Dual audio decoding electronic circuit

DTMF (Dual Tone Multi Frequency) dual tone multi-frequency signal decoding circuit is an integrated circuit currently widely used in touch-tone phones (fixed phones, mobile phones), program-controlled switches and wireless communication equipment. It includes a DTMF transmitter and a DTMF receiver. The former is mainly used in touch-tone phones as a dual-tone signal transmitter to send a set of dual-tone multi-frequency signals to achieve audio dialing. Dual-tone multi-frequency signals are a set of combined signals that are superposed by high-frequency signals and low-frequency signals. Both CCITT and my country's national standards stipulate the corresponding relationship between telephone keyboard keys and dual-tone multi-frequency signals. The telephone remote control system adopts MITEL The produced MT8870 DTMF receiver serves as the core device for decoding DTMF signals. MT8870 is mainly used in program-controlled switches, remote control, wireless communication and broadcast systems to realize the separate filtering and decoding functions of DTMF signals and output four-bit parallel binary codes corresponding to 16 frequency combinations. MT8870 has dial tone suppression and adjustable analog signal input functions, so when designing the MT8870 DTMF decoding circuit, you only need to add some resistive and capacitive components.

The dual-tone multi-frequency DTMF signal decoding circuit of this system is mainly undertaken by MT8870. The connection of MT8870 is as shown in the figure. Its pins 2 and 3 receive the dual tone multi-frequency pulse signal from the telephone. The dual tone multi-frequency signal first passes through its internal dial tone filter to filter out the dial tone signal, and then passes through After the preamplification, it is sent to the dual audio filter, which separates the dual audio signal according to the high and low audio signals. It is then sent to the output decoding circuit through the high and low group filters and the amplitude detector. After digital operation, the data is The output terminal (pins 11~14) outputs the corresponding 8421 code. The data output terminals Q4~Q1 of MT8870 are connected to the microcontroller, and the microcontroller recognizes the 4-digit code. Telephone buttons and corresponding decoding (Q4~Q1) output. R/P, REDIAL, HOLD, HANDSFREE and other functions are used. In order to enable the microcontroller to obtain valid data in time, the CLD effective end of MT8870 is inverted and then connected to the INT0 pin of the CPU. When MT8870 obtains a valid dual-tone multi-frequency signal, the CLD level changes from low to high, and then inverts to low. After detection by the CPU, it indicates that the input port receives a valid binary code. Invalid dual audio signals (telephone line noise, people's voice signals, etc.) will not cause changes in the CLD end of MT8870. The peripheral circuit of the DTMF receiver is shown in the figure.