radio networks to provide signalling and transferring of small amounts of data.

When encoding DTMF signals the frequency should be within 1.5% of that stated, and when decoding any signals out of tolerance by more than 3.5% should be ignored. Likewise when encoding the duration of each tone should be at least 40ms, whilst when decoding tones less than 23ms should be ignored. The CCITT DTMF recommendation also states that the DTMF tone energy should exceed the levels of any other frequencies present by 30dB.

Table of DTMF frequecies