CD Writer – How the CD Burner works

CD burning is an amazing technology and its working principle is really fascinating. CD burners encode music and other datas on to the blank CD which can be retrieved through CD reading. CD stores the datas in digital form represented by a series of 1s and 0s. The 1s are represented by tiny bumps and the 0s as flat areas. Such millions of bumps and flats areas are present in a standard size CD.

A CD is made from 1.2 mm thick, almost-pure Polycarbonate plastic and weighs 15–20 grams. From the center outward components are at the center (spindle) hole, the first-transition area (clamping ring), the clamping area (stacking ring), the second-transition area (mirror band), the information (data) area, and the rim.

A CD is made up of a polycarbonate plastic known as Polymethyle Meta acrylic.The surface of the CD is coated with a thin layer of Aluminium to make it reflective, and is protected by a film of lacquer that is normally spin coated directly on top of the reflective layer, upon which the label print is applied.

The bumps and flat areas are present on the reflective side of the CD which is arranged in continuous tracks. These tracks measures about 0.5 microns and can stretch about 5 Kms. During CD reading, a ‘Read laser beam’ passes over the flat area in the track. The laser beam will reflect back which will be passed onto a photo sensor assembly. The Photo sensor interprets the reflected laser light as 1. When the laser light bounce back from the bumps, the photo sensor will not get it and the CD player recognize it as 0.

Blank CD

The bumps are arranged spirally starting from the centre of the CD to the periphery. The motor in CD player spins the CD in a steady speed. The laser assembly of the CD player moves from the center to the periphery of the CD at a steady speed to retrieve the data. The CD player shows a slow spinning speed as the laser assembly move outwards to keep the bumps moving past the laser at a constant rate. The digital information retrieved by the photo sensor will be then converted into audio or video by the amplifier circuit.

CD burning technique

CD writer is used to burn the CD to record the data in the digital format. The CD burner darkens microscopic areas in the reflective side of the ‘Blank CD’. During burning, both reflective and non reflective areas are created in the CD that can be interpreted as 1 and 0 by the CD player. The CD burner has a laser assembly similar to the CD reader. But the laser is ‘Write Laser’ with high power around 40 mW. The write laser interacts with the CD and alters its surface.

The movement of the Write laser assembly is exactly similar to the read laser assembly. It moves outward from the centre of the CD while the CD is spinning. The bottom layer of the ‘Blank CD’ has grooves pre pressed into it to guide the write laser in the correct path. By calibrating the spin speed and speed of the laser assembly, the burner will guide the laser assembly exactly through the track at a constant speed. During the movement, the laser diode will turn on and off in synch with the pattern of 1s and 0s in the track. The high power laser then darkens the CD material to encode 0 and leaves the material translucent to represent 1.

CD with Data. The dark shadow area in the middle shows data

The CD writing speed can be varied depending on the choice. The CD burner can write at multiple speeds. 1x is the writing speed exactly similar to the reading speed. That is ,it takes 60 minutes to write 60 minute duration data. At 2x rate, the burner will take 30 minute to write 60 minutes data. The general writing speeds are 8x, 16x, 24x, 32x etc. It is better to avoid high speed writing since at high speed, some of the data will not be stored especially when writing Music files. The normal writing speed is 16x or 24x.

Re – Writable CD

Rewritable CD is designed for ‘Write and Erase’ functions so that it is easy to erase the old data and record new data over it. Unlike the readable CD, the Rewritable CD has a’ Phase change compound ‘coated in its reflective surface. This phase change substance is the compound of silver, antimony, tellurium and indium. This compound changes its physical state on heating depending on the temperature applied. When the temperature rises above its melting point (around 600 degree) it changes to liquid form and in its crystallization point (around 200 degree) it becomes solid. Unlike the ordinary CD, in Rewritable CD, the bumps are represented by the phase changes in the compound. When it is in the ‘Crystalline form’, it remains translucent so that laser light can reflect back. When the compound becomes ‘Amorphous’ due melting, it becomes non translucent and laser light will not reflect back. These changes during melting can ‘lock’ the phase change in place.

The erasing process changes the crystalline and amorphous states of the compound through melting. The high temperature from the Laser beam will change the states of the compound so that the data in the form of translucent and non translucent areas in the reflective layer will be erased. During writing, the high power laser beam melts points corresponding to the bumps of the conventional CD. They block the read laser so that it will not reflect. These non reflective melted points remain as opaque and represent 0s The crystalline reflective areas represent 1s.

CD Writing Format

To control all the functions of the CD writer, software like Nero is used. The datas accessible for all types of CD players are encoded in the understandable form by the software. The programme also reduces the data errors during writing. This is achieved by incorporating a number of extra digital information with the recorded data and arranges them carefully. The format gives ‘Time Codes’ during writing so that the CD player can recognize which part is reading at a particular time. The format also gives ‘Table of contents’ at the beginning of the track (centre of CD) so that the CD player can recognize, which file is reading. ‘Extra data bits’ are also added so that the player can fix the mistakes if the laser misread the data bit. The recorded data is not arranged sequentially but in an ‘inter laced pattern’. This prevents the loss of complete data from the CD, if a portion of the CD is damaged.

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