Signal data can be used to represent any information, such as symbols, characters, sounds, images and so on. , can be divided into analog signals and digital signals. The difference between analog signal and digital signal can be determined according to whether the amplitude value is discrete or not. Analog signal means that the amplitude value is continuous (amplitude can be expressed by infinite value). Time-discrete analog signal is a kind of sampling signal, which is obtained by sampling the analog signal every t. Although its waveform is discontinuous in time, its amplitude is continuous, so it is still an analog signal, which is called pulse amplitude modulation (PAM) signal. Usually, the sound we hear and the TV images we see are analog signals. The conversion from analog signal to digital signal generally goes through three processes: sampling, quantization and coding, and finally becomes a series of pulse digital signals represented by 0 and 1. Digital signals were first used in communication, which triggered a revolution in the field of communication. 1. Analog communication: The advantages of analog communication are intuitive and easy to realize, but it also has important disadvantages. (1) Analog communication with poor security, especially microwave communication and wired communication, is easy to be eavesdropped. As long as the analog signal is received, it is easy to get the communication content. (2) The electric signal with weak anti-interference ability will be interfered by various noises inside and outside the communication system during transmission along the line, and it is difficult to separate the noise from the signal after mixing, thus reducing the communication quality. The longer the line, the more noise is accumulated. Everyone has experience: every time you copy tapes and videos, the quality of sound and images is poor, which is why. 2. Advantages of digital communication (1) Digital transmission and exchange ① enhance 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 analog signal through D/A conversion. ② Anti-interference ability is improved. Digital signals will be mixed with noise during transmission. The input signal voltage can be measured by mosfet (called threshold) composed of electronic circuits. Only when the voltage reaches a certain amplitude, the circuit will have an output value and automatically generate neat pulses (called shaping or regeneration). When the small noise voltage comes, it will be filtered out because it is lower than the threshold value, and it will not cause circuit action. Therefore, the regenerated signal is exactly the same as the original signal, and only when the interference signal is greater than the original signal will an error code be generated. In order to prevent the error code, the method of error detection and correction is set in the circuit, that is, when the error code occurs, the opposite party can be retransmitted by using the reverse signal. Therefore, digital transmission is suitable for long-distance transmission and lines with poor performance. ③ A complete digital communication network can be built. After adopting time division switching, transmission and switching are unified, and a complete digital communication network can be formed. (2) Disadvantages of digital communication ① It occupies a wide frequency band. Because the line transmits pulse signals, it needs 20-64kHz bandwidth to transmit digital voice information, while an analog voice channel only occupies 4kHz bandwidth, that is, a PCM signal occupies several analog voice channels. For a certain telephone line, its utilization rate has decreased, or its requirements for the line have increased in detail. ② The technical requirements are complex, especially the synchronization technology requires high precision. If the receiver can correctly understand the meaning of the sender, it must correctly distinguish each symbol and find the beginning of each information group, which requires strict synchronization between the sender and the receiver. If a digital network is formed, it will be more difficult to solve the synchronization problem. ③ The analog-to-digital conversion will bring quantization error. With the use of large-scale integrated circuits and the popularization of broadband transmission media such as optical fiber, more and more digital signals are used for information storage and transmission, so analog signals must be converted from analog to digital, and quantization errors will inevitably occur in the conversion. After the communication is converted from analog to digital, the benefits are well known: in the past, when the analog crossbar switch was used, the connection rate between a local telephone office was less than 20%, and when the rural cable broadcasting was opened, the rural telephone could not be called, all of which were interference sounds ... After using the all-digital program-controlled switch and digital transmission circuit, the telephone connection rate was greatly improved, and the transmission quality was almost inaudible, forming a local area with prefecture-level cities as the unit. A few years later, the digitalization of television was another revolution.