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h6. {color:#000000}Optical discs used originally for  storing and playback sound tracks but is developed later to store data,  rewrite-ability and much more features, and is the basic and popular form of data storage even when memory sticks or external hard drive are developed strongly. We will have a look at 3 most popular type: CD, DVD and Blu-ray Disc{color}


h1. Physical Detail

{color:#000000}&nbsp;{color}

{color:#000000}A standard disc is 1.2mm thick, 15-20grams in weight, diameter of 120mm. It consists of 4 layers as stated from the figure: a polycarbonate layer that contain data, aluminum or gold layer for reflective{color} {color:#000000}purpose, film/lacquer layer to provide protection to previous important parts and the most outer layer used for label printing{color}

{color:#000000}From the center outward, components are: the center spindle hole  (15&nbsp;mm), the first-transition area (clamping ring), the clamping area  (stacking ring), the second-transition area (mirror band), the program  (data) area, and the rim{color}
{color:#000000}Data is represented as tiny indentations known as "pits", encoded in a  spiral track moulded into the top of the polycarbonate layer. The areas  between pits are known as "lands"{color}

h2. {color:#000000}CD{color}

{color:#000000}The elongated bumps that make up the track are each 0.5 µm wide,  a minimum of 0.83 µm long and 125 nm high. They look something like this:{color} !cd-crosssection.gif|align=right! !cd-bumps.gif|align=left,border=1!

{color:#000000}You will often read about  "pits" on a CD instead of bumps. They appear as pits on the aluminum  side, but on the side the laser reads from, they are bumps.{color}

{color:#000000}The  incredibly small dimensions of the bumps make the spiral track on a CD  extremely long. If you could lift the data track off a CD and stretch it  out into a straight line, it would be 0.5 microns wide and 5 km long{color}

h2. {color:#000000}DVD{color}

Each writable layer of a DVD has a spiral track of data. On single-layer  DVDs, the track always circles from the inside of the disc to the  outside. {color:#000000}One track is{color} just 740  nanometers seperate from the next.the elongated bumps that make up the  track are each 320 nanometers wide,  a minimum of 400 nanometers long  and 120 nanometers high.

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;



&nbsp;&nbsp;&nbsp;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp;&nbsp; &nbsp; &nbsp; !dvd1.jpg|align=left!

The microscopic dimensions of the bumps make the spiral track on a DVD  extremely long. If you could lift the data track off a single layer of a  DVD, and stretch it out into a straight line, it would be almost *7.5 miles* long\! That means that a double-sided, double-layer DVD would have *30 miles* (48 km) of data\!

h3. DVD Video

When movies are put onto DVDs, they are encoded in MPEG-2 format and  then stored on the disc. This compression format is a widely accepted  international standard. Your DVD player contains an MPEG-2 decoder,  which can uncompress this data as quickly as you can watch it.

Here are the typical contents of a DVD movie:
* Up to 133  minutes of high-resolution video, in letterbox or pan-and-scan format,  with 720 dots of horizontal resolution (The video compression ratio is  typically 40:1 using MPEG-2 compression.)
* Soundtrack presented in up to eight languages using 5.1 channel Dolby digital {color:#000000}surround sound{color}
* Subtitles in up to 32 language

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h3. DVD Audio

DVD audio recordings can provide far better sound quality than CDs. The  chart below lists the sampling rate and accuracy for CD recordings and  the maximum sampling rate and accuracy for DVD recordings. CDs can hold  74 minutes of music. DVD audio discs can hold 74 minutes of music at  their highest quality level, 192kHz/24-bit audio. By lowering either the  sampling rate or the accuracy, DVDs can be made to hold more music. A  DVD audio disc can store up to two hours of 6-channel, better than CD  quality, 96kHz/24-bit music.

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; !dvd 2.gif|border=1!


h2. {color:#000000}BLURAY{color}

{align:left}
!blu-ray-5.gif|align=right,thumbnail,border=1!
&nbsp;
Unlike current DVDs, which use a *red laser* to read and write data, Blu-ray uses a *blue laser* (which is where the format gets its name). A blue laser has a *shorter wavelength* (*405 nanometers*)  than a red laser (650 nanometers). The smaller beam focuses more  precisely, enabling it to read information recorded in pits that are  only *0.15 microns* (µm) (1 micron = 10^-6\^ meters) long \-\- this is more than twice as small as the pits on a DVD. Plus, Blu-ray has reduced the *track pitch* from 0.74 microns to *0.32 microns*.  The smaller pits, smaller beam and shorter track pitch together enable a  single-layer Blu-ray disc to hold more than 25 GB of information \-\-  about five times the amount of information that can be stored on a DVD.{align}
{align:right} !blu-ray-5.gif|border=1!{align}
Each Blu-ray disc is about the same thickness (*1.2 millimeters*)  as a DVD. But the two types of discs store data differently. In a DVD,  the data is sandwiched between two polycarbonate layers, each 0.6-mm  thick. Having a polycarbonate layer on top of the data can cause a  problem called [birefringence|http://dictionary.reference.com/search?q=birefringence&r=67],  in which the substrate layer refracts the laser light into two separate  beams. If the beam is split too widely, the disc cannot be read. Also,  if the DVD surface is not exactly flat, and is therefore not exactly  perpendicular to the beam, it can lead to a problem known as *disc tilt*, in which the laser beam is distorted. All of these issues lead to a very involved manufacturing process.

h2. {color:#000000}How optical discs are read{color}

{color:#000000}The common principle for optical discs is the reflection of laser in interaction with the polycarbonate data surface. The change in height  between pits and lands results in a difference in the way the light is  reflected. By measuring the intensity change with a photo-diode, the data can be read from the disc.&nbsp;{color}&nbsp;
{multimedia:name=dvd-read.swf|width=880|height=300|autostart=true}
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!express-cd-on.gif|align=left!





{color:#000000}Each string of 1s and 0s  corresponds to an electrical signal (a voltage). The DAC  (digital-to-analog converter) turns the numbers into voltages.{color}
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