The solution of DTMF in VoIP

Dual tone Multi-frequency DTMF (Dual Tone multi-frequency) signaling, which provides a higher dialing rate, quickly replaces the dial-pulse signaling used in traditional turntable telephones. DTMF is also used in interactive control, such as language menus, language mail, telephone banking, and ATM terminals. DTMF is still playing an important role in VoIP because of its extensive use in traditional communication field.

A DTMF signal is composed of two frequencies of audio signals superimposed. Each pair of such audio signals uniquely represents a number or symbol. The generation of DTMF signal, is the use of two different frequency sine wave superimposed after the formation of the waveform, decoding using an improved Goertzel algorithm, from the frequency domain search two sine wave existence. There are 16 kinds of DTMF signals, 0-9, * #ABCD.

The solution of DTMF in VoIP:

Since the communication transmission in the IP network is based on packet switching (packet switch) instead of the traditional domain of line switching (circuit switch) and the unstable characteristics of the IP network, the solution and tradition of DTMF application in VoIP are different, and are not unified for the time being. There are a variety of solutions.

1. Use the info method of the SIP signaling to carry the DTMF signal.

The method uses the info method of SIP signaling to define the DTMF signal. This method is also discussed in the discussion, some experts think that it does not apply, the main drawback is that the SIP control signaling and media transmission (RTP) is separated transmission, it is easy to cause DTMF signals and media packets out of sync.

For example, in the voice mail application, the user enters a DTMF signal according to the cue tone and then begins to leave a message. The server starts to save the user's message after receiving the DTMF signal. However, since the DTMF signal is transmitted through the SIP signaling, and the media stream is transmitted through RTP, it is possible for the user to message the RTP packet first, and the information message of the DTMF signal is delayed, resulting in the server does not save the user's voice message until the info message is received.

2. Carry DTMF signals in the RTP media transmission.

This method is the same as the DTMF signal and the media stream, with the RTP packet transmission, so there is no DTMF signal and the media stream is not synchronized, VOIP using H323 signaling is the use of this method, relatively mature.

And among them is in band and out of band (RFC2833) two kinds.

In Band dtmf:in Band DTMF refers to the direct DTMF audio digital signal without any processing directly into the RTP packet in the IP network transmission. It may be transferred together with the user's voice media Stream Mix (mix). Program to know which packet has DTMF signal, what is the DTMF signal, must be in real-time check each RTP packet inside the media stream data, analysis of its frequency domain.

Out of Band DTMF (RFC 2833): Out of Band DTMF is a DTMF signal identified with a dedicated RTP packet, in the head domain of the RTP packet can be learned that the packet is a DTMF packet, and know what DTMF signal. RFC2833 specifically defines this.

3. Dynamic generation of DTMF audio signals

When the program needs to produce a DTMF audio digital signal, of course you can read the recorded files.

4. Detect DTMF signals based on the audio signal in the RTP packet

In the input signal to detect DTMF signal and convert it to the actual number, this decoding process is essentially a continuous process, it is necessary to continuously search the input data signal stream of the existence of DTMF signal spectrum.

The solution of DTMF in VoIP