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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">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

Physical Detail


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 purpose, film/lacquer layer to provide protection to previous important parts and the most outer layer used for label printing.

From the center outward, components are: the center spindle hole (15 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
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".

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Figure 1. Layers of a CD

                                                                                                                      Copied from http://en.wikipedia.org/wiki/Compact_Disc

                                                                                                                                                                                                                               

CD">CD

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:

           Figure 2. Cross section of a CD. Copied from                                      Figure 3. CD bumps. Copied from

           http://electronics.howstuffworks.com/cd1.htm                                http://electronics.howstuffworks.com/cd1.htm                                            

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.

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

DVD">DVD

A spiral data track which is 740 nanometers apart from the next stays in each writable layer of DVD. The tracks are composed of the elongated bumps whose size is 320 nanometers wide, 400 nanometers long and 120 nanometers high each at minimum.

                                                                                                                                        

                                                                                   

DVD Video

Before movies are stored on the disc, they are encoded to MPEG-2 format first which is widely accepted and a international standard. When you buy a DVD player, it already contains a MPEG-2 decoder used to immediately uncompress the encoded data while you are watching the movie.

Here are the typical contents of a DVD movie:

  • The DVD can contains 133 minutes of high-resolution video, in letterbox or pan-and-scan format, with 720 dots of horizontal resolution ( The MPEG-2 format compresses the video with a typical ratio of 40:1)
  • It can present the soundtrack in eight languages utilizing 5.1 channel Dolby digital surround sound
  • It supports audience with subtitles in up to 32 languages




Figure 4. DVD pits layout. Reprinted from

http://electronics.howstuffworks.com/dvd2.htm

DVD Audio

DVD can take a higher sampling rate than CD so DVD quality is much better than CD in recordings. About 74 minutes of songs can be stored in CDs. By being able to sample the signal at higher rate, DVD can store a much better quality of the same amount of music compared to CDs. The highest quality level is 192kHZ/24-bit audio. A longer amount of time of music can be stored in DVD by decreasing the quality of the songs. To decrease the quality, sampling rate or the accuracy need to be lowered. For example, at 96kHZ/24-bit music, DVD can hold a longer amount of music which is up to two hours of 6-channel and its quality is still better than CD at the same level. Below is the chart that shows us the sampling rate and accuracy for CD recordings and the maximum sampling rate and accuracy for DVD recordings

                                                                                                       

                                                                                                    Figure 5. Comparison of a raw audio signal to the CD audio and DVD audio output.

                                                                                                        Reprinted from http://electronics.howstuffworks.com/dvd5.htm

The sampling frequency specifies how often a signal is sampled meaning how many times a signal is sampled per second. The accuracy of the sampling process is determined by the number of bits because the sampled values are converted into binary. The amount of information in every sampling is determined by the number of bits. The more bits we have, the higher the accuracy. The quality of the music at 8-bit system is not good. Therefore, the music is often coded on the disc in 16 bits system.

Blu-ray Disc">Blu-ray Disc

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  • Blu-ray disc (BD) uses a blue laser to read and write data. A blue laser has a shorter wavelength (405 nanometers) than a red laser (650 nanometers). The smaller beam reading information recorded in pits that are only 0.15 microns (µm) (1 micron = 10^-6^ meters) long -- 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.ㅤ

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  • Each Blu-ray disc is about 1.2 millimeters in thickness. Having a polycarbonate layer on top of the data can cause a problem called birefringence, 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.

                                                                                                                                                                                     

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Figure 6. Building a Blu-ray Disc.

Reprinted from http://electronics.howstuffworks.com/blu-ray1.htm

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Fun facts: The name

The Blu-ray name is a combination of "blue", for the color of the laser that is used, and "ray", for optical ray. The "e" in "blue" was purposefully left off, according to the manufacturers, because an everyday word cannot be trademarked.

How optical discs are read">How optical discs are read

Figure 7. Comparison of how different types of dics are read. Reprinted from http://en.wikipedia.org/wiki/File:Comparison_CD_DVD_HDDVD_BD.svg

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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.  

Figure 8. How data is read inside DVD player. Reprinted from http://electronics.howstuffworks.com/hd-dvd.htm

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

Laser beam scans the compact disc. The laser is reflected from a land to a photo-electric cell. The current is created from the cell and a 1 is registered. No current is emitted when the beam shines on a pit because half of the light is reflected from the surface and half from the depth of the pit and they destroy the original beam by interference. The photo- electric cell emits no current, and a 0 is registered.

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REFERENCE:

* CD Wikipedia http://en.wikipedia.org/wiki/Compact_Disc
* DVD Wikipedia http://en.wikipedia.org/wiki/DVD
* Bluray Wikipedia http://en.wikipedia.org/wiki/Blu-ray_Disc
* CD HowStuffWork http://electronics.howstuffworks.com/cd.htm
* DVD HowStuffWork http://electronics.howstuffworks.com/dvd.htm
* Bluray HowStuffWork http://electronics.howstuffworks.com/blu-ray.htm
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