What does Quadrature Amplitude Modulation (QAM) combine?

Quadrature Amplitude Modulation (QAM) combines amplitude and phase variations to transmit data efficiently. In QAM, two amplitude-modulated signals are sent simultaneously, each out of phase with the other by 90 degrees—hence the term "quadrature." This allows for the representation of multiple bits of information per symbol, making QAM highly effective for digital communications.

By varying the amplitude of both signals, QAM can encode a greater number of bits into the transmitted signal compared to simpler modulation schemes. This combination of phase and amplitude manipulation enables QAM to achieve higher data rates while maintaining a reasonable level of resilience against noise and interference, which is crucial in communication systems.

The other options do not accurately describe the functionality of QAM: it does not exclusively rely on frequency variations, solely amplitude changes, nor does it pertain directly to a combination of digital and analog signals. Instead, QAM is rooted in its unique ability to manipulate both amplitude and phase, which is fundamental to its design and application in varying communication scenarios.