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 (CD, DVD and Blu‑ray) and be able to compare them with magnetic and solid‑state storage.

1. What is an optical drive?

An optical drive reads and writes data by using a laser beam to detect (and create) a pattern of pits and lands on the surface of a rotating disc.

• Laser diode – wavelength determines the minimum spot size; a shorter wavelength → smaller spot → more data per unit area.
• Spindle motor – rotates the disc at a constant speed (≈ 200‑500 rpm for CD, up to 12 000 rpm for Blu‑ray).
• Optical pickup assembly – contains the laser, lenses (numerical aperture), and a photodiode detector.
• Controller electronics – convert the reflected‑light signal into binary data and manage error‑correction.

2. How data is stored on an optical disc

1. The disc is coated with a thin reflective layer (aluminium or gold).
2. During recording a high‑power laser melts (recordable) or alters (rewritable) the coating to form pits (low reflectivity) surrounded by lands (high reflectivity).
3. During playback a low‑power laser reads the pattern; the photodiode measures changes in reflected light, which are decoded as binary 0 / 1.

3. Technical characteristics common to all optical media

• Disc size & thickness: 120 mm diameter, 1.2 mm thick (standard). Mini‑discs (80 mm) are also used for CD.

Error‑correction codes (ECC)

• CD – CIRC (Cross‑Interleaved Reed‑Solomon Code): detects and corrects burst errors caused by scratches or dust.
• DVD – RSPC (Reed‑Solomon Product Code) with EDC/ECC: provides stronger correction for higher‑density data.
• Blu‑ray – LDPC + BCH (Low‑Density Parity‑Check combined with Bose‑Chaudhuri‑Hocquenghem): offers very high error‑resilience needed for high‑definition video.

4. Optical disc formats

4.1 CD (Compact Disc)

• Laser wavelength: 780 nm (infra‑red)
• Standard single‑layer capacity: 700 MB (≈ 80 min audio)
• 1× data rate: 150 KB s⁻¹ (≈ 1.2 Mbps)
• Physical structure: single‑layer, 120 mm diameter, 1.2 mm thick
• Typical uses: audio albums, software distribution, small backups

4.2 DVD (Digital Versatile Disc)

• Laser wavelength: 650 nm (red)
• Standard single‑layer capacity: 4.7 GB
• Dual‑layer (single‑sided) capacity: 8.5 GB
• 1× data rate: 1.385 MB s⁻¹ (≈ 10.5 Mbps) – raw data rate ≈ 1.32 MB s⁻¹
• Typical uses: movies, large software packages, data archives

4.3 Blu‑ray Disc (BD)

• Laser wavelength: 405 nm (blue‑violet)
• Standard capacities:
  • Single‑layer (BD‑R/BD‑RE) – 25 GB
  • Dual‑layer (BD‑R DL/BD‑RE DL) – 50 GB
  • Triple‑layer (BD‑XL) – 100 GB
  • Quad‑layer (BD‑XL) – 128 GB
• 1× data rate: 36 Mbps (≈ 4.5 MB s⁻¹); commercial drives usually 2× – 6×.
• Typical uses: high‑definition (1080p) and Ultra‑HD (4K) video, large backups, game‑console media.

4.4 Recordable vs. rewritable media – why it matters

All three formats exist in three “write‑status” types:

• Read‑only (ROM) – pressed during manufacturing; ideal for mass‑produced products (music CDs, commercial DVDs, Blu‑ray movies) because the data are permanent and highly reliable.
• Write‑once (recordable, e.g., CD‑R, DVD‑R, BD‑R) – can be recorded a single time. Useful for archival copies or one‑off distribution where the data must not be altered after burning.
• Rewritable (RW) – can be erased and re‑recorded many times (≈ 1 000 cycles for CD‑RW, ≈ 10 000 cycles for DVD‑RW/BD‑RE). Suited for temporary data exchange, testing, or projects that require frequent updates.

4.5 Summary of formats

5. Comparison of CD, DVD and Blu‑ray

6. Optical media versus magnetic and solid‑state storage

7. Handling, safety and storage conditions

• Store discs vertically in their jewel cases, away from direct sunlight.
• Ideal environment: 15 °C – 25 °C, relative humidity 30 % – 50 %.
• Handle by the outer edge or centre hub; never touch the data surface.
• Clean with a soft, lint‑free cloth, wiping from centre outward.
• Static electricity can damage rewritable media – ground yourself before handling.
• Scratches deeper than the protective lacquer cause read errors despite ECC.

8. Emerging issues

• Declining sales – cloud storage and high‑capacity flash are replacing optical media for most everyday uses.
• Archival value – optical discs remain immune to magnetic fields and, when stored correctly, can survive decades; “archival‑grade” discs such as M‑Disc claim > 100 years lifespan.
• Environmental concerns – manufacturers are developing biodegradable or recycled‑plastic discs to reduce plastic waste.

9. Summary checklist for revision

1. State the laser wavelength for CD, DVD and Blu‑ray and explain why a shorter wavelength (combined with a higher numerical aperture) reduces the spot size, allowing smaller pits and tighter track pitch.
2. Recall the standard single‑layer capacities: 700 MB (CD), 4.7 GB (DVD), 25 GB (Blu‑ray).
3. Describe how “layers” (single, dual, triple, quad) increase capacity and give one example for each format.
4. List at least three typical applications for each disc type (e.g., audio CD, DVD movie, Blu‑ray 4K video).
5. Compare the 1× data rates and discuss the practical impact on read/write performance.
6. Identify one advantage and one disadvantage of optical media compared with magnetic and solid‑state storage.
7. Recall the correct handling and storage conditions to maximise disc lifespan.
8. Explain one current trend or issue affecting the future use of optical media.