UnderStanding CD Writers
CD-ReWriter FAQ

Q1. What is CD-Recordable(CD-R)?

CD-R is a WRITE-ONCE recordable media that can be used to create one's own CD with a compatible drive and recording software. Being able to hold approx. 74 min. of audio or 650 MB of data it is suitable for storing data files as well as crystal-clear audio.

Q2. What is CD-ReWritable(CD-RW)?

With both the WRITABILITY and the ERASIBILITY, CD-RW has been designed to be a WRITE-MANY-TIMES media for digital data storage. Like CD-R, one can write audio and data files onto CD-RW; on the other hand, files or audio tracks on a CD-RW that users considered to be obsolete can be erased anytime using appropriate software. Being more flexible than CD-R.

Q3. What is the difference between factory-press CD, CD-R and CD-RW ?

On a factory-press CD, data is stored in transition between pits and lands (low and high reflectance) so that Laserdiode of the CD-ROM drive can pick up and recognize the signal.

On a CD-R, on the other hand, a pregrooved (physical tracks) Photosensitive Organic Dye is used to record digital information. Laser beam with high energy can change the reflectance of the photosensitive dye so that holes and lands (higher and lower reflectance) can be made hence picked up by CD-ROM drive's laserdiode.(see figure below) However since CD-RW discs have a reflectance of only around 15~25%, which is about a third to that of pressed CD or CD-R, they must use a laserdiode that is more optically sensitive. Therefore the CD-RW discs cannot be read by ordinary CD-ROM laserdiode, but those with a wider range of reflection acceptance .This kind of CD-ROM drive is Multi-Read capable. Nowadays newer CD-ROM drives come with this feature.

Philips, the originator of the compact disc storage solution, has more detailed explanation on both disc format and media. Visit the Philips Optical Storage for more information.

Q4. Why CD-R or CD-RW ?

With tens of million of CD-ROM drives already in the end users hand, the answer is obvious - Compatibility. Although there are alternative storage solutions in the market like MO discs, for vital data that is portable and sharable among just about anyone with a PC, CD-R or CD-RW will have to be the most ideal solution.

Q5. What CD standards are now available in the industry? Other than the original Red Book Compact Disc Digital Audio, there are a number of CD standard available in the industry now:
Format Application Variety
Red Book Audio CDs None
Yellow Book Data CDs Mode 1 - original data format
Mode 2 - further categorized into
two forms: Form 1 and Form 2
Green Book CD-i (Interactive CD) None
Orange Book Physical format for Recordable CDs Part I : CD-MO (Magneto-Optical)
Part II : CD-WO(Write-Once or commonly know as CD-R: recordable; this also includes "hybrid" spec for PhotoCD by Kodak)
Part III: CD-RW (ReWritable)
White Book Video CD None
Blue Book CD Extra None

Q6. Overview of File Systems

Not all CDs can be read by all operating systems; much depends on what file system and file naming option are used when the disc is created. If you need to make a disc readable on more than one operating system, please consult the table below to determine the best file naming option for the combination of platforms the disc will be read on.
For further introduction to the file systems.

File system/filename written: Operating systems on which disc will be read
DOS Windows 3.1 Windows 95 Windows NT 3.51 Windows NT 4.0
ISO 9660 8+3 chars Yes Yes Yes Yes Yes
Any MS-DOS 8+3 chars Yes Yes Yes Yes Yes
Joliet Short name Yes No Yes
Joliet (CD Creator 2.x) No No Yes No Yes
Long filename No No Yes Yes Yes
Romeo No No Yes Yes Yes
UDF (Direct CD) No No Yes, if Direct CD or No UDF reader installed Yes, if Direct CD or UDF reader installed
ISO 9660 Level 3 (DirectCD for Windows) No No Yes Yes Yes


Q7. How does a CD Writer Write CD's?

First of all we need to understand how a CD (compact disc) works, a good place to look at is How Stuff Works: http://www.howstuffworks.com/cd1.htm

Then we need to think about how a CD-R (CD - Recordable) works. These are written in a different way to factory 'burned' CDs. Again try http://www.howstuffworks.com/question287.htm for a more detailed explanation, but basically CD-R's have an extra layer made of dye between the plastic bottom protective layer and the reflective aluminium or gold layer, which can be modified by a laser to create non-reflective areas or dots on the disc. The laser writes these dots onto the disc by heating up the dye which causes it to change and so it nolonger allows the light to be reflected. These areas that do not reflect the light is what both CD and CD-R machine read to get the information from the CD.

When you write data to a CD-R, the writing laser (which is much more powerful than the reading laser) heats up the dye layer and changes its transparency. The change in the dye creates the equivalent of a non-reflective area. This is a permanent change and both CD and CD-R drives can read the modified dye as a bump later on.

This dye is fairly sensitive to light - it has to be in order for a laser to modify it very quickly. Therefore you want to avoid exposing CD-R discs to sunlight.

The CD-R is a write-once disc that cannot be written over whereas CD-RW (CD - ReWriteable) can be written over multiple times. CD-RW discs look the same as CD-R's, but have different structure - three bottom layers, where a CD-R has one. Basically, the middle layer is heated by a laser to change its structure, after which it can be erased and written again. Very similar to the CD-R but it is the structure change that makes the big difference.

This structure change is actually achieved by using materials in the recording layer that, when irradiated by a laser beam, can change phase (be written - like the CD-R) and back again (be erased - not like the CD-R).

The recorded areas are irradiated by a high-powered laser beam (higher power than the read laser beam), this causes the recording material to rise to a temperature approaching the melting point. It then cools rapidly. Through this, the molecules are frozen in a random form, causing it to become amorphous or non-crystalline and not reflect light so well. This produces a dots which appears the same as ones written on a CD-R. To erase the dots the temperature is raised again and then cooled slowly so that the molecules have got time to organise themsleves into crystals and so they reflect the light. This means that the dots disappears and has been erased. This is how the amorphous phase is changed to the crystalline phase and enables repeated overwriting of data 1,000 or more times.

In the recorded area, the recording material is in an amorphous (non-crystalline) phase, which has a comparatively low reflectance ratio. Conversely, the erased areas or non-recorded areas are in a crystalline phase, with a comparatively high reflectance ratio. Playback is performed by reading the differences in the reflectance ratio of these two phases while tracking the groove.



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