Pit & Byte Guide

Microscopic pits representing encoded data are first formed by exposing photo-resist material with a laser beam. This beam is modulated according to the input source data. After the exposed areas are developed away by conventional methods to produce pits, a rigid metal negative to the master, called the Father or Master stamper, is produced by an electoplating process (see Figure 2). Multiple positive image Mother may be electroplated from the Father stamper. In turn, negative image Son stampers are plated from each Mother to produce multiple copies of the original master. Mass replication of the source begins by mounting a Father or Son in a molding press. Melted plastic is injected into the cavity and allowed to cool. The pits from the stamper are accurately reproduced in a plastic substrate, forming the original positive image. Each disc is finished by successively applying a reflective layer, protective lacquer, and printed labeling (see Figure 3). CD Readback System A compact optical head in a CD player "reads" the pits. Light emitted from a semiconductor laser is collected by a collimating lens and focused by an objective lens through the clear disc substrate onto the pits. Light of high intensity (between pits) or low intensity (on a pit) is reflected from the metallized surface back through the objective lens. The reflected light then passes through a beamsplitter, which redirects the light onto an optical detector. Electronics amplify and decode the detector signal to produce a computer bit stream (see Figure 3).

How "real" computer data is generated from a CD-ROM

The pits which are formed during the mastering and molding processes are read by a small optical system which includes a laser and an optical detector.

The disc and corresponding pit structures rotate under the laser beam during playback. The pits cause the amount of reflected light traveling from the disc to the detector to fluctuate, creating a high frequency signal. The drive determines points in time where this signal crosses a certain threshold value. These crossings, signifying a logical "1", occur at each pit edge.

Due to physical limitations of pit size, it is necessary to maintain at least two "0"s between each "1". An encoding scheme called Eight-to-Fourteen Modulation (EFM) translates every 8 bit combination to a 14 bit code to be written to the disc. Three merging bits are used between each byte to further ensure the separation of "1"s.