M2M Machine to Machine

DTMF encoding by GSM modem

It is not easy to detect and recognize DTMF with satisfactory precision. Often, dedicated integrated circuits are used. It is rather complicated, so it is used only marginally. Most often, a MT8870 or compatible circuit would be used.
The MT8870 is a complete DTMF receiver integrating both the band split filter  and digital decoder functions. The filter section uses switched capacitor  techniques for high and low group filters; the decoder uses digital counting  techniques to detect and decode all 16 DTMF tone-pairs into a 4-bit code.  External component count is minimised by on chip provision of a differential input amplifier, clock oscillator and latched three-state bus interface.

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