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Second harmonic test: The objective of this test is to reject speech that has harmonics close to f k so that they might be falsely detected as DTMF tones. Since DTMF tones are pure sinusoids, they contain very little second harmonic energy. Speech, on the other hand, contains a signi cant amount of second harmonic. To test the level of second harmonic, the detector must evaluate the second harmonic frequencies of all eight DTMF tones. These second harmonic frequencies (1394, 1540, 1704, 1882, 2418, 2672, 2954, and 3266 Hz) can also be detected using the Goertzel algorithm. Digit decoder: Finally, if all ve tests are passed, the tone pair is decoded and mapped to one of the 16 keys on a telephone touch-tone keypad. This decoded digit is placed in a memory location designated D(m). If any of the tests fail, then 1 representing no detection is placed in D(m). For a new valid digit to be declared, the current D(m) must be the same in three successive frames, i.e., D(m 2) = D(m 1) = D(m). There are two reasons for checking three successive digits at each pass. First, the check eliminates the need to generate hits every time a tone is present. As long as the tone is present, it can be ignored until it changes. Second, comparing digits D(m 2), D(m 1), and D(m) improves noise and speech immunity.
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9.3 Internet Application Issues and Solutions
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Voice coders have been designed for transmission of low-bit-rate signals while retaining reasonable audio quality and robustness over the networks. However, most of the vocoders overlook the importance of passing in-band tonal signals including DTMF. The approach for DTMF signaling over the networks can either use vocoders that are capable of passing in-band tones or use out-of-band signaling. RFC2833 is a document for carrying DTMF signals and telephony events using RTP packets over the Internet. A separate RTP payload format is used due to the concern of low-rate vocoders that may not guarantee to reproduce the tone signals accurately for automatic recognition. The separate RTP payload format can be considered as the out-of-band channel. Using separate payload formats also permits higher redundancy while maintaining a low-bit rate. Figure 9.9 shows an example for Internet applications using DTMF generator and detector. The end user can use an automated computer system that recognizes the DTMF signaling and controls the access to the database, such as electronic bank accounts, voicemail systems, and automatic billing systems. Using the separated RTP payload allows the receiving side to recognize the in-band DTMF tones and regenerate these tones locally if necessary. RFC2833 also covers fax tones, modem tones, country-speci c subscriber line tones, and trunk events. The RTP payload type for vocoders will be discussed in 11. The DTMF typically uses dynamic payload type. The dynamic payload type means the type is negotiated using session description protocol between the two sides de ned in RFC3551. Table 9.3 gives an example of the DTMF packet. The rst 12 bytes are RTP header and the last 4 bytes are DTMF event. In the DTMF event data, the rst byte
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Table 9.3 Example of RTP packet of DTMF digit 1 ; ; ; ; ; ; ; ; ; ; ; Data 80
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80 62 f4 62 00 24 cb ac ac 24 a8 7a 01 08 00 a0 Packet summary lines Real-Time Transport Protocol 10.. .... = Version: RFC 1889 Version (2) ..0. .... = Padding: False ...0 .... = Extension: False .... 0000 = Contributing source identifiers count: 0 0... .... = Marker: False .110 0010 = Payload type: Unknown (98) Sequence number: 62562 Timestamp: 2411436 Synchronization source identifier: 2888083578 RFC 2833 RTP event Event ID: DTMF One 1 (1) 0... .... = End of event: False .0.. .... = Reserved: False ..00 1000 = Volume: 8 Event duration: 160
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