Quadrature amplitude modulation
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A Modulation Scheme where both Phase and Amplitude are used to encode a symbol. The attached number specifies how many different states can get encoded, e.g. QAM-64 encodes 64 States in the Phase Diagram, QAM-256 encodes 256 States.
The more states are encoded for a given allowed signal Bandwidth the more susceptible the signal is to noise.
Quadrature Amplitude Modulation is usually used for DVB-C and DVB-T, typically QAM-16, QAM-64 or (in finnish cable networks) QAM-128 are used.
U.S. networks also use 8VSB Modulation then and when.