Optical Storage
Part of Data Storage
Reading and writing data with laser light — how CDs, DVDs, and their industrial cousins store hundreds of gigabytes on polycarbonate discs.
Why This Matters
Optical storage occupies a distinctive niche: it combines large per-disc capacity, complete mechanical independence between disc and drive (unlike magnetic disks where the head almost touches the platter), and remarkable longevity. A factory-pressed CD-ROM exposed to reasonable conditions is expected to remain readable for over a century. This permanence makes optical storage one of the best archival media available.
For rebuilders, optical media has a practical advantage: billions of CD, DVD, and Blu-ray discs exist in the salvageable pool. Books, program libraries, encyclopedia collections, and scientific databases were widely distributed on optical discs. Even if all recording drives fail, previously pressed discs can be read with any surviving drive and the digital content recovered.
Understanding how optical storage works lets you evaluate which discs are worth preserving, diagnose read errors, and understand the limits of recordable (versus pressed) disc longevity.
Physical Principles
All optical storage uses the same basic principle: encode data as variations in the reflectivity of a disc surface, then read the disc by measuring how much laser light reflects from each location as the disc spins.
Pressed discs (CD-ROM, DVD-ROM): Data is encoded as microscopic pits and lands molded into the surface of a polycarbonate disc during manufacturing. The pits are slightly below the land surface (about 110 nm deep for CD, 90 nm for DVD). When the laser beam illuminates a transition between a pit and a land, partial destructive interference reduces the reflected light — this registers as a transition in the signal. When the beam illuminates a flat land or the flat bottom of a pit (no transition), it reflects strongly — no transition in the signal.
Note: it is transitions that encode data, not pits themselves. This is an important distinction for understanding recording codes (EFM — Eight to Fourteen Modulation — ensures transitions never occur too close together or too far apart, maintaining tracking and clock synchronization).
Recordable discs (CD-R, DVD-R): These use an organic dye layer between the polycarbonate substrate and the reflective layer (usually gold or silver). In the unrecorded state, the dye is transparent and the laser reflects off the metallic layer — reads as a land. During writing, a high-power laser pulse heats a small spot on the dye, permanently decomposing it and creating a semi-opaque spot with different reflective characteristics — reads as a pit. CD-R is write-once; once a spot is burned, it cannot be restored.
Rewritable discs (CD-RW, DVD-RW): Use a phase-change alloy (germanium-antimony-tellurium, GeSbTe) instead of organic dye. This material has two phases: crystalline (reflective) and amorphous (less reflective). A high-power laser melts and quench-cools a spot to create the amorphous phase (a “pit”). An intermediate-power laser slowly heats the same spot, allowing it to recrystallize to the reflective phase (erasing the “pit”). CD-RW discs can be rewritten approximately 1,000 times.
Disc Structure and Spiral Track
Unlike magnetic disks which have concentric circular tracks, optical discs have a single spiral track starting at the inner hub and spiraling outward to the near edge. The spiral has a pitch of approximately 1.6 micrometers for CD, 0.74 micrometers for DVD.
Data is read at a constant linear velocity (CLV): the disc spins faster when reading the inner tracks (smaller circumference) and slower when reading outer tracks (larger circumference), maintaining constant pit speed past the laser. This differs from magnetic disk’s constant angular velocity (CAV) and complicates seeking — changing to a different track requires changing rotation speed, adding to seek latency.
Modern CD drives use Constant Angular Velocity (CAV) or Partial CAV modes during data access to improve seek times, accepting variable pit density across the disc.
Track following: The laser beam must follow the single spiral track despite disc wobble, warping, and manufacturing imperfections. A split photodiode sensor detects when the beam drifts off-track, and a servo motor adjusts the objective lens position to re-center the beam. This tracking servo is one of the most complex parts of an optical drive.
The Optical Pickup Unit (OPU)
The optical pickup unit (OPU) is the assembly that generates, focuses, and detects the laser beam. Its components:
Laser diode: A semiconductor diode emitting coherent light at a specific wavelength. CD uses 780 nm (infrared), DVD uses 650 nm (red), Blu-ray uses 405 nm (blue-violet). Shorter wavelength allows smaller spot size and thus higher data density.
Collimating lens: Converts the diverging laser output into a parallel beam.
Beam splitter (half-silvered prism or diffraction grating): Separates the returning reflected beam from the outgoing beam, routing reflected light to the photodetector.
Objective lens: High numerical aperture lens that focuses the beam to a spot of approximately 0.8–1.7 micrometers on the disc surface. This lens is mounted on a voice coil actuator that moves it radially for track seeking and axially for focus control.
Quad photodiode detector: A 2×2 array of photodiodes that detects the reflected beam. The sum of all four provides the data (RF) signal. Differences between halves provide tracking error and focus error signals for the servo systems.
Focus servo: Adjusts the objective lens height to keep the beam precisely focused on the data layer. An out-of-focus beam becomes larger, reducing signal contrast and causing read errors. The focus range is typically ±0.5 mm.
Error Correction and Data Integrity
Optical discs suffer from several error sources: scratches, fingerprints, dust, disc warping, and dye degradation (on recordable discs). To combat this, optical storage uses multiple layers of error correction far more robust than magnetic storage systems.
Cross-Interleave Reed-Solomon Coding (CIRC) on CD: Data is encoded across two levels of Reed-Solomon code (C1 and C2), with interleaving between them. A burst error (scratch) causes scattered single-byte errors in the deinterleaved data, which the Reed-Solomon decoders can correct individually. Correctable burst error length: approximately 7.5 mm of missing track.
Concealment: If errors are too large to correct, the CD-Audio specification allows concealment — interpolating the correct value from neighboring samples. For data CDs, concealment is not acceptable; uncorrectable errors are reported to the host as hard errors.
DVD and Blu-ray use more advanced Reed-Solomon product-code (RS-PC) error correction, enabling correction of larger error bursts. Blu-ray additionally specifies a hard-coating on the disc surface to reduce scratch susceptibility.
Testing disc quality: A sector error rate test (supported by DVD drives with software like DVDInfoPro or K-Probe) reports uncorrectable error counts per sector. A disc with zero uncorrectable errors is in good health. Rising error counts predict future unreadability.
Archival Considerations
Pressed discs: The polycarbonate substrate is extremely durable. The reflective layer (aluminium for most CDs, gold for archival variants) is sandwiched between protective lacquer layers and is not in contact with the environment. Failure mode: lacquer delamination exposing the aluminium to oxidation (“bronze disease”). High-quality discs stored in cases in cool, dry conditions are expected to outlast all current magnetic storage technologies.
Recordable discs (CD-R, DVD-R): The organic dye layer degrades over time, especially under UV light and heat. Cheap CD-R discs may become unreadable in 5–10 years; good quality discs in proper storage may last 50+ years. Gold-reflective archival CD-R variants (Mitsui/MAM-A Gold CD-R) claim 100-year lifetimes — the gold layer resists oxidation better than silver.
Practical archival protocol:
- Use pressed discs (replication) for truly archival content, if volumes justify the pressing cost.
- Use high-quality gold-reflective CD-R or DVD-R for smaller runs; verify immediately after burning with a sector error rate test.
- Store in jewel cases or archival sleeves (not paper sleeves that abrade the surface) in a cool, dark, dry environment.
- Verify readability every 5–10 years; refresh (copy to new media) any disc showing rising error rates.
- Keep at least two copies of irreplaceable content on different physical discs in different locations.