This circuit detects the dial tone (DTMF Tone) from a telephone line and decodes the keypad pressed on the remote telephone. The dial tone we heard when we pick up the phone set is call Dual Tone Multi-Frequency (DTMF). The name was given because the tone that we heard over the phone is actually make up of two distinct frequency tone, hence the name dual tone. The DTMF tone is a form of one way communication between the dialer and the telephone exchange. A complete communication consist of the tone generator and the tone decoder. In this article, we are use the IC MT8870, the main component to decode the input dial tone to 5 digital output. These digital bits consists of pressed key code 4-bit and 1-bit to tell that data is available. It can be interface to a microcontroller for further application such as remote control, IVRS (Interactive voice control systems) etc.

The MT8870 is a full DTMF Receiver that integrates both bandsplit filter and decoder functions into a single 18-pin DIP or SOIC package. Manufactured using CMOS process technology, the M-8870 offers low power consumption (35 mW max) and precise data handling. Its filter section uses switched capacitor technology for both the high and low group filters and for dial tone rejection. Its decoder uses digital counting techniques to detect and decode all 16 DTMF tone pairs into a 4- bit code. External component count is minimized by provision of an on-chip differential input amplifier, clock generator, and latched tri-state interface bus. Minimal external components required include a low-cost 3.579545 MHz color burst crystal, a timing resistor, and a timing capacitor.

DTMF Decode Circuit

DTMF Decode circuit main component is CM8870CP it gives decoded output, Output can be viewed on LED and also you can connect it to microcontroller using connector. This circuit is useful when making IVRS system using GSM Modem. you can connect speaker output of GSM Modem to the DTMF IN.

DTMF Decode PCB Layout

Single sided PCB is designed in Eagle software, size of the PCB is kept optimum. Download Printable Layout from Here “DTMF Decode PCB Layout”

DTMF Tone Decoder

Decoder The M-8870 decoder uses a digital counting technique to determine the frequencies of the limited tones and to verify that they correspond to standard DTMF frequencies. A complex averaging algorithm is used to protect against tone simulation by extraneous signals (such as voice) while tolerating small frequency variations. The algorithm ensures an optimum combination of immunity to talkoff and tolerance to interfering signals (third tones) and noise. When the detector recognizes the simultaneous presence of two valid tones (known as “signal condition”), it raises the Early Steering flag (ESt). Any subsequent loss of signal condition will cause ESt to fall.

CP8870 Pin Functions

 Telephone Keypad

The DTMF telephone keypad is laid out in a 4×4 matrix of push buttons in which each row represents the low frequency component and each column represents the high frequency component of the DTMF signal. Pressing a key sends a combination of the row and column frequencies. For example, the key 1 produces a superimposition of tones of 697 and 1209 hertz (Hz). Initial push button designs employed levers, so that each button activated two contacts. The tones are decoded by the switching center to determine the keys pressed by the user.

Decoding

DTMF was originally decoded by tuned filter banks. By the end of the 20th century, digital signal processing became the predominant technology for decoding. DTMF decoding algorithms often use the Goertzel algorithm to detect tones.