Design of trunking radio communication module based on ISD4004

　　The hardware environment for debugging this system is a combination of a concealed communication module and a MOTOROLfA MTH850 handheld radio. The software environment uses the KEIL, C programming environment and the STC-ISP microcontroller burning program. The specific contents are as follows:

　　In the hardware debugging environment, in addition to the general settings, you also need to pay attention to the following points:

　　(1) Selection of the four wires for voice input, voice output, PPT and grounding in the cluster radio. Cluster radios are generally equipped with a dedicated data interface. The corresponding signal leads can be taken out according to the instructions. However, general cluster radios do not have obvious data input and output ports, and radios of different brands usually use different technical standards. However, whether it is an analog cluster radio or a digital cluster radio, it usually has a wired headphone microphone interface. By cutting off the wiring and studying it, we can know the definition of the four wires and then determine the specific locations of the four wires for voice input, voice output, PPT and grounding of the radio.

　　(2) The MIC receiving sensitivity of the clustered radio needs to be adjusted. The so-called MIC receiving sensitivity of the radio refers to the sensitivity at which the built-in/external microphone of the radio can be turned on. If the sensitivity is too low, the PPT function of the clustered radio cannot be effectively triggered. Experiments have shown that if the PPT function of the radio needs to be triggered through the headphone interface of the radio, two conditions must be met at the same time. First, the external PPT line needs to be set to 0 (this function is realized through the play() function in C language programming). Second, there must be sound input on the external MIC line, that is, ISD4004 has sound output. Both conditions must be met at the same time to effectively trigger the PPT function of the radio. Therefore, in the radio burning software, the MIC receiving sensitivity can be appropriately increased to avoid the situation where the PPT cannot be triggered due to insufficient sensitivity.

　　(3) Adjustment of output sound quality. In the voice output part, a single capacitor is usually used for noise filtering. After experiments, the noise filtering effect of a single small capacitor is average. Therefore, by connecting two capacitors, one large and one small, in parallel, the high-pitched noise and the low-pitched noise in the sound output process are filtered out respectively. In this project, the capacitors are selected as 2.2μf/50V and 22μf/35V respectively. Experiments have shown that after connecting two capacitors in parallel, the voice output quality of this product is effectively improved.

　　(4) Implementation of working power supply. The system mainly involves the working voltage of the microcontroller and the working voltage of the ISD4004 chip, which are 5V and 3.3V respectively. The 5V working voltage can be obtained by connecting 4 1.3V rechargeable nickel-hydrogen batteries in series. After testing, its working voltage and working time meet daily requirements. The 3.3V voltage is dedicated to the ISD4004 voice recording and playback chip. Since the chip includes the interchange process of analog and digital signals, it has higher requirements for power supply. This system uses the HT7133 voltage-stabilized power supply chip to provide a stable 3.3V DC output. Of course, if conditions permit, you can also choose a microcontroller with a 3V working voltage to simplify the circuit.

　　During software debugging, there are several key points:

　　(1) By calling the at89x52.h function library, the external interface of the microcontroller and the internal actual address are effectively mapped. This greatly reduces the programming complexity of the main function and functional function, and improves the readability and maintainability of the program.

　　(2) By effectively setting the microcontroller burning program, the software editing and burning work is completed. The software selected in this project is STC-ISP burning software, which is STC's microcontroller burning software. The interface is shown in Figure 6.

　　After completing C programming in KEIL C and generating a valid HEX hexadecimal file, just select MCU Type in sequence: select the corresponding CPU type, here select STC89C52RC;

　　COM port selection: Select the COM port corresponding to the computer;

　　Baud rate: maximum 115200, minimum 1200;

　　Double speed: Select 12T/single speed

　　Other options can be left as default. After completing the option settings, select "Open Program File" to download the corresponding source program, and then click "Download" to complete all the MCU code burning work.

　　5 Conclusion

　　This system makes full use of the headphone and microphone integrated jacks of various trunking radios, combining the high-definition voice recording and playback of ISD4004 with the single-chip control function. It has low cost and good compatibility. It is suitable for covert communication applications of various analog/digital trunking equipment and can play a special role in specific occasions.