This system uses a single-chip microcomputer as the main control chip, and ISD4004 as a voice recording and playback chip, and cooperates with relevant peripheral circuits and control switches to achieve covert communication in special occasions. The user only needs to record the required voice in advance, and press the corresponding button during work to send the voice from the trunk station. As long as it can provide voice input, voice output, PPT and grounding four wirings, various cluster trolleys can be used with this module.
1 Working principle
The system is mainly divided into two parts. One is the voice input part before use. That is, users can record as needed according to their actual needs. Through the peripheral buttons of the system, after pressing the recording button, the single-chip microcomputer inputs through the SPI serial to tell the ISD4004 chip to enter the recording mode and the recording start address. The sound input by the microphone is filtered and amplified and input to the ISD4004 to achieve recording. Another part is that in the process of use, when the staff needs to speak, they can press the corresponding button to play the voice. When the user acts, the MCU inputs through the SPI serial and tells the ISD4004 chip to enter the playback mode and the starting address of the playback. At the same time, the single-chip computer triggers the PPT control pin of the cluster radio to realize the input and forwarding of the voice from the ISD4004 chip to the cluster radio. Figure 1 Schematic diagram of working mode
2 Circuit design
The circuit design of this system is shown in Figures 2, 3, and 4.
When working in the voice input mode, press the record button 1, the sound is input from the microphone, and the input to the transistor is effectively amplified. Then filter and input to the ANA IN + pin of ISD4004. The voice signal of ISD4004 is input by the coupling capacitor, the maximum amplitude is 32mV peak, the coupling capacitor and the input impedance of the resistor at the local end determine the low-end cut-off frequency of the chip band. At this time, the SS pin is connected to the P00 pin of the single-chip microcomputer and used as a chip select signal. The MOSI (serial input) pin is connected to the P01 pin of the single-chip microcomputer, and the single-chip microcomputer writes 0xB0 through the software, that is, commands the ISD4004 to work in the "record from the specified address" state to record. After recording, press button 1 again to stop recording.
When working in the key forwarding mode, press button 2 and the sound will be output via the AUD OUT port of the ISD4004 and connected to the MIC input signal line of the trunk station. Among them, two capacitors of different sizes can be connected in parallel (2.2μf and 22μf are selected in the test) to filter high-frequency and low-frequency noise in the line respectively. The MOSI (serial input) pin is connected to the P01 pin of the single-chip microcomputer. The single-chip microcomputer writes 0xE0 through software, that is, the ISD4004 is commanded to work in the "play from the specified address" state. address. ISD4004 will achieve automatic playback until it encounters EOM. In the playback state, the P10 port of the single-chip microcomputer directly drives the PPT input signal of the cluster radio, and sets P10 to a low level through the software to realize the PPT function and sound forwarding of the cluster radio. It is worth noting that the input impedance of different MICs in different clusters is different, and the output impedance of ISD4004 needs to be adjusted if necessary.
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