When the baseband transmission is used, the frequency of the original transmission code is not changed. At this time, the signal contains a wide frequency range, and thus the transmission distance and speed are affected by the quality of the transmission line. The use of baseband transmission can only be used for close range, and the line is simple and economical. A slightly longer transmission can be done by changing the form of the signal, such as increasing the voltage amplitude of "0", "1" level or using differential methods.
Since the frequency range of the general public telephone network is 340HZ-3.4kHz, it is not suitable for baseband transmission. However, wireless communication cannot be used for baseband transmission. In this case, a frequency band transmission method can be adopted, that is, a digital signal is converted into an analog signal transmission by a modem, and the modulated signal can be transmitted by using a telephone line or radio communication, which is usually adopted. Modulation methods include amplitude modulation, frequency modulation, and phase modulation.
In the amplitude modulation mode, "0" and "1" are represented by the amplitude of two different carrier signals, or by the presence or absence of a carrier of constant amplitude. The amplitude modulation is susceptible to sudden interference and is usually used for low speed data. transmission. In the frequency modulation mode, "0" and "1" are represented by two different carrier frequencies, and their anti-interference ability is stronger than amplitude modulation, but the frequency utilization rate is not high. In the phase modulation method, "0" and "1" are represented by different carrier phase changes.
In digital modulation, the digital circuit's switches can be used to control amplitude, frequency, and phase changes, namely amplitude keying (ASK), frequency keying (FSK), and phase keying (PSK).
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