Know and understand optical drives including CD, DVD, Blu-ray

Storage Devices and Media – Optical Drives

Learning Objective

Know and understand the principles, formats and applications of optical drives, specifically CD, D \cdot D and Blu‑ray.

1. What is an Optical Drive?

An optical drive reads and writes data by using a laser beam to detect the pattern of pits and lands on the surface of a disc. The key components are:

• Laser diode (different wavelengths for each format)
• Rotating spindle motor
• Photodiode detector
• Optical pickup assembly

2. How Data Is Stored on an Optical Disc

1. The disc surface is coated with a reflective layer.
2. During recording, the laser creates microscopic pits (low reflectivity) and lands (high reflectivity).
3. During playback, the laser’s reflected light is measured; changes between pits and lands are interpreted as binary data.

3. CD (Compact Disc)

Developed in the early 1980s for audio, later adapted for data (CD‑ROM, CD‑R, CD‑RW).

• Laser wavelength: 780 nm (infra‑red)
• Standard capacity: \$700\text{ MB}\$ (≈ \$80\$ min of audio)
• Data transfer rate: 150 KB/s (1× speed); higher speeds are multiples of this base rate.
• Physical structure: single layer, 1.2 mm thickness, 120 mm diameter.

4. D \cdot D (Digital \cdot ersatile Disc)

Introduced in the mid‑1990s, offering higher capacity by using a shorter‑wavelength laser and tighter track spacing.

• Laser wavelength: 650 nm (red)
• Typical capacities:

  • Single‑layer, single‑sided (D \cdot D‑5): \$4.7\text{ GB}\$
  • Dual‑layer, single‑sided (D \cdot D‑9): \$8.5\text{ GB}\$
  • Double‑sided variants (D \cdot D‑10, D \cdot D‑18) double these amounts.

• Standard data rate: 1.385 MB/s (1× speed); 1× D \cdot D speed equals \$1.32\text{ MB/s}\$ of raw data.
• Can store video, software, and large data sets.

5. Blu‑ray Disc (BD)

Designed for high‑definition video and large data volumes, using a blue‑violet laser.

• Laser wavelength: 405 nm (blue‑violet)
• Typical capacities:

  • Single‑layer, single‑sided (BD‑R/RE): \$25\text{ GB}\$
  • Dual‑layer, single‑sided (BD‑R DL/RE DL): \$50\text{ GB}\$
  • Triple‑layer (BD‑XL): \$100\text{ GB}\$
  • Quad‑layer (BD‑XL): \$128\text{ GB}\$

• Standard data rate: 36 Mbps (≈ \$4.5\text{ MB/s}\$) for 1× speed; commercial drives often operate at 2×–6×.
• Supports HD video (1080p) and Ultra‑HD (4K) with appropriate codecs.

6. Comparison of CD, D \cdot D and Blu‑ray

7. Advantages and Disadvantages of Optical Media

• Advantages

  • Portable and relatively inexpensive.
  • Read‑only formats (CD‑ROM, D \cdot D‑ROM) provide protection against accidental modification.
  • Long‑term archival potential when stored properly (cool, dry, dark).
  • Standardised formats – compatible across many devices.

• Disadvantages

  • Limited capacity compared with modern flash or HDD solutions.
  • Susceptible to scratches, dust, and U \cdot degradation.
  • Write speeds are slower than solid‑state media.
  • Physical discs add to waste and require careful handling.

8. Summary Checklist for Revision

1. Identify the laser wavelength for each format and explain why a shorter wavelength allows more data.
2. Recall the standard single‑layer capacities: \$700\text{ MB}\$ (CD), \$4.7\text{ GB}\$ (D \cdot D), \$25\text{ GB}\$ (Blu‑ray).
3. Explain the concept of “layers” and how they increase capacity.
4. List at least three typical applications for each type of disc.
5. Compare the 1× data rates and discuss the impact on read/write performance.