DTMF (Dual Tone Multifrequency): Dual Tone Multifrequency. DTMF codec converts keystrokes or digital information into dual-tone signals and sends them when encoding, and detects whether there are keystrokes or digital information in the received DTMF signals when decoding. DTMF signal consists of two audio signals with superimposed frequencies. The frequencies of these two audio signals come from two pre-assigned frequency groups: line frequency group or column frequency group. Each pair of such audio signals uniquely represents a number or symbol. A telephone usually has 16 keys, including 10 numeric keys 0-9 and 6 function keys *, #, A, B, C and D. According to the combination principle, there should be 8 different tone signals. So there are eight frequencies that can be used, so it is called multi-frequency, and because two frequencies are used for coding, it is also called "8 out of 2" coding technology. According to CCITT's suggestion, there are eight kinds of frequencies used internationally, which are 687Hz, 770Hz, 852Hz, 94 1.209 Hz, 1336Hz, 1477Hz and 1633Hz. These eight frequencies can form 16 different combinations, thus representing 16 different numbers or function keys. See table 1 for specific combinations.
Frequency shift keying: frequency shift keying. Is to use digital signals to modulate the carrier frequency. It is an early modulation method for information transmission, and its main advantages are: easy implementation, good anti-noise and anti-attenuation performance. It has been widely used in low-speed data transmission.