Fundamentals Of Modems
The average person's voice frequency range is 300-3400Hz, in order to carry out the voice signal in the ordinary telephone system transmission, on the line to allocate a certain bandwidth, international standards take 4KHz as a standard road to occupy the band width. In this transmission process: voice signal to 300-3400Hz frequency input, the sender's telephone to turn this voice signal into analog signal, this analog signal through a frequency division multiplexing, so that the circuit can transmit multi-channel analog signal at the same time, when it reaches the receiver after a frequency of the process to restore it to the original frequency range of analog signals, and then by the receiver telephone to the analog signal converted to sound signals.
The information in the computer is composed of "0" and "1" digital signals, but on the phone line is transmitted only analog electrical signals. Do not take any measures to use analog channel to transmit digital signals will inevitably occur a lot of errors (distortion), so the ordinary telephone network transmission data, the digital signal must be transformed into the telephone network originally designed to require the audio spectrum within (ie 300Hz-3400Hz).
Modulation is used to control a certain parameter of the wave shape with baseband pulses and form a signal suitable for transmission line.
Demodulation is that when the modulated signal reaches the receiver, the analog signal transformed by the modulator is removed from the carrier to the original baseband digital signal.
The use of modems can also convert audio signals to higher frequencies and to convert higher frequencies to audio signals. Therefore, another purpose of modulation is to facilitate line reuse in order to improve the utilization of the line.
Based on the three main parameters of the carrier signal, the modulation can be divided into three kinds: amplitude modulation, FM and phase modulation.