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

Storage Devices and Media – Optical Discs

Learning Objective

Know and understand the characteristics, uses, advantages, disadvantages and safe handling of the three main families of storage devices (magnetic, optical, solid‑state) with a particular focus on optical discs: CD, DVD and Blu‑ray.

1. Overview of Storage‑Device Families

Modern computers use three broad types of storage. Each family has distinct physical media, typical capacities, speed characteristics and handling requirements.

2. What Is an Optical Disc?

An optical disc stores data as a continuous spiral of microscopic pits (represent binary 0) and lands (binary 1) on a reflective polymer layer. A low‑power laser in the drive shines on the disc; a photodiode detects changes in reflected light intensity and converts them back into digital data.

3. Types of Optical Discs

3.1 Compact Disc (CD)

• Original purpose: Audio; later adapted for data (CD‑ROM, CD‑R, CD‑RW).
• Physical size: 120 mm diameter, 1.2 mm thick.
• Capacity: 700 MB (≈ 80 min of audio).
• Laser wavelength: 780 nm (infra‑red).
• Speed: 1× = 150 KB s⁻¹. Commercial drives commonly support 1×‑48× for reading; write speeds are usually lower (e.g., CD‑R 24×, CD‑RW 16×).
• Typical uses: Music albums, software distribution, small backups.

3.2 Digital Versatile Disc (DVD)

• Purpose: Video and larger data files.
• Physical size: 120 mm (same as CD).
• Capacity options:

  • Single‑layer, single‑sided (DVD‑5): 4.7 GB
  • Dual‑layer, single‑sided (DVD‑9): 8.5 GB
  • Dual‑layer, double‑sided (DVD‑18): 17 GB

• Laser wavelength: 650 nm (red).
• Speed: 1× = 1.385 MB s⁻¹. Typical commercial drives read at 8×‑16×; write speeds are lower (e.g., DVD‑R 8×, DVD‑RW 4×).
• Typical uses: Movies, video games, larger software packages.

3.3 Blu‑ray Disc (BD)

• Purpose: High‑definition video and high‑capacity data storage.
• Physical size: 120 mm (same as CD/DVD).
• Capacity options:

  • Single‑layer, single‑sided (BD‑R/BD‑RE): 25 GB
  • Dual‑layer, single‑sided (BD‑XL): 50 GB
  • Triple‑layer (BD‑XL‑3L): 100 GB (less common)

• Laser wavelength: 405 nm (blue‑violet).
• Speed: 1× = 4.5 MB s⁻¹. Common commercial drives read at 4×‑12×; write speeds are typically 4×‑8× for BD‑R.
• Typical uses: 1080p/4K movies, large software suites, archival backups.

Why Laser Wavelength Matters

Data density on an optical disc is limited by the diffraction limit of the laser light. Shorter wavelengths (e.g., 405 nm for Blu‑ray) can focus to a smaller spot, allowing pits to be packed more closely together and therefore increasing the disc’s capacity. This principle explains the progression from CD (780 nm) → DVD (650 nm) → Blu‑ray (405 nm).

Real‑World Example for Capacity Choice

A school video project that records 2 hours of 1080p footage requires roughly 6 GB of storage. This exceeds a single‑layer DVD (4.7 GB) but fits comfortably on a single‑layer Blu‑ray (25 GB). The example illustrates when a higher‑capacity optical medium is justified.

4. Media Characteristics Summary

*All longevity figures assume storage in a clean, dry environment at 15‑25 °C and < 70 % relative humidity.

5. Advantages & Disadvantages – Three‑Column Comparison

6. Handling and Care of Optical Discs

• Storage: Keep discs in jewel cases or anti‑static sleeves, away from direct sunlight. Ideal environment: 15‑25 °C, < 70 % relative humidity.
• Handling: Touch only the centre hub or the outer edge. Never touch the data surface – fingerprints alter reflectivity and cause read errors.
• Cleaning: Use a soft, lint‑free cloth. Wipe in a straight line from the centre outward; do not use circular motions or abrasive cleaners.
• Scratch protection: Store discs vertically (spine down) to minimise edge damage. Avoid stacking heavy objects on top of a disc stack.
• Labeling: Write on the non‑reflective side with a permanent‑ink marker. Avoid adhesive stickers that can unbalance the disc during rotation.
• Temperature & humidity: Do not expose discs to temperatures below 0 °C or above 60 °C, and avoid rapid humidity changes which can cause warping.
• Read vs Write speeds: For CD‑R/RW, DVD‑R/RW and BD‑R/RE the write speed is usually a fraction of the maximum read speed (e.g., a DVD‑R 8× write ≈ 11 MB s⁻¹, while DVD read at 16× ≈ 22 MB s⁻¹). When selecting a medium for a task, consider the required *write* speed as well as the *read* speed.

7. Safety & Handling of All Storage Media

• Always power down a computer before inserting or removing any removable storage device.
• Magnetic media (HDDs, tapes): Handle by the edges, avoid dropping, and keep away from strong magnets.
• Solid‑state media: Static discharge can damage flash cells. Ground yourself (e.g., touch a metal part of the chassis) or wear an anti‑static wrist strap. Work on an anti‑static mat when handling bare circuit boards.
• Backup strategy (AO2 requirement): Keep at least two copies of important data on different media types (e.g., one on an optical disc and another on an SSD) and store them in separate locations.
• Dispose of obsolete media responsibly – recycle according to local e‑waste guidelines.

8. Summary

Optical discs (CD, DVD, Blu‑ray) remain valuable for distributing audio, video and moderate‑size data sets because they are inexpensive, portable and have a long shelf‑life when cared for correctly. Understanding how they compare with magnetic (hard‑disk, tape) and solid‑state (SSD, flash) storage helps learners select the most appropriate medium for a given task, balance cost against capacity and speed, and apply safe handling practices to preserve data integrity.