QAM, or Quadrature Amplitude Modulation, is a type of modulation scheme. In QAM, data is conveyed by changing the amplitude of two carrier waves through modulation.
The two carrier waves involved in QAM are normally sinusoids, functions that describe a wavelike quality of time. The QAM carrier waves are out of phase with each other by around 90 degrees, hence the name.
The structure of QAM can be seen in a setting that includes a transmitter and a receiver. The QAM transmitter first splits into two parts the flow of bits it will transmit. These parts should always be equal to ensure that two independent signals would be transmitted. Through this process, a 90-degree phase is created between the two channels.
The receiver then accepts the signals sent by the transmitter. Once it takes in the signals, the receiver carries out an inverse version of the transmitter’s process. There is, however, a phase delay occurring between the transmitter and the receiver. This is compensated for by the receiver’s synchronization.
QAM has a number of forms:
- Analog QAM, for instance, is used in PAL and NTSC television systems;
- Compatible QAM is used in AM stereo radio. In this system, Compatible QAM is used to carry information regarding the stereo difference;
- Quantized QAM is used in certain hardware devices such as cable modems and digital cable television.