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

Feature	CD	D \cdot D	Blu‑ray
Laser wavelength	780 nm (infra‑red)	650 nm (red)	405 nm (blue‑violet)
Typical single‑layer capacity	$700\text{ MB}$	$4.7\text{ GB}$	$25\text{ GB}$
Maximum single‑sided capacity	$700\text{ MB}$	$8.5\text{ GB}$ (dual‑layer)	$128\text{ GB}$ (quad‑layer)
Standard 1× data rate	150 KB/s	1.385 MB/s	4.5 MB/s
Typical uses	Audio, small software	Video movies, software distribution	HD/4K video, large backups, game consoles
Typical drive compatibility	CD‑only, CD/D \cdot D, CD/D \cdot D/BD	CD/D \cdot D, CD/D \cdot D/BD	CD/D \cdot D/BD (requires Blu‑ray drive)

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.