Analysis of the principle of DTMF

Transferred from: http://blog.csdn.net/wangwenwen/article/details/8264925

1. DTMF principle

DTMF (double Tone mulitifrequency, dual tone multi-frequency) as a technology to achieve fast and reliable transmission of telephone numbers, it has strong anti-jamming ability and high transmission speed, so it can be widely used in telephone communication system. But most of the audio dialing is used as a telephone. In addition, it can be widely used in data communication system to achieve a variety of data flow and voice information such as remote transmission.
DTMF is a coding technique that uses two specific single-audio combination signals to represent digital signals to realize its function. The frequency of two single audio is different, the number represented or the function of the implementation is different. There are usually 16 keys in this telephone, including 10 numeric keys 0~9 and 6 function keys *, #, A, B, C, D. As a result of the combination principle, there should be 8 different kinds of single audio signal generally. Therefore, the frequency can be used there are 8, it is called multi-frequency, and because it uses any of the 8 frequencies from any 2 combinations to encode, so also known as the "8 to take 2" coding technology.
According to CCITT's recommendations, the international use of a variety of frequencies 697Hz, 770Hz, 852Hz, 941Hz, 1209Hz, 1336Hz, 1477Hz and 1633Hz and other 8 kinds. The 8 frequencies can be used to form 16 different combinations, representing 16 different numbers or function keys, and the specific combinations are shown in table 1.

Table 1 dual tone multi-frequency combination function

HF Group Hz Function Low Frequency group Hz	1209	1336	1477	1633
697	1	2	3	A
770	4	5	6	B
852	7	8	9	C
941	*	0	#	D

2. The solution of DTMF in VoIP

(http://blog.chinaunix.net/uid-27122534-id-3260925.html)

There are three common ways to transmit DTMF signals:

A. Transmission via communication protocol (SIP signaling)

Use the Info method of the SIP signaling to carry the DTMF signal.

For out-of-band detection, a DTMF signal is carried through the info method of the SIP signaling. There is no uniform implementation standard, and the DTMF keys are identified by the signal field in the Sipinfo package with the Cisco sipinfo standard . Note that when DTMF is "*" The different standards implement the corresponding signal=* or signal=10. The advantage of Sipinfo is that it does not affect the transmission of RTP packets, but because SIP control signaling and Media transfer (RTP) are transmitted separately, it is easy to cause DTMF signals and media packets to be 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.

B. Data content transfer via RTP (Inband)

For in-band detection mode, in band refers directly to the DTMF audio digital signal without any processing directly into the RTP packet in the IP network transmission. It may be mixed with the user's voice media stream to transmit. Program to know which packet has a DTMF signal, what is the DTMF signal, the RTP packet must be extracted for spectral analysis, through the spectrum analysis to get high frequency and low frequency, and then look up the table to get the corresponding keys. In the selection of compression ratio is very high rate of codec, such as g.723.1 and g.729a. The main defect is the loss of DTMF signal due to the influence of network drops, and the DTMF sound is mixed in the voice packet, which is prone to deviation and distortion of signal.

C. Transfer via rfc2833 rules and format package

For in-band detection mode, through RTP transmission, the special Rtppayloadtype is teleponeevent to mark the RFC2833 packet. The same DTMF key will usually correspond to multiple RTP packets, the timestamps of these RTP packets are the same, this can be used as a basis for identifying the same key, the last packet of RTP packet end Flag 1 indicates The DTMF data ends. In addition, many SIP UA including IAD provide teleponeevent setting functions such as 3CX Phone,billion-iad,zte-iad and other default Teleponeevent are 101, but can be artificially modified, this requires prior to RFC2833 DTMF detection needs to obtain the teleponeevent parameters of the SDP negotiation .

Analysis of the principle of DTMF