Optical Storage Media: CD-ROM, DVD & Blu-Ray Architectures

Explanation

Optical storage density is governed by optical diffraction limits: minimum focal spot size is proportional to wavelength / numerical aperture (lambda / NA). By shifting from infrared (780 nm) in CDs to red (650 nm) in DVDs and finally blue-violet (405 nm) in Blu-Ray, the laser spot diameter shrinks dramatically, allowing pit lengths and track pitches to contract. Transitions between pits and lands represent binary 1s (using Non-Return to Zero Inverted - NRZI encoding), while uniform pit or land lengths represent strings of binary 0s.

Key points

• CD-ROM: 780 nm infrared laser; track pitch 1.6 um; standard capacity 650-700 MB; uses Eight-to-Fourteen Modulation (EFM).
• DVD-ROM: 650 nm red laser; track pitch 0.74 um; dual-layer and double-sided configurations; capacity 4.7 GB to 17 GB.
• Blu-Ray Disc: 405 nm blue-violet laser; track pitch 0.32 um; capacity 25 GB (single-layer) to 50 GB (dual-layer).
• Error Correction: Reed-Solomon Cross-Interleaved Code (CIRC) and Cross-Interleaved Reed-Solomon Code (CIRC) recover data damaged by scratches and surface dust.

Example

Code examples

/*
Optical Discs Physical Specifications:
Format   | Laser Wavelength | Numerical Aperture | Track Pitch | Standard Capacity
CD-ROM   | 780 nm (Infrared)| 0.45               | 1.6 um      | 700 MB
DVD      | 650 nm (Red)     | 0.60               | 0.74 um     | 4.7 GB (Single Layer)
Blu-Ray  | 405 nm (Blue)    | 0.85               | 0.32 um     | 25.0 GB (Single Layer)
*/

Exam questions

• [8 Marks] Compare CD, DVD, and Blu-Ray optical storage technologies across laser wavelength, track pitch, numerical aperture, and capacity.
• [5 Marks] Explain how binary data (1s and 0s) is physically encoded and read using pits and lands on an optical disc.