The average human voice frequency range is 300-3400 Hz. In order to transmit the voice signal in an ordinary telephone system, it is allocated a certain bandwidth on the line. The international standard takes 4KHz as the bandwidth occupied by a standard voice channel. In this transmission process: the voice signal is input at a frequency of 300-3400 Hz, and the transmitter's telephone converts the voice signal into an analog signal, and the analog signal is changed by a frequency division multiplexer so that the line can be simultaneously transmitted. The multi-channel analog signal, after reaching the receiving end, is restored to the original frequency range of the analog signal by a frequency-resolving process, and then the receiver's telephone converts the analog signal into a sound signal. [1] The information in the computer is composed of "0" and "1", and only the analog electric signal is transmitted on the telephone line. If you do not take any measures to use the analog channel to transmit digital signals, there will inevitably be a lot of errors (distortion). Therefore, when transmitting data on a normal telephone network, the digital signals must be converted into the audio spectrum required by the original design of the telephone network (ie, 300 Hz). -3400Hz). Modulation is the use of baseband pulses to control a certain parameter of the carrier waveform to form a signal suitable for line transmission. Demodulation means that when the modulated signal arrives at the receiving end, the analog signal transformed by the modulator is removed and the carrier is restored to the original baseband digital signal. It is also possible to convert an audio signal into a higher frequency signal and a higher frequency signal into an audio signal using a modem. Therefore, another purpose of modulation is to facilitate line multiplexing in order to improve line utilization. Based on the three main parameters of the carrier signal, the modulation method can be divided into three types: amplitude modulation, frequency modulation and phase modulation.
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