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Signal modulation and demodulation technology

Source: Shangpin China | Type: website encyclopedia | Time: June 20, 2014
Digital transmission is superior to analog transmission in many aspects. Even analog data can be converted into digital data, encoded into digital signals, and then transmitted on digital channels. However, there are a lot of Website production Communication uses analog channel technology. Therefore, in an analog transmission system, how digital signals are modulated (demodulated in reverse) into analog signals is briefly introduced as follows.

1. Modulation/demodulation

Modulation: The process of converting a digital signal into an analog signal.

Demodulation: The process of converting analog signals into digital signals.

For example, if the telephone line is used to connect to the Internet, the necessary equipment for network communication, namely Modem, is a device with waveform transformation and waveform recognition, which can "modulate" the digital signal output by the computer into a pulse signal that can be transmitted on ordinary telephone lines; At the other end of the line, it is "demodulated" by another Modem as a digital signal.

2. Modulation mode

The basis of analog signal transmission is carrier wave, which has three major elements: amplitude, frequency and phase.

Modulation mode: a fixed frequency sine wave is used as the carrier signal, and the digital signal to be transmitted is used as the control signal to make certain physical characteristics (such as amplitude, frequency or phase) of the carrier signal change accordingly according to the control signal. Based on the three elements of carrier signal, there are roughly three traditional modulation methods, as shown in Figure 2-4.
The basis of analog signal transmission is carrier wave
(1) Amplitude Modulation (AM): refers to that the amplitude of the carrier changes with the change of the digital signal. For example, digital signal 1 corresponds to carrier output, digital signal. Corresponding to no carrier output. Amplitude modulation can also use the amplitudes of different carrier signals to represent 1 and of digital signals For example, the high amplitude carrier signal represents 1, and the low amplitude carrier signal represents 0.

(2) Frequency Modulation (FM): refers to the change of carrier frequency with the change of digital signal. For example, digital signal 1 corresponds to carrier high frequency: digital signal 0 corresponds to carrier low frequency medium.

(3) Phase Modulation (PM): refers to that the initial phase of the carrier changes with the change of the digital signal. If the phase change is 180 degrees, it means 1, and the phase change. Degree (i.e. no change) This modulation mode with only two phases (such as 0 degrees or 180 degrees) is called two-phase modulation. In order to improve the transmission rate of information, there are four phase modulation and/or phase modulation in practical applications. The digital information and phase distribution of the four phase and eight phase modulation modes are shown in Figure 2-5.

The digital information and phase distribution of its four phase and eight phase modulation modes are shown in the figure


It can be seen from Figure 2-5 that four phase modulation uses four different phases to represent 00, 01, 10, 11 respectively, or each modulation can transmit two bits of information; Eight phase modulation can transmit 3 bits of information per modulation, obviously both of them improve the transmission rate of information.

With the development of high-capacity and long-distance digital communication technology, the three traditional digital modulation methods mentioned above can no longer meet the needs of practical applications. In order to achieve a more efficient information transmission rate, it is necessary to adopt more complex, multi system and hybrid modulation methods in technology, which will not be described here.
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