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数字传输系统分为基带传输系统和频带传输系统两大类。数字终端设备编码产生的数字信号（基带信号）直接在信道上传输称为基带传输。频带传输系统也叫数字调制系统，它对基带信号进行调制，使其频谱搬移到适合在信道上传输的频带上，然后在信道上传输。 数字调制与模拟调制都是
Digital transmission system is divided into baseband transmission system and frequency band transmission system two broad categories. Digital terminal equipment encoded digital signal (baseband signal) directly transmitted on the channel called baseband transmission. The band transmission system, also called the digital modulation system, modulates the baseband signal so that its spectrum is shifted to the band suitable for transmission on the channel and then transmitted on the channel. Digital modulation and analog modulation are