说明：  双音多频 DTMF（Dual Tone Multi Frequency），双音多频，由高频群和低频群组成，高低频群各包含4个频率。
(Dual-tone multi-frequency signaling (DTMF) is used for telecommunication signaling over analog telephone lines in the voice-frequency band between telephone handsets and other communications devices and the switching center. The version of DTMF that is used in push-button telephones for tone dialing is known as Touch-Tone. It was developed by Western Electric and first used by the Bell System in commerce, using that name as a registered trademark. DTMF is standardized by ITU-T Recommendation Q.23. It is also known in the UK as MF4.)