Before you start
Objectives: Learn which versions of CDs exists and which technologies they are using.
Prerequisites: no prerequisites.
Key terms: cd-r, rom, rw, data, disk, drive, speed, standard, write, read, laser, book
CD was originally designed to hold 74 minutes of audio. Now we can also buy a CD which can hold 80 minutes of audio. A CD is 120 millimeters in diameter and it is 1.2 millimeters thick. There are three different types of CD optical media drives used today. Those are CD-ROM, CD-R and CD-RW.
CD ROM stands for Compact Disk Read Only Memory. On the bottom of every CD there are a series of pits and lands. Pits and lands are used to store binary (zero and one) data.
Image 326.1 – CD Surface
How CD-ROM Works
In general a pit doesn’t reflect while a land (flat spot), does reflect. If we focus a laser beam to the CD, some parts of the CD would reflect light and other parts of it wouldn’t. Laser continuously shines its beam onto the bottom surface of the CD. If it hits a pit, the beam won’t reflect back. If it hits a land, we will get a reflection. By spinning the CD and by moving the laser head across the disk, we can read binary information on the whole disk. Reflection is captured by a photo sensitive device which then converts the pulse of laser light into the binary PC data. CD can store approximately 650 megabytes of data, but there is also a version which can store 700 megabytes. Remember that we can only read data from the CD-ROM.
The first CD ROM drives transferred data at a rate of 150 kilobits per second. 150 kilobits data rate is used by audio CD player, but for PCs that was really slow. Improvements in CD ROM drive speeds were measured as multiples of the original speed. To calculate our CD ROM drives transfer rate we can simply multiply it’s speed by 150 kilobyte. For example a 2X drive would transfer data at 300 kilobits per second, a 8X drive would transfer data at 1200 kilobits per second, and a 52X CD ROM drive would transfer data at 7800 kilobits per second.
CD-R allows us to read from a traditional CD-ROM, as well as from CD-R disks, but it also allows us to write to a CD-R disk. Writing to a CD is called burning. CD-R disk can only be written to one time. After that the CD-R disk works like a typical CD-ROM disk.
How CD-R Works
CD-R doesn’t use pits and land. Instead of that, CD-R disk is coated with a photo reactive dye. A CD-R drive has a read laser (just like a regular CD ROM drive does), but it also has a second, high power write laser. When the write laser hits the photo reactive dye on the bottom of the CD-R disk, it changes the pigment to a darker color. Darker color acts like pit on a CD ROM disk, it doesn’t reflect. Anywhere where the write laser hasn’t hit, reflects and it acts like a land. Using this system of dark and light spots a CD R can store about 650 megabytes of data, just like a CD ROM, but we can also purchase CD-R disks that can store 700 megabytes.
CD-R drives are also rated according to their speed. The difference is that instead of having just one parameter for speed, we have two. We have the write speed and we have the read speed. Both of these parameters are multiples of the original 150 kilobyte per second speed of a 1X CD ROM drive. For example if we have a CD-R drive that is rated 24X48 that means that that drive can write data at 3600 kilobytes per second (24*150), and that it can read data at 7200 kilobytes per second (48*150). Remember, the limitation of CD-R is that we can only write to them once.
CD-RWs can be written to multiple times. It can be written to, erased, and then again written to. They also have a capacity of about 650 megabytes.
How CD-RW Works
The bottom surface of a CD-RW disk is coated with a photo reactive crystalline coating. CD-RW drive also has a second high power write laser. When this laser hits the bottom of the photo reactive material on the bottom of the CD-RW disk it causes crystals to form. Wherever a crystal is formed on the bottom of a CD-RW disk it’s like a land on a CD ROM disk, which means that it reflects light. CD-RW drives can burn and erase CD-RW disks, but they can also burn a CD-R disk and read a typical CD ROM disk.
CD-RW drive has a speed rating which consists of CD-R write speed, CD-RW write speed, and a read speed. Again, all of these are multiples of the original 150 kilobytes per second 1X speed defined by the first CD ROM drives. For example if we have a 10x8x40 CD-RW drive that means that it can write as a CD-R at 1500 (10*150) kilobytes per second, it can write to a CD-RW disk at 1200 (8*150) kilobytes per second and it can read at 6000 (40*150) kilobytes per second.
CD-ROM is just one type of CD format. Each format is a different way to organize the data stored on the disc, and that allows the disc to be used for a specific purpose. CD drive must be compatible with that format in order to play a CD created using a particular format. Compact disc formats are categorized by book color. For example, Red book describes Audio CDs, CDDA being the specific standard. Yellow book describes data CDs. CD-ROM generally refers to a data CD created using a Yellow book standard. Several specific Yellow book standards exist. Those specific standards are:
- CD-ROM – the original data CD standard
- ISO 9660 – a more standardized format than the original CD-ROM standard. This standard is not operating system-specific.
- CD-ROM XA is an extension to the ISO 9660 standard. It allows multiple types of data (audio, data, video) to be put on the same CD.
We also have a Green book which describes interactive CDs, CD-I being the specific standard. CD-I is a proprietary standard for creating CDs with built-in interactivity. White book describes Video CDs, Video CD being the specific standard. Video CD is a standard for structuring digitized MPEG video files.Orange book describes defines standards for multi-session CDs. Specific standards are CD-R and CD-RW.
We can only read from CD-ROM disks. The original CD-ROM speed was 150 Kb/s. CD-R disk can only be written to one time. CD-R has a speed rating which consists of the write speed and the read speed. CD-RWs can be written to multiple times. CD-RW drive has a speed rating which consists of CD-R write speed, CD-RW write speed, and a read speed. CD-ROM generally refers to a data CD created using a Yellow book standard.