Signal data can be used to represent any information, such as symbols, text, voice, images, etc., and can be divided into two categories in terms of expression: analog signals and digital signals. The difference between analog signals and digital signals can be determined based on whether the amplitude is discrete or not.

An analog signal means that the amplitude value is continuous (the amplitude can be represented by an infinite number of values). Time-continuous analog signals, continuously changing image (TV, fax) signals, etc., as shown in Figure 1-1(a). A time-discrete analog signal 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 time T. Although its waveform It is discontinuous in time, but its amplitude value is continuous, so it is still an analog signal, which is called pulse amplitude modulation (PAM, referred to as pulse amplitude modulation) signal.

Digital signals refer to amplitude values ??that are discrete, and the amplitude representation is limited to a limited number of values. Binary code is a digital signal. Binary codes are less affected by noise and can be easily processed by digital circuits, so they have been widely used.

1. Analog Communication

Analog communication has the advantage of being intuitive and easy to implement, but it has two major disadvantages.

(1) Poor confidentiality

Analog communications, especially microwave communications and wired open-wire communications, are easily eavesdropped. As long as the analog signal is received, the communication content is easily obtained.

(2) Weak anti-interference ability

During the transmission process along the line, electrical signals will be interfered by various noises from the outside and within the communication system. It is difficult to mix the noise and signals. separation, resulting in poor communication quality. The longer the line, the more noise accumulates.

2. Digital communication

(1) The advantages of digital transmission and exchange

① Enhance the confidentiality of communication. After the voice signal is A/D converted, it can be encrypted first and then transmitted. After being decrypted at the receiving end, it can be restored to an analog signal through D/A conversion.

The digital encryption process can be briefly described as follows. Y1 represents the digital signal Y1=1011101100001 turned into by voice, and uses an 8-bit password C=10001101. Before sending to the transmission line, the password is "added" to the voice code, X=Y1+C (password C is repeated continuously), then the transmitted digital signal is

X=Y1+C=1011101100001 Y1

＋1000110110001 C

——————————————

0011011010000 X

Obviously X≠Y1, even if someone eavesdrops Even if you reach code X, you can’t get code Y1 right away. At the receiving end, as long as the same password C is added to the digital number /p>

——————————————

1011101100001 Y1

It can be seen that voice digitization provides very favorable conditions for encryption processing, and The more digits there are in a password, the more difficult it is to decipher.

② Improved anti-interference ability. Digital signals will be mixed with noise during the transmission process. The threshold voltage (called the threshold) formed by the electronic circuit can be used to measure the input signal voltage. Only when a certain voltage amplitude is reached, the circuit will have an output value and automatically generate a neat Impulse (called shaping or regeneration). When the smaller noise voltage arrives, it is filtered out because it is lower than the threshold and will not cause circuit action. Therefore, the regenerated signal is exactly the same as the original signal, and bit errors will occur unless the interference signal is larger than the original signal. In order to prevent bit errors, error checking and error correction methods are set up in the circuit. That is, when a bit error occurs, the backward signal can be used to cause the other party to retransmit. Therefore, digital transmission is suitable for longer distance transmission and can also be applied to lines with poor performance.

③ A comprehensive digital communication network can be constructed. After adopting time division switching, transmission and switching are unified to form a comprehensive digital communication network.

(2) Disadvantages of digital communication

① Occupies a wider frequency band. Because the line transmits pulse signals, transmitting one channel of digital voice information requires a bandwidth of 20 to 64 kHz, while an analog voice channel only occupies a 4 kHz bandwidth, that is, one PCM signal occupies several analog voice channels. For a certain session, its utilization rate has decreased, or its line requirements have increased.

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 code element and find the beginning of each information group. This requires both the sender and the receiver to strictly synchronize. If a digital network is formed, Synchronization issues will be more difficult to resolve.

③ Quantization error will occur when performing analog/digital conversion.

With the use of large-scale integrated circuits and the popularization of broadband transmission media such as optical fibers, digital signals are increasingly being used to store and transmit information. Therefore, analog signals must be converted from analog to digital. Quantization errors inevitably occur.