how cds works

Compact Disc Technology: Principles and Operation

The Compact Disc (CD) is a digital optical disc data storage format, initially developed for storing and playing digital audio recordings. Later adapted for data storage (CD-ROM), its core functionality relies on optical principles for reading and writing information.

Data Encoding and Physical Structure

Digital information is encoded on the CD as a series of microscopic pits and lands arranged in a continuous spiral track. The pits are indentations pressed into the reflective surface of the disc, while the lands are the flat areas between the pits. The transition between a pit and a land, or vice-versa, represents a binary '1', while no transition indicates a binary '0'. The spiral track begins at the center of the disc and extends outward.

Reading Mechanism: Optical Pickup Unit (OPU)

The OPU, also known as the laser pickup, is the key component for reading data. It consists of:

  • Laser Diode: Emits a focused laser beam, typically with a wavelength of 780 nm (near-infrared).
  • Objective Lens: Focuses the laser beam onto the reflective surface of the disc, through the polycarbonate substrate.
  • Beam Splitter: Directs the reflected laser beam towards the photodiode array.
  • Photodiode Array: Detects the intensity of the reflected laser beam. Variations in intensity, caused by the pits and lands, are converted into electrical signals representing the digital data.
  • Tracking Mechanism: Ensures the laser beam remains precisely aligned with the spiral track.
  • Focusing Mechanism: Maintains the laser beam's focus on the disc surface, compensating for imperfections and disc wobble.

Signal Processing and Error Correction

The electrical signal from the photodiode array undergoes several processing steps:

  • Amplification: Boosts the weak signal to a usable level.
  • Equalization: Compensates for signal distortion caused by the optical system.
  • Clock Recovery: Extracts the timing information from the signal to synchronize data reading.
  • Demodulation: Converts the encoded signal back into the original digital data.
  • Error Correction: Uses error-correcting codes (ECC), such as Cross-Interleaved Reed-Solomon Coding (CIRC), to detect and correct errors caused by imperfections on the disc surface or reading inaccuracies.

Manufacturing Process

CD replication involves several stages:

  • Mastering: Creating a glass master disc with a photoresist coating. A laser beam etches the pits onto the master disc.
  • Electroplating: Coating the master disc with a metal layer (typically nickel) to create a negative mold (stamper).
  • Injection Molding: Using the stamper to mold polycarbonate discs with the pit structure.
  • Metallization: Applying a thin reflective layer (typically aluminum) to the disc surface.
  • Protective Coating: Applying a lacquer coating to protect the reflective layer.
  • Labeling: Printing the label information on the top surface of the disc.