CDs for burning
The reason that regular CDs weren’t initially able to write CD media is that the ones and zeros are encoded using physical changes to the disk - as pits that are physically etched into the plastic substrate. CD-R and later CD-RW technology was faced with the difficult task of finding a way to conveniently create these pits, without the special equipment required to etch the pits, because the CD presses cost a lot of money.

CD-R technology
CD-R media addresses this issue by doing away with pits and lands entirely and using a different kind of media. CD-R media starts with a polycarbonate substrate, just like regular CDs, but instead of physically etching pits into this substrate, it is stamped with a spiral pre-groove. This is similar to the spiral found on a regular CD except that it is intentionally ‘wobbled’ or distorted. This groove is what the CD-R drive uses to follow the data path of the disk during recording. If the disk weren’t wobbled but totally blank, then writing the spiral tracks would be a very complex process, especially because the pits are spaced only 1.6 microns apart.

On top of the polycarbonate, a special light-sensitive dye layer is deposited; on top of that a metal reflective layer is applied (such as a gold or silver alloy) and then finally, a plastic protective layer. It is these different layers that give CD-R media their different visual appearance from regular CDs.

The key feature of CD-R is the dye layer and the special laser used in the drives. It is made in such a way that when light from a specific type and intensity of laser is applied to it, it heats up rapidly and changes its chemical composition. In fact when we talk about creating a CD-R as ‘burning’ a disk that is exactly what we do.

As a result of this change in chemical composition, the area ‘burned’ reflects less light than the areas that do not have the laser applied. This system is designed to mimic the way light reflects cleanly off a ‘land’ on a regular CD, but is scattered by a ‘pit’, so an entire disk is created from burned and non-burned areas, just like how a regular CD is created from pits and lands. The result is that the created CD media will play in regular CD players as if it were a regular CD.

Since the media is being physically altered by a process of heat and chemistry, the change is permanent and irreversible. Once any part of the CD has been written, the data is there forever. Some drives allow you to record some information in one sitting, and then more information later on, if the disk is not yet full. This is called multi-session recording, and requires a CD player capable of recognizing multi-session disks in order to use the burned disk. A CD-R disk can generally hold about 650MB of Data.

CD-RW technology
CD-RW media are formed in the same basic way that CD-R media are; they start with a polycarbonate base and a moulded spiral pre-groove to provide a base for recording. There are several layers applied to the surface of the disk, with one of them being the recording layer where ones and zeroes are encoded. The recording layer for CD-RW is different of course than it is for CD-R. The problem with CD-R is that the dye layer used is permanently changed during the writing process, which prevents rewriting.

CD-RW media replaces this dye with a special phase-change recording layer, comprised of a specific chemical compound that can change states when energy is applied to it, and can also change back again. Much the way water can change to steam, or ice, depending on its temperature, there are some types of chemicals that can not only change their state after having heat or other conditions applied, but even retain that state when the heat is removed. They can later be returned to their original state through another, different process.

The material used in CD-RW disks has the property that when it is heated to one temperature and then cooled, it will crystallize, while if it is heated to a higher temperature and then cools, it will form a non-crystalline structure when cooled. (Many metals are like this; in fact, different types of iron are formed by controlled heating and cooling to modify its internal structure).

When the material is crystalline, it reflects more light than when it doesn’t; so in the crystalline state it is like a “land” and in the non-crystalline state, a “pit”. By using two different laser power settings, it is possible to change the material from one state to another, allowing the rewriting of the disk. The change of phase at each point on the disk’s spiral is what encodes ones and zeros into the disk. The spiral and other structures are the same as for CD-R; what changes is how the pits are encoded.

CD-RW media have one very important drawback: they don’t emulate the pits and lands of a regular CD as well as the dye layer of a regular CD-R, and therefore, they are not backward compatible to all regular audio CD players and CD-ROM drives. Also, the fact that they are written multiple times means that they are multi-session disks by definition, and so are not compatible with non-multi-session-capable drives.

There are many compatibility issues associated with CD-RW. First is the fact that CD-RW media are not backward-compatible with many regular CD-ROM drives. Due to the lower reflectivity of the CD-RW media, regular drives can have problems reading them. In essence, the CD-RW media just does not emulate the pits and lands of a regular pressed CD well enough to fool a standard reader.

Source: Hardware guide - McH
Next week we will take a break from our explorations inside the computer to catch up on what is going on in the world outside. Keep those emails rolling in with your questions.

Improve your computer literacy
System resource - A tool used by either hardware to alert software of a need or by software to control a function of hardware. Hardware and software need a way to communicate with each other, and they do so by using a combination of four system resources:

• IRQ - Hardware devices use the IRQ bus on a motherboard to signal the CPU for attention.
• Port addresses - Software addresses a hardware device using the device’s port, or I/O, address. The device ‘listens’ to the bus to determine if it is being requested.
• Memory addresses - Software communicates with physical memory located in either RAM or ROM chips using memory addresses.
• DMA channel - Data travels back and forth between memory and a hardware device using this channel.

Financial Information Exchange - Abbreviated as FIX, a vendor-neutral standardized message format protocol for describing real-time security transactions, FIX is a public-domain specification owned and maintained by FIX Protocol, Ltd. The protocol supports the following electronic conversations between brokers and other financial institutions:

• Equity order submissions, cancellations and replacements
• Equity execution reporting
• Equity order status
• Equity trade allocation
• Indication of interest communication
• Completed trade advertisements
• Directed e-mail and news messaging

FIX is similar to OFX in that the two protocols are used to communicate financial information. However, OFX is focused more on retail transactions and is a query-response protocol much like HTTP while FIX is focused on institutional business and is a connected session-based protocol.

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