Quadrature amplitude modulation

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The Quadrature Amplitude Modulation or QAM is 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 following pictures give you an idea what happens.

Principle of QAM64 Principle of QAM256

The more states are encoded for a given allowed signal Bandwidth the more susceptible the signal is to noise. On the other hand, the more states are possible the more bandwith is in theory possible. But only if the signal quality is good enough to decide between the different states because amplitude noise and phase noise will disturb the signal.

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 seem to use 8VSB Modulation then and when.