Because most radio stations use transparent transmission mode, as long as the frequency band is the same, they can receive the voice signal sent on the same frequency band, so it is impossible to accurately identify whether the signal is useful. The radio station with voice encryption function can avoid this situation, but in some areas or places requiring high confidentiality. Therefore, this paper introduces a system based on C8051F microcontroller and DTMF coding to identify the identity of radio station. The system is simple to use, without changing the original characteristics of the radio station, it only needs to connect with the expansion port of the radio station to realize the identification of the radio station. Even if the other party has cracked the voice signal, the identity of the radio station can be distinguished by address coding. The system has a certain application prospect in the high security field.
DTMF encoding / decoding
The DTMF codec is MT8880. Mt8880c is a single chip DTMF signal transceiver with call processing filter. The device adopts iso-cmos technology with low power consumption and high stability, a high-performance receiver with variable gain internal amplifier, a transmitter with pulse counter and a standard microprocessor interface to access the internal registers of MT8880. The internal register includes one status register, two data registers and two control registers. The circuit is shown in Figure 4.
The system adopts single input mode, in – is the input of op amp, R1 and R2 adjust the input signal gain. Tdne is the audio output, used to send DTMF signal. OsCl is the input of clock / oscillator, osc2 is the output of clock, MT8880 adopts 3.579 5 MHz working clock. IRQ / CA is an interrupt request signal with open drain output. When DTMF signal is input, IRQ / Ca sends an interrupt request to C8051F020. D0-d3 is a microcomputer data bus, compatible with TTL. It outputs the DTMF signal measured and inputs the DTMF signal to be sent. It is connected with the I / O interface of C8051F020. The 12 pin CP2 terminal is the system clock input, which completes the read-write operation of MT8880 in cooperation with the read-write control terminal R / W. the rs0 pin is the register selection, and the TTL level is compatible. The address code can be composed of 0-9 and A-D. Because of the particularity of the application field of the system, the address code is encrypted. When the terminal is used as a transmitter, the DTMF encoding / decoding part encrypts the address code of the terminal with a certain algorithm, modulates it into a DTMF signal, and then sends it to the radio station, so as to facilitate the identification of other radio stations in the network. When the terminal is used as the receiver, the DTMF encoder / decoder demodulates the received DTMF signal into a digital code, and then obtains the address code through the decryption algorithm. Considering that the impedance of the system may not match the impedance of the radio audio I / O interface, an audio transformer should be added between the two.
ISD4004 of ISD company is selected as the voice storage and playback module. The speech device adopts the patented technology of multi-level direct analog memory (chip corder). The sound does not need a / D conversion and compression. Each sampling value is directly stored in the on-chip flash memory, so as to avoid quantization noise and metal sound caused by quantization and compression of general solid state recording circuit. Figure 6 shows the principle circuit of voice playing. Voice input is directly connected with ISD4004 pin in + and in – via mic. The voice output is amplified by LM386 and connected to the voice output of the radio station. The system adopts keying subsection recording and playback mode. Through the recording key, 10 different voice segments can be recorded. Each recording corresponds to a key on the keyboard. According to the received voice signal, different voice can be played through 10 keys, which is convenient and flexible.