*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 that combines two amplitude-modulated signals into a single channel, effectively allowing for the transmission of more data.

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

- QAM works by varying the amplitude of two carrier waves that are 90 degrees out of phase (also known as quadrature). This allows for the encoding of two different signals simultaneously, effectively increasing the data rate. 
- The two carriers can be thought of as being represented on a constellation diagram, where each point represents a unique combination of amplitudes for the two carriers.

### Incorrect Answer Explanations:
**A: A technique for digital data compression used in digital television which removes redundancy in the data by comparing bit amplitudes.**
- This describes data compression, not modulation. QAM is specifically about modulating signals, not compressing data.

**C: A method of performing single sideband modulation by shifting the phase of the carrier and modulation components of the signal.**
- Single sideband modulation (SSB) is a different technique that reduces bandwidth by eliminating one sideband and the carrier. QAM does not involve this process; it modulates two carriers simultaneously.

**D: A technique for analog modulation of television video signals using phase modulation and compression.**
- While QAM can be used in television signals, this description mixes concepts. QAM modulates amplitude, not just phase, and is not primarily about compression.

### Summary:
- QAM is a powerful modulation technique that allows for efficient data transmission by using two carriers that are out of phase.
- Understanding the distinction between modulation techniques is crucial for the exam.

### Useful Concepts:
- **Constellation Diagram**: A graphical representation of the modulation scheme, showing how different amplitude and phase combinations correspond to different data values.
- **Bandwidth Efficiency**: QAM is known for its ability to transmit more bits per symbol, making it bandwidth efficient compared to simpler modulation schemes. 

This understanding of QAM will help you in both the exam and practical applications in amateur radio.