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

With the progress of human civilization and the rapid development of electronic technology, video communication , as an extension of human vision, has been widely used in all walks of life. The digital image processing technology that emerged has also developed rapidly. At present, due to its fast computing speed, rich on-chip resources and the ability to implement complex linear and nonlinear algorithms, DSP has become a basic device in the fields of communication, computer and consumer electronics , especially in digital image processing technology. However, since all electronic devices including DSP itself are interference sources, and there are many external interference sources in the working environment of the system , and digital image processing technology is very sensitive to signal noise, the anti-interference problem of the system must be considered in the system design . Otherwise, it will at least affect the processing results of the system, and even cause more serious consequences. 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 DSP-based digital image processing system 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 heating of electronic components such as resistors , 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 electromagnetics 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 channels of the DSP-based digital image processing system mainly include power lines, input / output lines, ground lines, electromagnetic induction, electrostatic induction, circuit common impedance, power supply anomalies, etc. The proportion of various interference channels in this system is shown in Table 1.

2. Anti-interference measures



Based on 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 source; ③ Cut off the interference path and other three solutions . 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 refers to the fact that when the power, electronic, communication equipment or its system is in working state at the place where it is set, it will not affect its surroundings, nor will it be affected by the electromagnetic environment around it, and will not cause malfunctions and performance degradation, and obtain its working capacity as designed. In other words, the equipment or system does not generate electromagnetic interference to the outside world, and its normal working ability is not affected by the electromagnetic interference in the environment.

2.2 Hardware anti-interference technology

Hardware anti-interference has the characteristics of high efficiency, which 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 the video communication system. 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 bus structure in the form of a three-state gate and connect a pull-up resistor to the bus to make the bus at a stable high level in an instant to avoid the bus being suspended. The bus must be buffered.

(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 controller 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 also helps to suppress interference.

(3) Suppress the cross-mode interference of digital signals. This type of cross-mode interference is caused by adjacent signal lines during signal transmission, and mostly occurs on parallel conductors on printed circuit boards. The strength of cross-mode 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: the length of the signal line should be shortened as much as possible; when transmitting multiple level signals, the level signals with similar leading and trailing edge times should be grouped together for transmission; a larger ground area should be arranged 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 equipment, 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.

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2.3 Software Anti-interference Technology

Software can also be used to improve the anti-interference ability of DSP digital image processing system. There are mainly: ① Use digital filters to filter out interference; ② Use 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 digital filter (DF) on image signal is shown in Figure 4. The image signal first passes through the sampling/holding circuit (S/H), sent to the analog/digital converter (ADC) to convert into digital quantity, and then the interference signal is filtered out by 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, which is characterized by the fact 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), which is characterized by the fact that the output of the filter is only related to the input signal, but not to the past output value. A recursive filter is used in this system. The software design methods of digital filters include: 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 and other 6 methods. According to the needs, this system selects the median filtering method. Assume that 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). Median filtering is performed 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 proved that the above anti-interference method 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 anti-interference ability and reliability of the system.