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Modulation
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Modulation


    Transmission of digital data over an analog line is achieved using a technique called modulation, where the digital bit stream is modulated over an analog carrier signal. A modem (modulator and demodulator) is a commonly used device which employs this technique. A modem converts the outgoing digital bit stream from a device into an analog signal and converts the incoming analog signal into a digital bit stream.


Modulation

    Three basic types of modulation are possible (see Figure for a visual comparison)

    1. Amplitude Modulation (AM). In AM, the carrier signal's amplitude is changed according to the modulating digital signal's bit value. For example, two amplitude sizes (a small and a large one) may be used to, respectively, represent bit values 0 and 1. AM's main weakness is its susceptibility to distortion.

    2. Frequency Modulation (FM). In FM, the carrier signal's frequency is changed according to the modulating digital signal's bit value. For example, two frequency values (a low and a high one) may be used to, respectively, represent bit values 0 and 1. FM is more resistant to distortion than AM.


    3. Phase Modulation (PM). In PM, the carrier signal's phase is changed according to the modulating digital signal's bit value. A change in the carrier signal's phase indicates a change in the modulating digital signal's bit value from 0 to 1 or from 1 to 0.

Basic-Modulation