DTMF（Dual Tone Multi Frequency）Recognition Algorithm

代码说明：

       Dual Tone Multi Frequencies (DTMF tones) are two different tones at two ends of a spectrum that are used to send information in telephonic communication mediums. The tones represent numbers 0-9 and the symbols * and #. A series of these numbers may be assigned to individuals as points of contact, as in a phone number, or the numbers can be used to relay other sorts of information.       While it may seem confusing, it should be noted that all numbers on a telephone are assigned two DTMF tones, and in all, seven different frequencies are assigned. Some numbers will have the same lower frequency, but different higher frequency, while others will have the same higher frequency but a different lower frequency. The range is between 697 Hz and 941 Hz for the lower frequencies and 1209 Hz and 1633 Hz for