*Question E8C01:
What is Quadrature Amplitude Modulation or QAM?*  

*Correct Answer is  B: Transmission of data by modulating the amplitude of two carriers of the same frequency but 90 degrees out of phase*  

### AI Response: (may contain errors)  

Quadrature Amplitude Modulation (QAM) is a modulation technique used to transmit data by varying the amplitude of two carrier waves. These carriers have the same frequency but are 90 degrees out of phase with each other. This is why the correct answer is:

**B: Transmission of data by modulating the amplitude of two carriers of the same frequency but 90 degrees out of phase**

### Explanation:

1. **Correct Answer (B):**
   - QAM combines two amplitude-modulated signals into a single channel, effectively using two carriers that are in quadrature (90 degrees apart). This allows for the transmission of more data over the same bandwidth compared to simple amplitude modulation (AM).

2. **Incorrect Choices:**
   - **A:** This choice describes data compression, not modulation. QAM is not about removing redundancy but about efficiently using bandwidth.
   - **C:** This describes a method related to single sideband (SSB) modulation, not QAM. QAM involves two carriers, not just phase shifting.
   - **D:** This choice incorrectly associates QAM with analog modulation of video signals using phase modulation and compression, which is not how QAM operates.

### Useful Concepts:

- **Quadrature:** Refers to the 90-degree phase difference between the two carriers. This phase difference allows the two signals to be combined without interfering with each other.
  
- **Amplitude Modulation:** In QAM, each carrier's amplitude is modulated independently, allowing for the representation of multiple bits per symbol.

- **Constellation Diagram:** A graphical representation of QAM signals, where each point represents a unique combination of amplitudes for the two carriers. More points allow for more data to be transmitted per symbol.

QAM is widely used in digital communication systems, including cable television and broadband internet, due to its efficient use of bandwidth.