What is DTMF and How Does it Work?
Dual Tone Multi-Frequency is a signal tone generated when buttons are pushed on a telephone keypad – hence the term ‘Touchtone Phone’. DTMF was invented by the Bell Telephone Company in 1963. Over the following decades, it gradually replaced the earlier Pulse Dialing or Rotary Dialing technology. ‘Touch-Tone’ remained a registered trademark of Bell Systems, and later AT&T, until 1984.
With DTMF, each key generates two simultaneous tones of specific frequencies, designed so that a human voice can’t replicate the tones. One tone is high frequency while the other is low frequency. It is standardized under ITU-T Recommendation Q.23. The list below shows the dual tones used to represent each standard telephone key.
Digit Low Frequency High Frequency
1 697 Hz 1209 Hz
2 697 Hz 1336 Hz
3 697 Hz 1477 Hz
4 770 Hz 1209 Hz
5 770 Hz 1336 Hz
6 770 Hz 1477 Hz
7 852 Hz 1209 Hz
8 852 Hz 1336 Hz
9 852 Hz 1477 Hz
0 941 Hz 1336 Hz
* 941 Hz 1209 Hz
# 941 Hz 1477 Hz
In-Band and Out-of-Band Signaling
Traditional DTMF is an in-band signaling system, meaning the signals are transmitted using the same channel as the voice traffic. But in voice over IP, DTMF signals can be transmitted in-band or out-of-band. Out-of-band VoIP DTMF signaling can be implemented using protocols like SIP and MGCP, whereby special message types are defined for the transmission of digits.
The standard in-band method is to simply transmit the tones along with the audio, but this can result in unreliable signals due to codec compression, packet loss or audio interference. In-band DTMF transmission is only reliable when the uncompressed G.711 codec is used. If G.729 or G.723 are used, signaling normally fails as a result of the compression.
DTMF Tones Explained
Alternatively, DTMF tones can be digitally removed from the audio RTP stream at the source and separately encoded as digital data payloads known as a Named Telephone Events (NTE), transmitted on the same network path alongside the other RTP packets. This is a more reliable method than transmitting the tones as part of the audio.
To learn more about touch-tone technology with VoIP, contact us today.