CD-ROM (Compact Disc Read-Only Memory) is an optical disc storage medium that revolutionized data storage and distribution in the late 20th century. Initially developed for audio playback, the technology was adapted to store digital data, allowing for the distribution of software, multimedia content, and extensive data archives.

Structure of a CD-ROM

A CD-ROM is composed of several layers, each serving a specific purpose:

1. Polycarbonate Layer: The base of the disc, made from clear plastic, provides structural support and houses the data.
2. Data Layer: Embedded within the polycarbonate layer, the data layer consists of a spiral track of microscopic pits and lands. Pits are tiny indentations, while lands are the flat areas between pits.
3. Reflective Layer: A thin layer of aluminum or gold is applied above the data layer, reflecting the laser beam used to read data.
4. Protective Coating: A layer of lacquer protects the reflective layer from scratches and environmental damage. A label can be printed on top of this coating.

Components of CD-ROM

The components of a CD-ROM system include various hardware parts that work together to read data from the disc. Here’s a detailed look at each component:

1. Laser Diode

• Function: Emits a laser beam that is focused onto the data layer of the CD-ROM.
• Details: Typically operates at a wavelength of around 780 nm (near-infrared). The laser beam is essential for reading the pits and lands on the disc.

2. Optical Pickup Unit (OPU)

• Function: Houses the laser diode, lens system, and photodetectors. It focuses the laser beam onto the data layer and detects the reflected light.
• Components:
  • Laser Diode: The source of the laser beam.
  • Lens System: Focuses the laser beam onto the data layer and helps in reading the reflected light.
  • Photodetectors: Sensors that detect the intensity of the reflected light and convert it into an electrical signal.

3. Spindle Motor

• Function: Spins the CD-ROM at a controlled speed to ensure the data track is read correctly.
• Details: The motor typically rotates the disc at a constant angular velocity (CAV) or constant linear velocity (CLV), depending on the design of the drive.

4. Tracking Mechanism

• Function: Ensures the laser beam accurately follows the spiral track of the disc.
• Details: Involves servo motors and feedback systems that adjust the position of the laser to maintain alignment with the data track.

5. Focusing Mechanism

• Function: Maintains the laser beam's focus on the data layer to ensure accurate reading.
• Details: Uses feedback from the photodetectors to adjust the position of the lens system to keep it focused on the correct depth of the disc.

6. Data Decoder

• Function: Converts the electrical signal from the photodetectors into binary data by decoding the EFM-encoded patterns.
• Details: Includes a signal processing circuit that interprets the reflected light intensity variations to reconstruct the original data.

7. Error Correction Unit

• Function: Detects and corrects errors in the data read from the CD-ROM.
• Details: Utilizes Cross-Interleaved Reed-Solomon Coding (CIRC) to correct errors and ensure data integrity.

8. Interface Controller

• Function: Manages communication between the CD-ROM drive and the computer’s interface (e.g., ATA, SATA, USB).
• Details: Responsible for transferring the decoded and corrected data from the drive to the computer system.

9. Drive Housing

• Function: Encloses and protects all internal components of the CD-ROM drive.
• Details: Provides structural support and contains the necessary connections for power and data communication.

Working Principle

1 Disc Rotation and Laser Scanning:

• Inside your CD player, there's a small laser and a light detector. When you press play, a motor spins the CD at high speed.
• The laser aims for the light detector. It is set up at an angle such that if there is an mirror in place of the disc then whole the light straightly reflected to the light detector.
• The laser scans the CD from the center to the edge.

2 Laser and Data Reading

• The laser shines onto the underside of the CD, which has a pattern of tiny bumps (pits) and flat areas (lands).
• The flat areas (lands) reflect the laser light directly to the light sensor | detector, without any obstruction. While the bumps (pits) scatter the light.

3 Data Detection

• When the light reaches the light detector, it senses it and sends a signal to the electronic circuit.
• If the light reflects and reaches to the light detector, it means the laser is over a land, and the circuit records a 1. If the light get scatter causing it to be less reflected, it means the laser is over a pit and the circuit records a 0.

Reading Process

1. Laser Emission and Reflection:
   • The laser diode emits a laser beam that is focused onto the data layer.
   • As the laser beam hits the disc, it reflects off the lands and scatters off the pits.
2. Detection of Reflected Light:
   • Photodetectors in the OPU sense the intensity of the reflected light. Strong reflections indicate lands, while weak reflections indicate pits.
3. Signal Processing:
   • The reflected light is converted into an electrical signal.
   • The signal is processed to distinguish between pits and lands, effectively decoding the EFM-encoded data.
4. Error Correction:
   • Using CIRC, the drive's controller corrects any detected errors, ensuring data integrity.
5. Data Output:
   • The decoded and corrected data is sent to the computer's interface (e.g., ATA, SATA, USB), where it can be accessed by the operating system or applications.

Data Representation

• Pits and Lands: Transitions between pits and lands represent binary data. A transition indicates a binary 1, while no transition indicates a binary 0.
• Track Structure: Data is stored in a single continuous spiral track divided into sectors, typically 2,352 bytes each, containing user data, synchronization, and error correction information.