DTMF

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Intro

640px-66a3aDTMFpad.jpg
DTMF keypad frequencies
1209 Hz 1336 Hz 1477 Hz 1633 Hz
697 Hz 1 2 3 A
770 Hz 4 5 6 B
852 Hz 7 8 9 C
941 Hz * 0 # D

https://en.wikipedia.org/wiki/Dual-tone_multi-frequency_signaling

Goertzel

https://en.wikipedia.org/wiki/Goertzel_algorithm


Stuffs