Chapter VI, Digital Storage: Lesson I CD Technology… How it Works http://www.kodak.com/country/US/en/digital/dlc/book3/chapter6/lesson1/p01.shtml

Optical storage technology uses the principles of light and optics to capture and retain large amounts of binary information in a way that can be accessed and accurately retrieved later. Optical storage offers some distinct advantages over systems that use magnetic recording techniques, such as hard drives and floppy discs.

Because there can be some distance between the record/playback head They include:                                             In optical storage, the record/playback head never touches the media. Therefore, it is not subject to the same mechanical wear and tear as magnetic media. Because there can be some distance between the record/playback head and the recording layer, data can be encased in a protective substrate. Optical storage is removable. Discs can resist far more dust, dirt, and physical handling than magnetic media, which typically must be sealed to prevent contamination. Optical storage media can be replicated very quickly for mass distribution of information. 700 megabytes of data can be duplicated using injection molding in as little as 3 seconds. Efficient mass production of optical storage media has made it one of the most affordable digital storage options available today.

Optical storage technology has been around for more than thirty years, being used for such things as interactive video discs and archiving large amounts of computer data. However, it wasn't until the introduction, in the mid 1980's, of the Compact Disc (CD) that this type of digital storage became widely accepted. What distinguishes the Compact Disc from earlier forms of optical storage is that standards have been established that describe such things as: The physical properties of the disc How information is organized and where it is placed on the disc How playback errors are corrected

Over time, standards have been expanded and have evolved to accommodate additional features and applications. These standards have been grouped into "books" and have been assigned colors to designate different ways specific types of information can be stored on the CD. There are currently 8 "standards books" that cover a variety of CD formats, some more common then others. The three formats that we will discuss here are: CD-Digital Audio (Red Book) CD-ROM (Yellow Book) CD-R and CD-RW (Orange Book)

The "Red Book", the first standard to be adopted, describes the physical properties of the Compact Disc and how digital audio information is encoded and organized, recorded and duplicated for standardized playback in stand alone CD players. The Compact Disc-Digital Audio format has become the industry standard for distributing commercially produced audio

The "Yellow Book" establishes a standard for a Compact Disc format called CD-Read Only Memory (ROM). CD-ROM is used to store computer data. These discs are read using a CD-ROM drive, special software, and in some cases, a special peripheral card. Because of the popularity and universal compatibility of the CD-ROM format, CD-ROM drives have become a standard peripheral in practically every new computer system sold.

The CD-ROM standard (Yellow Book) shares many of the same specifications found in the CD-Digital Audio (Red Book) standard. As a result, all CD-Digital Audio discs can be played in all computer CD-ROM drives (if the computer has been equipped with the appropriate sound card). However not all CD-ROM discs will be playable on the stand alone CD-Digital Audio players found in most audio sound systems.

CD-Digital Audio and CD-ROM formats are very similar CD-Digital Audio and CD-ROM formats are very similar. They are both examples of "read-only memory" (ROM) storage. Digital information is written once to a master disc. This disc is used in the mass production of hundreds, if not hundreds of thousands, of CD-ROM copies. It's these "Read Only" copies that are read (played back) by a computer's CD-ROM drive or an Audio CD player. In the late 1990's manufactures introduced affordable CD recording hardware. This hardware, sometimes referred to as a CD writer or CD burner, is used with special recording media to create a "write once-read many" (WORM) type Compact Disc.

All computer manufactures include this equipment as part of their standard system configuration.                                The CD-Recordable Disc format, also called CD-R, is described in the Orange Book specifications. It makes it possible for consumers to create Compact Discs that can be played in both computer CD-ROM drives and Audio CD players. Users can easily and affordably store a wide variety of digital information including: Digital Audio files Digital Picture files Virtually any type of computer data that can be stored on a floppy disc or hard drive

The CD-Rewritable Disc format also, called CD-RW, takes the features of CD-R one-step further. CD-RW allows consumers to record information then later erase and record new information on the same disc. With this capability CD-RW operates like a hard drive or floppy disc, yet still offers all the advantages of optical storage mentioned earlier

CD-ROM - How it works CD-ROM discs are mass-produced using an injection molding process. The substrates are 120mm in diameter and made of a plastic polycarbonate material . As each disc is molded a spiral track, radiating out from the center, is stamped into the disc. Subsequently a reflective layer is vacuum deposited over the track. Labeling Surface Protective Layer Reflective Layer Recording Layer Polycarbonate Substrate

As each disc is molded a spiral track, radiating out from the center, is stamped into the disc. Subsequently a reflective layer is vacuum deposited over the track. The surface of the metabolized disc bears a series of microscopic depressions (called "pits") that are inter-spaced with flat reflective regions (called "lands"). These pits and lands represent stored data in the binary form of 1's and 0's.

The CD-ROM drive or the Audio CD player reads the Compact Disc by shining a focused beam of low powered laser light onto the reflective surface of the CD-ROM disc as it spins. When laser light strikes a "land" region it is reflected back to a sensor, which generates an electrical pulse. When the light strikes a "pit" region, the light is scattered, preventing it from reflecting back to the sensor. The result is a stream of electrical pulses that a computer or audio CD player recognizes as the binary data recorded on the disc.

The spiral sequence of pits stamped into the CD-ROM disc provides a track for the laser beam to follow as the disc spins. "Pits" and "lands" along this data track are grouped into equal length segments called sectors. Each sector impresses a fixed length along the track. the rotation speed of the disc changes depending on the radius so that data sectors at the center of the disc are read at the same rate as sectors located on the outside edge. 0.6µm Track Width 1.6µm Track Distance (Pitch)

As the disc is read (played back) reflected light is measured to provide feedback to a servo system that keeps the laser beam centered and focused on the spiral data track as the disc spins. This feature helps ensure reliable playback of discs that may have experienced varying amounts of wear and tear.

CD-R - How it Works The CD-R format supports both playback (read) and record (write) operations. This type of Compact Disc requires the use of: CD Writer hardware Computer software CD recording media

A CD writer is similar to any other CD-ROM player A CD writer is similar to any other CD-ROM player. It reads CD-Digital Audio and type CD-ROM discs exactly the same way. However, a CD writer also contains a high power laser light source and additional electronics that allow it to burn marks into blank recording media. In the past, the cost of this type of hardware has been prohibitive. However, in recent years substantial price drops have made CD writers a popular computer peripheral for many consumers.

Special computer software is required to control a CD writer Special computer software is required to control a CD writer. Software gathers a variety of computer data, organizes it in specific ways for later retrieval, and controls the operation of the write (record) head to create "pits" in the proper location along the spiral data track of the recording media. We will discuss recording operations in greater detail in lesson 3.

CD recording media contains most of the physical features of other types of Compact Discs. A special recording dye layer and a gold or silver reflective layer are added. The dye formula and type of material used in the manufacture of these discs is an important factor in the long-term stability of the disc.

During recording, the write head focuses laser light onto the dye recording layer. The dye interacts with the light and either fades the dye away or creates a small capsule of air, causing a "mark" to be formed in this location. During playback, the read head shines laser light through the dye-recording layer onto the reflective layer. If the light passes through a non pitted ("non-marked") area of the dye layer, the light is reflected back to a sensor. When light passes through an area where a "mark" was created, the light is scatter and isn't reflected back to the sensor. CD-R's created with a CD writer will be compatible (readable) with computer CD-ROM drives and Audio CD players (if the audio files are set up properly with the recording software - see lesson 3 for more details).

CD-RW - How it works The basic difference between CD-R and CD-RW formats is the type of material used in the recording layer of the media. CD-RW uses special phase change material that can be changed from a high-reflectivity crystalline state to a low-reflectivity non-crystalline form when heated with a high energy laser beam During recording, marks are formed when laser light changes material in the recording layer to a non-crystalline form. During playback, non-crystalline areas will not be as reflective as crystalline areas .

Recorded information can later be changed in a CD-RW disc by erasing pits during an "annealing process". In this procedure, pits are heated (using an intermediate laser power) causing the area to re-crystallize to its original, high reflectivity state. Once erased, the disc can be recorded again using a high energy laser light source to change the record layer material back to a non-crystalline state, forming new "marks".

CD-RW media is generally more expensive then CD-R media CD-RW media is generally more expensive then CD-R media. In addition, CD-RW discs need to be read by CD-ROM drives that are more sensitive to subtle changes in reflectivity. Therefore, CD-RW discs may not be compatible with all CD-ROM players, particularly older models. In optical storage, the record/playback head never touches the media. Therefore, it is not subject to the same mechanical wear and tear as magnetic media. However, compared to non-removable types of digital storage media, CD's are subject to a lot more physical abuse, including dust, dirt, and scratches.

To ensure that discs are read reliably, optical storage uses a technique called error correction. Error correction is a sophisticated way of repeating data, based on complex mathematical methods of building redundancy. As much as 30% of the information on a CD is dedicated to this purpose. In addition, as a way of reducing the likelihood of a continuous piece of data being damaged because of a scratch or some other type of damage, information is highly interleaved (spread out over the surface of the disc). Error correction is a critical part of any CD format standard. Because the correction method is encoded during the recording process then decoded during playback, all manufacturers must use the exact same methods.