Compact Disc Read‑Only Memory (CD‑ROM) is an optical data storage format standardized by ISO/IEC 11174. It uses a 12‑inch (30‑cm) polycarbonate disc with a thin metal layer (usually aluminum) coated by a protective lacquer. The data is encoded as pits and lands by a laser during manufacturing, and the disc can only be read, never written to.
Key Characteristics
- Geometry: 12‑inch diameter, 0.6‑mm thickness, 5 × 10⁶ track pitch.
- Laser wavelength: 650 nm red laser.
- Encoding: Modified Frequency Modulation (MFM) with Compact Disc Redundancy Code (CIRC) error correction.
- Capacity: 700 MB per disc.
- File system: ISO/IEC 9660 (UDF for larger payloads).
Physical & Optical Features
The disc’s optical read mechanism requires the laser to focus on the reflective metal layer. Because the recording layer is pre‑imprinted during manufacturing, the disc is fully read‑only; this gives it inherent stability but also means that any physical defect is permanent.
Applications
- Software distribution (OS, applications, firmware).
- Long‑term data archiving.
- Audio/video discs (music, movies, educational content).
- Embedded firmware storage.
- Educational and reference materials.
Limitations
- Physical degradation: scratches, delamination, exposure to UV.
- Capacity constraints: 700 MB limits multi‑gigabyte datasets.
- Obsolescence: many modern PCs lack optical drives.
- Read reliability: aging lasers or poor alignment can cause tracking errors.
Environmental Considerations
- Materials: polycarbonate, aluminum, lacquer (sometimes hazardous).
- Manufacturing energy: high‑temperature processes, laser deposition.
- Disposal: hazardous chemicals, limited recyclability.
Future Outlook
- Hybrid backup systems: combining optical media with SSD or cloud storage.
- Improved error‑correcting codes (LDPC, Polar Codes) for greater durability.
- Enhanced recycling technologies to recover polycarbonate and metal.
- Digital preservation initiatives maintaining discs as reference copies.
External Resources
- ISO/IEC 11174 – Compact Disc Specifications
- ISO/IEC 9660 – File System for CD‑ROMs
- UDF – Universal Disk Format
- National Archives – Digital Preservation
Category
Storage media
```
No comments yet. Be the first to comment!