QAM
(1) (Quality Assessment Measurement) A system used to measure and analyze voice transmission.
(2) (Quadrature Amplitude Modulation) A modulation technique that employs both phase modulation (PM) and amplitude modulation (AM). Widely used to transmit digital signals such as digital cable TV and cable Internet service, QAM is also used as the modulation technique in orthogonal frequency division multiplexing (see OFDM). The "quadrature" comes from the fact that the phase modulation states are 90 degrees apart from each other.
Analog QAM
Analog QAM uses two carriers 90 degrees out of phase with each other. Each carrier is modulated by an analog signal, and the resulting modulated waves are combined.
Digital QAM
In digital QAM, the input stream is divided into groups of bits based on the number of modulation states used. For example, in 8QAM, each three bits of input, which provides eight values (0-7) alters the phase and amplitude of the carrier to derive eight unique modulation states (see example below). In 64QAM, each six bits generates 64 modulation states; in 128QAM, each seven bits generates 128 states, and so on. See modulation.
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| Analog QAM |
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| Analog QAM modulates two carriers 90 degrees out of phase with each from two analog input streams. The modulated carriers are combined and transmitted. |
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| Digital QAM |
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| In this 8QAM example, three bits of input generate eight different modulation states (0-7) using four phase angles on 90 degree boundaries and two amplitudes: one at 50% modulation; the other at 100% (4 phases X 2 amplitudes = 8 modulation states). QAM examples with more modulation states become extremely difficult to visualize. |
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| Quadrature PSK |
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| Quadrature PSK (QPSK) is like 4QAM without amplitude modulation. QPSK uses four phase angles to represent each two bits of input; however, the amplitude remains constant. |