Decoders are digital ICs which are used for decoding. In other words the decoders decrypt or obtain the actual data from the received code, i.e. convert the binary input at its input to a form, which is reflected at its output. It consists of n input lines and 2^n output lines. A decoder can be used to obtain the required data from the code or can also be used for obtaining the parallel data from the serial data received.
The MT8870C/MT8870C-1 is a DTMF decoder IC to integrating the band split filter and digital decoder operations. The filter section uses switched capacitor techniques for high and low group filters; the decoder uses digital counting techniques to detect and decode each of the 16 DTMF tone pairs into a 4-bit code. Dual-tone multi-frequency is the audible sounds we hear when we press keys on our phone. DTMF decoder is used for remote control applications.
DTMF is a strategy for sending and receiving control qualified information over a communications channel. The viewer is likely generally acquainted with DTMF tones as heard on a modern push-button telephone. Each number on the keypad is generates the corresponding DTMF tone. When a number is pressed on the keypad then it encoded and transmitted over a medium. The receiver receives it and decodes the DTMF tone back into its two particular frequencies and after that the processing circuit will act appropriately.
Working of DTMF DECODER MT8870:
From the application circuit, it uses a DTMF decoder MT8870 that uses a crystal of 3.57 MHz for generating appropriate frequency for comparing the input audio tones at its pin2 to generate 4 bit BCD code at its output from pin 11 to 14. This BCD data is passed through HEX CMOS inverters the output of which are duly pulled up and connected to port-3 pin 10 to 14 as buffer between the DTMF IC and the microcontroller. While tone commands arrive from a telephone line after a call is established, it first reaches the DTMF decoder IC MT8870. For example if button 1 is pressed the output develops 0001 at pin 11-14 which are inverted and fed to the microcontroller input ports. For digit 2 the output developed accordingly provides 0010 and so on for rest of the digits. The microcontroller program while executed develops specific output for each number.
2. HT9170B DTMF Decoder IC:
The HT9170B is a Dual Tone Multi Frequency (DTMF) receiver integrating a digital decoder. The HT9170 series all use the digital counting techniques to detect and decode all DTMF input into a 4-bit code output. The high precise filters are designed to separate tone signals into low and high level frequency signals. Basically it is an 18 pin IC.
The input arrangement is at pin no 2 with a RC circuit connection. The system oscillator comprises of an inverter, a bias resistor and fundamental load capacitor on IC. A standard 3.579545MHz crystal oscillator is connected with X1 and X2 terminals to execute the oscillator function. D0, D1, D2, D3 are the data outputs terminals. In this we used a keypad of any telephone or cell phone, normally a matrix 4×3 keypad. When we press the one on the key pad it gives a binary output of 0001, similarly for 2-0010, 3-0011, 4-0101, 5-0101, 6-0110, 7-0111, 8-1000 and 9-1001. When the decoder receives an effective tone signal, the DV pin goes high and the tone code signal is transformed to its internal circuitry for decoding. After that the OE pin goes high, the DTMF decoder will appear on output pins D0-D3.