Anti-interference Design in Digital Image Processing System Based on DSP

This article introduces the anti-interference design in the digital image processing system based on DSP.

　　1. System interference sources and interference paths

　　The interference sources in the digital image processing system based on DSP mainly include noise caused by the basic properties of light and electricity, noise generated by the mechanical movement of electrical appliances, atmospheric noise caused by lightning discharge, atmospheric noise caused by the movement of sunspots, thermal noise caused by the heat generated by electronic components such as resistors during operation, grid interference caused by 50Hz power frequency power grid, ignition system interference caused by automobile ignition device, radio frequency interference caused by wireless communication system, and interference caused by some malicious human factors. Among all the interference sources, high-frequency pulse noise is the most harmful to the digital signal processing system.

　　The interference sources mentioned above are all electromagnetic interference (EMI). The principle of electromagnetism tells us that as long as there is current, a magnetic field will be generated, and as long as there is voltage, an electric field will be generated. The amount of magnetic field and electric field generated over time is the root cause of electromagnetic interference. The concept of electromagnetic interference is shown in Figure 1.

　　The interference pathways of the DSP-based digital image processing system mainly include power lines, input/output lines, ground lines, electromagnetic induction, electrostatic induction, common impedance of the circuit, power supply anomalies, etc. The proportion of various interference pathways in this system is shown in Table 1.

　　2. Anti-interference measures

　　According to the analysis of the system itself, interference sources and interference paths, the anti-interference measures of this system are mainly: ① improve the electromagnetic compatibility of the system itself; ② isolate the interference sources; ③ cut off the interference paths. Based on this, this paper proposes some hardware anti-interference technologies and software anti-interference technologies suitable for this system.

　　2.1 Electromagnetic compatibility

　　Electromagnetic compatibility means that when the power, electronic, communication equipment or its system is in working condition at the place where it is installed, it will not affect its surroundings, nor be affected by the electromagnetic environment around it, will not cause malfunction and performance degradation, and obtain its working capacity as designed. In other words, the equipment or system will not generate electromagnetic interference from the outside world, and its normal working ability will not be affected by the electromagnetic interference in the environment.

　　2.2 Hardware Anti-interference Technology

　　Hardware anti-interference has the characteristics of high efficiency and can keep most interference out. Hardware anti-interference technology is the first choice in design. It can effectively suppress interference sources and block interference channels. As long as the relevant parameters are reasonably arranged and selected, appropriate hardware anti-interference measures can suppress most interference in video communication systems. Common hardware anti-interference technical measures include: filtering (passive filtering and active filtering) technology, decoupling technology, isolation technology and grounding technology.

　　Since high-frequency pulse noise is the most harmful to this system, in order to improve the anti-interference performance of the system, the following measures can be taken in the system:

　　(1) Increase the anti-interference ability of the bus. Use a three-state gate bus structure and connect a pull-up resistor to the bus to keep the bus at a stable high level in an instant and avoid the bus being suspended. A buffer must be added to the bus.

(2) Improve the anti-interference ability of the system control signal. There are usually control lines such as RESET and STB in the system. When the transmission distance between the CPU and its control device is far and the control line impedance is high, it is easy to be interfered by pulse noise. A 20pF capacitor can be connected in parallel to the input end of the controlled device, and a 0.01μF capacitor can be connected in parallel to the control signal line such as RESET. Adding a buffer to the control line to lower the impedance of the control line can also help suppress interference.

　　(3) Suppress the crosstalk interference of digital signals. This crosstalk interference is caused by adjacent signal lines during signal transmission, and mostly occurs on parallel conductors on printed circuit boards. The strength of crosstalk interference is related to the coupling impedance between two adjacent signal lines and the impedance of the signal itself. Therefore, in this system, the following should be done: shorten the length of the signal line as much as possible; when transmitting multiple level signals, try to group the level signals with similar leading and trailing edge times into one group for transmission; and arrange a larger ground area on the back of the double-sided printed circuit board to absorb and shield the high-frequency pulse noise generated by the components.

　　(4) Use electromagnetic interference filters (EMI Filters) to eliminate power supply interference. With the emergence and widespread popularity of electronic devices, computers, and household appliances, power grid interference is becoming increasingly serious and has become a public nuisance. In particular, transient electromagnetic interference has a high voltage amplitude (several hundred volts to thousands of volts), a fast rise rate, a short duration, and strong randomness, which can easily cause serious interference to digital circuits and even damage equipment. Electromagnetic interference filters, also known as power supply noise filters (PNFs), can effectively suppress power grid noise, improve the anti-interference ability of equipment and the reliability of the system. The application of electromagnetic interference filters in the system is shown in Figure 2.

　　(5) Use the hardware watchdog function to improve the system's anti-interference ability. The watchdog circuit composed of the dedicated chip MAX692 is shown in Figure 3. The system uses few peripheral components. MAX692 is a microsystem monitoring circuit chip with functions such as backup battery switching, power-off detection, and watchdog monitoring. Among them, WDI is the watchdog detection input terminal, which is connected to a dedicated I/O port or a bus port of the DSP. It is the reset signal output terminal, which is connected to the reset terminal of the DSP. The WDI timing cycle of MAX692 is 1.6s, and the reset pulse width is 200ms. If WDI remains high or low for more than the "watchdog" timing cycle (1.6s), a negative pulse with a width of 200ms (minimum 140ms) will occur at the terminal to reset the DSP.

　　2.3 Software Anti-interference Technology

　　Software can also be used to improve the anti-interference ability of the DSP digital image processing system. The main methods are: ① using digital filters to filter out interference; ② using software watchdog, multiple sampling technology, timed refresh output port and other technologies to suppress interference. The following mainly introduces the application of digital filters in this system.

　　The processing process of the digital filter (DF) on the image signal is shown in Figure 4. The image signal first passes through the sample/hold circuit (S/H), is sent to the analog/digital converter (ADC) to be converted into a digital quantity, and then the interference signal is filtered out through the digital filter, and finally the voice signal output is obtained through the digital/analog converter (DAC).

　　According to the different mathematical models used, digital filters can be divided into two categories: one is a recursive filter, whose characteristic is that the output of the filter is not only related to the input signal, but also to the past output value; the other is a non-recursive filter (such as first-order and second-order low-pass filters), whose characteristic is that the output of the filter is only related to the input signal, but not to the past output value. The recursive filter is used in this system.

　　There are six methods for software design of digital filters: program judgment filtering method (limited filtering method), median filtering method, arithmetic average filtering method, recursive average filtering method, pulse interference prevention average filtering method, first-order lag filtering method, etc. According to the needs, this system selects the median filtering method.

　　Suppose the input signal of the digital filter is a one-dimensional sequence l, 2, ..., n, and the window length (number of points) is m (m is an odd number). Perform median filtering on it, which means extracting m numbers iv, ..., i-1, i, i+1, ..., i+v from the input sequence (where f2 is the window center value, v=(m-1)/2), and then sorting these m points according to their numerical values, and taking the number with the center point as the filter output. It 
can be expressed as: yi=Med{iv, ..., i, ..., i+v} i∈N, v=(m-1)/2

　　3. Conclusion

　　Experiments have shown that the above anti-interference methods can fully suppress interference from outside the system and the system itself in the digital image processing system based on DSP, and can effectively improve the system's anti-interference ability and reliability.