How is information stored on CDs

CD-ROM (Compact Disc)

The CD-ROM is a mass storage medium with a storage capacity of 650 to 879 Mbytes. The compact disc (CD), which was developed as a digital medium for music in the early 1980s, served as a template.

In the emerging multimedia age it became necessary to store the extensive computer data in a meaningful way. The old 1.4 Mbyte floppy disk was no longer sufficient. With the music CD, a reliable data carrier for digital data was already on the market. It was therefore obvious to use the compact disc as a storage medium in computer technology as well.

Compact Disc / CD-ROM

CD-ROM drive

Structure of a CD-ROM

The diameter of a CD-ROM is 12 cm. The CD is made of polycarbonate with a thickness of 1.2 mm. Inside is a reflective aluminum layer. It is written with data on one side and has a storage capacity of 650 Mbytes or 74 minutes of playback time.

The data is stored in pits on a spiral-shaped, inside-out, track. These depressions are called pits (valleys). They are 0.2 µm deep, 0.6 µm wide and a maximum of 0.9 µm long. The distance between the tracks is 1.6 µm. The spaces between the pits are called lands.

Structure of a CD-ROM

The data on the audio CD is stored in tracks. The individual tracks are controlled via a table of contents, the table of content (TOC). A track consists of several sectors that are identified with a time code. This technique is good for the audio CD. For the use as a data CD, a few extensions had to be made:

  • Additional headers
  • File system for managing the files

The data on a CD-ROM is stored in sectors of the same size. These sectors are accommodated on a single track that is laid out in a spiral.

Reading the data

The laser diode generates a laser beam that is also bundled by a lens. This laser beam hits the underside of the CD-ROM, penetrates the protective layer and hits the aluminum layer. There it is reflected. The prism forwards the laser beam to the photocell. The laser beam that hits the photocell generates a low electrical voltage.
If the laser beam hits a transition between pit and land, the laser beam is deflected, which causes a different electrical voltage in the photocell. This creates a series of ones and zeros.

Two methods are used to read data from a CD-ROM. The CLV process is typical for audio CDs. The CAV method is used for CD-ROMs. To improve performance, a mixture of CLV and CAV methods is used to read CD-ROMs.

CLV - Constant Linear Velocity

A constant linear velocity is used to read the data (Constant Linear Velocity - CLV). This means that the speed of rotation becomes slower the closer the laser beam approaches the outer area of ​​a CD-ROM. This results in a constant reading speed per sector. For example, an audio CD has a constant data transfer rate and thus a constant reading speed.
The limit for this technique has been reached from a rotation speed of 16 times (16x). With an internal rotation speed of 3,200 and outside of 8,500 revolutions per minute, there are problems with the accuracy of fit and the concentricity of the CD-ROM.

CAV - Constant Angular Velocity

In the case of CD-ROMS (data CDs) there is an additional circumstance which means that the read head has to jump from the outer edge to the inner edge. The data is not read sequentially like an audio CD. The data is apparently randomly distributed across the entire CD-ROM. With the CLV process, the drive would have to constantly change the speed of rotation. No data can be read during this adjustment phase.
The CAV method was developed to counteract this problem. Here the CD-ROMs spin at the same speed as hard drives. Regardless of whether you read on the inner or outer edge. There are no waiting times due to braking of the turning speed. However, this has the disadvantage that the data transmission rate in the inner area is lower than in the outer area. According to its name, an x-drive drive would only do justice to reading outdoors. In the interior, the data transfer rate is reduced by a good half.

Read speed of CD-ROM drives

An audio CD is read out at a constant data rate of 150 kByte / s. This data throughput is referred to as "1x". Drives that have a 52-fold data throughput reach 7800 kByte / s. This information relates to the maximum data throughput, which can only be achieved with the CAV method in the outer area of ​​a CD-ROM.

Transfer rate
Access time
1x150 kByte / s 400 ms
2x300 kByte / s 300 ms
4x600 kByte / s 150 ms
8x1200 kByte / s 100 ms
12x1800 kByte / s 100 ms
16x2400 kByte / s 90 ms
24x3600 kByte / s 90 ms
32x4800 kByte / s 85 ms
44x6600 kByte / s 85 ms
52x7800 kByte / s -

Dependency of the reading speed

Average speed of rotation of the drive

A CD-ROM drive with a high maximum rotational speed in the outer area is of little use if it has only low data transfer rates in the middle or inner area of ​​a CD-ROM. This is due to the rotation speed being too low.

Bug fix

Over time and due to improper handling, every CD-ROM gets flawed areas (scratches, cracks). These places cause a CD-ROM drive to repeat the reading process many times at a reduced speed. If the error correction works well, these processes are less frequent. This has a positive effect on the reading speed.

Access time

A high speed takes a relatively long time to get up to speed. The data cannot be accessed during this time. The access time is correspondingly high.

CD formats

Different formats have been developed for different applications.

CD + GCompact Disc and Graphics for audio and graphics with a very low distribution.
CD + midiCompact disc with digital interface for musical instruments for audio and midi information with very little distribution.
CD-DACompact Disc Digital Audio is the classic music CD with a playing time of 74 minutes.
CD extraEnhanced Music Compact Disc is the enhanced music CD. Successor to the CD-Plus for audio and data.
CD-ICompact Disc Interactive is the multimedia CD with the possibility of actively intervening in the process (interactive) for audio, video, photos and games.
CD-MOMagneto-optical data CD with little distribution.
CD-RCompact Disc Recordable with the option of writing audio or data once.
CD-RWCompact Disc Rewriteable with the possibility of writing multiple times with audio or data.
CD-ROMCompact Disc Read Only Memory is read-only memory for data.
CD-ROM XACD-ROM eXtended Architecture is a read-only memory for data, text, graphics, images, audio and video.
Photo CDPhoto CD for storing digital images in real colors.
Video CDVideo CD for films with a maximum playing time of 74 minutes in MPEG audio and video format. Video quality in VHS format.

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The computer technology primer is a book about the basics of computer technology, processor technology, semiconductor memories, interfaces, data storage devices, drives and important hardware components.

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Everything you need to know about computer technology.

Computer technology primer

The computer technology primer is a book about the basics of computer technology, processor technology, semiconductor memories, interfaces, data storage devices, drives and important hardware components.

I want that!