2010年1月3日 / 96次阅读
In electronics, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with respect to a modulating signal (which typically contains information to be transmitted).
This is done in a similar fashion to a musician modulating a tone (a periodic waveform) from a musical instrument by varying its volume, timing and pitch. The three key parameters of a periodic waveform are its amplitude ("volume"), its phase ("timing") and its frequency ("pitch"), all of which can be modified in accordance with a low frequency signal to obtain the modulated signal. Typically a high-frequency sinusoid waveform is used as carrier signal, but a square wave pulse train may also occur.
A modem's purpose is to convert digital information to analog signals (modulation), and to convert analog signals back into useful digital information (demodulation).
A Brief Introduction to Modem Technology
[Kwa:m] / /kæm/ Quadrature amplitude modulation.
Quadrature amplitude modulation, or QAM, is a big name for a relatively simply technique. It is simply a combination of amplitude modulation and phase shift keying.
The most common forms are 16-QAM, 64-QAM and 256-QAM. By moving to a higher-order constellation, it is possible to transmit more bits per symbol.
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