Analog signal means that the amplitude value is continuous (amplitude can be expressed by infinite value). Time-continuous analog signals and continuously changing image (TV, fax) signals are shown in figure 1- 1(a). A discrete analog signal in time is a sampling signal, as shown in figure 1- 1(b). It is a signal obtained by sampling the analog signal in figure 1- 1(a) every t. Although its waveform is discontinuous in time, its amplitude is continuous, so it is still an analog signal.
Digital signal refers to the discrete value of amplitude, and the amplitude representation is limited to a limited number of values. A binary code is a digital signal. Binary coding is less affected by noise and easy to be processed by digital circuits, so it has been widely used.
1. Analog communication
The advantages of analog communication are intuitive and easy to realize, but there are two main disadvantages.
(1) has poor security.
Analog communication, especially microwave communication and wired communication, is easily eavesdropped. As long as the analog signal is received, it is easy to get the communication content.
(2) Weak anti-interference ability
In the process of electric signal transmission along the line, it will be interfered by various noises inside and outside the communication system, 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.
2. Digital communication system
Advantages of (1) digital transmission and exchange
① The confidentiality of communication is strengthened. 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.
The digital encryption process can be simply described as: Y 1 digital signal y1=1011100001,with 8-bit cipher C. Before sending it to the transmission line, the password is "added" to the voice code, X = Y 1+C (password c is repeated constantly), so the transmitted digital signal is
x = y 1+C = 10 1 10 1 10000 1y 1
+100011kloc-0/10001celsius.
—————————————
00 1 10 1 10 10000 X
Obviously, X≠Y 1, even if someone eavesdrops on the X code, it is impossible to get the Y1code immediately. At the receiving end, as long as the same password C is added to the number X, the original phonetic symbol number Y 1 can be erected, that is
y 1 = X+C = 00 1 10 1 10 10000 X
+100011kloc-0/10001celsius.
—————————————
10 1 1 10 10000 1y 1
It can be seen that the digitization of speech provides a very favorable condition for encryption processing, and the more password digits, the more difficult it is to decipher the password.
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) occupies a wide frequency band. Because the line transmits pulse signals, it takes 20? 64kHz bandwidth, while an analog channel only occupies 4kHz bandwidth, that is, a PCM signal occupies several analog 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.