HT9170 is the series of Dual Tone Multi Frequency (DTMF) receivers. They employ digital counting techniques to detect and decode the 16 DTMF tones into 4 bit output code.
HT9170 series receivers do not require any external filters as they use highly accurate switched capacitor filters for filtering low and high frequency signals from the DTMF tones. They also support power down (PWDN) and inhibit (INH) modes. PWDN mode is used to power off the crystal, while INH mode to inhibit the A, B, C & D DTMF tones. The clock is provided by a 3.58 MHz crystal.
In simple terms, HT9170 IC detects and decodes the 16 DTMF tones into 4 bit output. In case the tones are not detected, the four output bits remain low. The DV pin goes high on detection of a valid tone.