The Compact Disc
By 1982, the final Compact Disc standard was released. The standard included specification details for recording, sampling, and most importantly the familiar 4.72-inch diameter disc size. Legend has it that the size was chosen because it was large enough to contain the entire of Beethoven's 70 minute "Ninth Symphony" without interruption. The final format of the disc would be capable of holding 74 minutes (approx 650MB) of audio or data, with a small amount of space reserved for error correction and other data.
Sony and Philips continued to cooperate through the 80's, as they announced and introduced new refinements to the technology and new standards, including the familiar CD-ROM drive.
The Compact Disc itself is made of a polycarbonate (plastic) wafer 1.2mm thick with a 15mm hole in the center for mounting to the playback device. The actual data is stored on a thin aluminum film, or "strata" that is "punched" with the digital data, and then covered with another protective plastic layer. The disc is read from by a low power laser beam, which penetrates the protective surface and reads the data from aluminum surface the disc. The disc is spun at varying rotational speeds depending on the disc encoding method, and location of the data being read.
A special LED (Light Emitting Diode) is used to generate the laser beam, which passes through a beam splitter. A small, computer controlled electric motor is used to move and position the laser lens head in the correct position to read the required data. A photo-detector picks up the reflections of the laser beam and interprets the data. A strong reflection indicates, "land" while a diffused or weak reflection indicates a "pit." In digital terms, the weak reflection of the pit is interpreted as a logical high, or 1. A strong reflection of land is interpreted as a logical low, or 0. These signals are then passed through a DAC (Digital To Analog Converter) circuit that translates the digital data into analog data. That information is then passed to the amplifier unit, which then amplifies the sound and sends it out to the speakers.
A CD is only readable from one side. A label is placed on the other so that you know which side is the data side of the disc. Even with the protective plastic coating, the disc is still just plastic, and as such can be easily scratched or gouged if mishandled. The CD player is able to cope with small scratches, but larger scratches will cause music to skip during playback, or may even render the disc completely unreadable. Smudges from fingerprints and dirt can be easily wiped off, but a scratch is pretty much permanent. Ironically, most folks tend to end up scratching the disk in an attempt to clean off smudges because they used a cloth and/or cleaner that was too abrasive.
You can easily end up doing more harm than good if you don't properly care for your discs. Of course it's best to not get the disc dirty in the first place; then you will never have to worry about accidentally scratching it while cleaning. A lens cleaning cloth can be purchased from any camera store, and is perfect for cleaning CDs without the risk of an accidental scratch.