Why does A/D converter convert analog signals to digital signals?

Signal data can be used to represent any information, such as symbols, text, voice, images, etc. The form can be summarized into two types: analog signals and digital signals. The difference between analog and digital signals can be determined based on whether the amplitude is discrete. Analog signals indicate that the amplitude value is continuous (the amplitude can be expressed by an infinite number ). Consecutive time-based analog signal continuously changing image (TV, fax) signals, etc., 1-1 (. The discrete analog signal in time is a sampling signal, as shown in 1-1 (B) even though the waveform is not continuous in time, the amplitude value is continuous, so it is still a simulated signal, it is called the pulse amplitude modulation (PAM) signal. Digital signals indicate that the value of the amplitude is discrete, and the value of the amplitude is limited to a finite number. Binary Code is a digital signal. Binary codes are less affected by noise and easy to process with digital circuits. Therefore, binary codes are widely used. 1. analog communication is intuitive and easy to implement, but there are two main disadvantages. (1) analog communication with poor confidentiality, especially microwave communication and wired plain-line communication, which are easily eavesdropped. As long as you receive the analog signal, you can easily get the communication content. (2) weak anti-interference capability electrical signals will be subject to various noise interference from the outside and the communication system during transmission along the line, and it is difficult to separate the noise and signal after mixing, this reduces the communication quality. The longer the line, the more noise accumulation. 2. Digital Communication (1) Advantages of digital transmission and exchange ① enhanced the confidentiality of communication. After A/D conversion, the voice signal can be encrypted first, then transmitted, decrypted at the receiving end, and then restored to a analog signal through D/A conversion. The following is a brief description of digital encryption. Y1 indicates the digital signal Y1 = 1011101100001, and the 8-bit password c = 10001101 is used. Before sending the password to the transmission line, add it to the voice code. x = Y1 + C (password C repeats continuously ), then, the digital signal transmitted is X = Y1 + c = 1011101100001 Y1 + 1000110110001 C ------------- 0011011010000 x obviously x = Y1. Even if someone steals the X code, they cannot get the Y1 code immediately. At the receiving end, you only need to add the same password C to the digital X to create the original voice digital Y1, that is, Y1 = x + c = 0011011010000 x + 1000110110001 C ------------- 1011101100001 Y1 is visible. Voice digitalization provides very favorable conditions for encryption, and the more digits the password has, the more difficult it is to crack the password. ② Improves the anti-interference capability. Digital signals are mixed with noises during transmission. The threshold voltage (called a threshold) composed of electronic circuits can be used to measure the input signal voltage. Only a certain voltage range is reached, the circuit will have an output value and automatically generate a neat pulse (called shaping or regeneration ). When the voltage of a small noise reaches, It is filtered out because it is lower than the threshold and will not cause circuit action. Therefore, the generated signal is exactly the same as the original signal, and the error code is generated only when the interference signal is greater than the original signal. In order to prevent the error code, a method for checking the error and correcting the error is set in the circuit, that is, when an error code occurs, you can use the backward signal to re-Send it to the other party. Therefore, digital transmission is applicable to long-distance transmission and poor-performance lines. ③ A Comprehensive digital communication network can be built. After the time division switch is adopted, the transmission and exchange can be unified to form an integrated digital communication network. (2) Disadvantages of digital communication: ① It occupies a wide band. Because the line transmits a pulse signal, it takes 20 to transmit a digital voice information? 64 kHz bandwidth, while a analog channel only occupies 4 kHz bandwidth, that is, one PCM signal occupies several analog channels. For a certain road, its utilization rate is reduced, or its requirements for the line are improved. ② Complicated technical requirements, especially high accuracy of synchronization technology. To correctly understand the meaning of the sender, the receiver must correctly distinguish each code element and find the beginning of each information group. This requires the sender and receiver to strictly synchronize the content, if a digital network is formed, it will be more difficult to solve the synchronization problem. ③ When the modulo/number conversion is performed, the quantization error will occur. With the use of large-scale integrated circuits and the popularization of broadband transmission media such as optical fiber, the storage and transmission of information are increasingly using digital signals, therefore, it is necessary to perform a modulo/number conversion on the analog signal, which inevitably produces a quantization error.