Development premise
Language is the first carrier of human transmission of information, is a tool for social communication and exchange of ideas, and is the most convenient, most complex, most common and most important in human society Information carrier system. With the development of production and the continuous progress of society, the second carrier of information appeared: text. There are more than 500 characters in use in the world. The invention of writing has made it possible for the storage (recording) and long-distance transmission of information, which is a major advancement for mankind. The invention of telegraph, telephone, and radio enabled a large amount of information to be transmitted at the speed of light, communicating the connection of the entire world, and human information activities have entered a new era. With the huge increase in the amount of information and the extensive exchange of information, a larger capacity information carrier is required. New information delivery vehicles such as computers, optical fibers, and communication satellites have become the main information carriers in the context of the new technological revolution. An optical fiber with a hair thickness can transmit hundreds of thousands of telephones or thousands of TVs at the same time. The new information carrier will surely lead to a new information revolution.
Introduction
Information itself is not an entity, but the content contained in messages, intelligence, instructions, data, and signals, which must be transmitted through a certain medium. The evolution of information carriers promotes the development of human information activities. In a sense, the signal revolution is the revolution of information carriers.
System development
Mankind began to use language in the primitive age. There are about 3,500 spoken languages in the world. Language is the first carrier of human transmission of information, and it is the means of social communication and exchange of ideas. Tools are the most convenient, most complex, most versatile, and most important information carrier system in human society. With the development of production and the continuous progress of society, the second carrier of information-text appears.
There are more than 500 characters in use in the world. The invention of writing has made it possible for the storage (recording) and long-distance transmission of information, which is a major advancement for mankind. The invention of telegraph, telephone, and radio enabled a large amount of information to be transmitted at the speed of light, communicating the connection of the entire world, and human information activities have entered a new era.
Electromagnetic waves and electrical signals have become the third information carrier of mankind. With the rapid increase in the amount of information and the extensive exchange of information, a larger capacity information carrier is required. New information delivery vehicles such as computers, optical fibers, and communication satellites have become the main information carriers in the context of the new technological revolution. An optical fiber with a hair thickness can transmit hundreds of thousands of telephones or thousands of TVs at the same time. Satellite communications can send information to any corner of the world. The new information carrier will surely lead to a new information revolution.
Signal
A signal is a physical quantity that represents a message. For example, an electrical signal can represent different messages through changes in amplitude, frequency, and phase. This kind of electric signal has two kinds of analog signal and digital signal. A signal is a tool for carrying messages and a carrier of messages. Broadly speaking, it includes optical signals, acoustic signals and electrical signals. According to the actual use, the signals include TV signals, broadcast signals, radar signals, communication signals, etc.; according to their time characteristics, there are deterministic signals and random signals.
Signal is a tool for carrying messages and a carrier of messages. Broadly speaking, it includes optical signals, acoustic signals and electrical signals. For example, the ancient people used the billowing smoke generated by lighting the beacon to transmit the news of the enemy invasion to the distant army. This is a light signal; when we speak, sound waves are transmitted to the ears of others, so that others understand our intentions. This belongs to sound. Signal;
Various radio waves traveling in space, electric current in the telephone network extending in all directions, etc., can be used to express various messages to a distant place. This is an electrical signal. People only know what the other party wants to express by receiving light, sound, and electrical signals.
Signal classification
There are many ways to classify signals. The signal is based on mathematical relationships, value characteristics, energy power, processing analysis, time function characteristics, and whether the value is a real number. It can be divided into deterministic signals and non-deterministic signals (also known as random signals), continuous signals and discrete signals (ie analog signals and digital signals), energy signals and power signals, time domain signals and frequency domain signals, time-limited signals Sum frequency limited signal, real signal and complex signal, etc.
Analog signal and digital signal
Analog signal refers to the signal waveform that simulates changes in information. Its main feature is that the amplitude is continuous, which is desirable Unlimited multiple values; and in time, it can be continuous or discontinuous.
Digital signal refers to a signal that is not only discrete in time, but also discrete in amplitude, and can only take a limited number of values. For example, telegraph signals, pulse code modulation (PCM, Pulse Code Modulation) signals, etc. are all digital signals. Binary signal is a kind of digital signal, which is represented by different combinations of two digits "1" and "0" to represent different information.
People divide the communication system into an analog communication system and a digital communication system according to whether the communication system transmits an analog signal or a digital signal. If the analog signal is sent into the transmission system, this communication method is analog communication. Most of the telephone and radio and television systems used today are analog communication methods.
If the analog signal is sampled, quantized, coded and converted into a digital signal before transmission, then this communication method is digital communication.
Compared with analog communication, although digital communication occupies a wider channel frequency band, it has strong anti-interference ability, no noise accumulation, convenient storage, processing and exchange, strong confidentiality, and easy large-scale integration. The realization of miniaturization and other advantages is becoming more and more widely used.
There can be mutual conversion between analog and digital signals: analog signals are generally quantized into digital signals by PCM pulse code modulation (Pulse Code Modulation), which means that different amplitudes of analog signals correspond to different binary values. For example, the use of 8-bit encoding can quantize the analog signal into 2^8=256 magnitudes. In practice, 24-bit or 30-bit encoding is often used; digital signals are generally converted into analog signals by phase shifting the carrier (Phase Shift) . Binary digital signals are used in computers, computer local area networks, and metropolitan area networks. In the computer wide area network, there are both binary digital signals and analog signals converted from digital signals. However, digital signals are more promising for application development.