l Introduction
At present, DSP has become a basic device in the fields of communication, computer and consumer electronics due to its fast computing speed, rich on-chip resources and the ability to implement complex linear and nonlinear algorithms. Among them, speech signal processing technology is particularly prominent. . However, since all electronic devices including the DSP itself are sources of interference, and there are many external sources of interference in the working environment of the system, and the speech recognition technology is very sensitive to signal noise, it must be designed in the system. Consider the anti-interference problem of the system. Otherwise, it will at least affect the processing results of the system, and even cause more serious consequences. This paper introduces the anti-jamming technology in DSP-based speech signal processing system.
2 System interference sources and interference pathways
The interference sources in the DSP-based speech signal processing system mainly include the atmospheric noise source caused by lightning discharge, the solar noise source caused by the sunspot movement, the thermal noise source caused by the heating of electronic components such as resistors, and the 50 Hz power frequency. Sources of grid interference caused by power grids, sources of interference caused by household appliances, sources of brush interference caused by motors, sources of interference from ignition systems caused by automobile ignition devices, sources of RF interference caused by wireless communication systems, and sources of interference caused by human maliciousness, etc. The high-frequency impulse noise of all sources of interference is the most harmful to digital signal processing systems (the audio interference caused by speech is not studied here).
The interference sources mentioned above are all electromagnetic interference (EMI). Electromagnetic principle: As long as there is current, a magnetic field is generated, and an electric field is generated as long as a voltage exists. The amount of change in the magnetic field and electric field with time is the root cause of electromagnetic interference. The concept of electromagnetic interference is shown in Figure 1.
The interference paths of DSP-based speech signal processing systems mainly include power lines, input/output lines, ground lines, electromagnetic induction, electrostatic induction, common impedance of circuits, and power supply anomalies. The ratio of various interference pathways in the system is shown in Table 1.
3 anti-interference measures
According to the analysis of the system itself, the interference source and the interference path, the main scheme of anti-interference measures is:
1 improve the electromagnetic compatibility of the system itself;
2 isolation interference source;
3 cut off the interference route.
Based on these three schemes, this paper proposes some hardware anti-interference technology and software anti-jamming technology suitable for this system.
3.1 Electromagnetic compatibility
Electromagnetic compatibility means that when power, electronics, communication equipment or its system is in working condition, it does not affect its surroundings, nor is it affected by the electromagnetic environment around it, and does not cause malfunction or performance degradation. Design to get the ability to work. That is to say, the device or system does not generate electromagnetic interference to the outside world, and it is not affected by electromagnetic interference in its environment to affect its normal working ability.
3.2 Hardware anti-jamming technology
Since high-frequency impulse noise is the most harmful to the system, in order to improve the anti-jamming performance of the system, the following measures can be taken in the system:
(1) Increase the anti-interference ability of the bus. The bus structure in the form of a three-state gate is adopted, and a pull-up resistor is connected to the bus, so that the bus is at a stable high level in an instant and the bus is suspended. The bus must be buffered.
(2) Improve the anti-interference ability of the system control signal. In the system, there are usually control lines such as RESET and STB. When the transmission distance between the CPU and its control device is long and the control line impedance is high, it is susceptible to impulse noise interference. A 20 pF capacitor can be connected in parallel with the input of the controlled device, and a 0.Ol μF capacitor can be connected in parallel with the control signal line such as RESET. The control line adds a level one buffer to make the impedance of the control line low, which also helps to suppress interference.
(3) Suppressing the serial-to-analog interference of digital signals. This type of series mode interference is the interference caused by adjacent signal lines during the transmission of signals, and most of them occur on parallel wires of printed boards. The strength of the series mode interference is related to the coupling impedance between the adjacent two signal lines and the impedance of the signal itself. Therefore, in this system, the length of the signal line should be shortened as much as possible; when transmitting a variety of level signals, the level signals with similar front and rear edge times should be classified as a group of transmissions; A large area of ​​the ground line, which absorbs and shields the high-frequency impulse noise generated by the component.
(4) Eliminate power supply interference by using an EMI filter. With the emergence and widespread popularity of electronic devices, computers, and household appliances, grid interference is becoming more serious and creating a public hazard. In particular, transient electromagnetic interference, its voltage amplitude is high (hundreds of volts to thousands of volts), rising rate block, short duration, randomness, easy to cause serious interference to digital circuits, and even damage equipment. The electromagnetic interference filter, also known as the power supply noise filter (PNF), can effectively suppress the noise of the power grid and improve the anti-interference ability of the equipment and the reliability of the system. The application of the electromagnetic interference filter in the system is shown in Figure 2.
(5) Use the hardware watchdog function to improve the anti-interference ability of the system. The watchdog circuit consisting of the dedicated device MAX692 is shown in Figure 3. The system uses fewer peripheral components. The MAX692 is a micro-system monitoring circuit with backup battery switching, power-down discrimination, and watchdog monitoring. The WDI is the watchdog detection input terminal, which is connected to a dedicated I/O port or a bus port of the DSP. RESET is the reset signal output terminal and is connected to the reset terminal RST of the DSP. The MAX692 has a WDI timing period of 1.6 s and a reset pulse width of 200 ms. If WDI remains high or low above the “watchdog†timing period (1.6 s), the RESET terminal will generate a negative pulse of 200 ms (minimum 140 ms) to reset the DSP.
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