CD-ROM

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CD-ROM

An Introduction

CD-ROM and DVD are optic readable media, contrary to hard disks, floppy disks and tapes, which are magnetic.

The optic storage media are read with a very thin and very precisely aimed laser beam. They supplement the magnetic media. They have clear advantages in the areas of data density and stability: Data can be packed much more densely in optic media than in magnetic media. And they have much longer life span. It is presumed that magnetic media, such as a hard disk or DAT (digital audio tape) can maintain their data for a maximum of five years. The magnetism simply fades away in time. Conversely, the life span of optic media are counted in tens of years.

Let us take a closer look at these disks, which are becoming increasingly popular for all types of information, education and entertainment. There are different types:

The Compact Disk

The compact disk (CD) was introduced by Philips and Sony in 1980 to replace LP records. It is a small plastic disk with a reflecting metal coating, usually aluminum. Myriads of tiny indentations are burned into this coating. These indentations contain the music in millions of bits. The CD is organized in tracks. Each track is assigned a number.

The big advantage of the CD is its high quality music reproduction and total absence of back ground noise as well as a great dynamic. During operation, the software in the drive can correct errors caused by such things as finger marks on the disk. All in all, CDs are excellent music storage media.

The CD-ROM

The CD-ROM (Read Only Memory) came as an extension of the CD in 1984. In principle, the media and the drives are the same. The difference is in the data storage organization. In a CD-ROM, the data are stored in sectors, which can be read independently - like from a hard disk.

The CD-ROM has become an important media in the PC world. It can hold 650 MB of data, and it is very inexpensive to produce. Today, there are three types of CD drives and DVD drives :

Drive type Name The drive can

CD-ROM Compact Disk Read Only Memory Read CD-ROM and CD-R

CD-ROM
multiread --''-- Read CD-ROM, CD-R and CD-E

CD-R Compact Disk Recordable Read CD-ROM and CD-R. Write once on special disks named CD R

CD-RW Compact Disk ReWritable Read CD-ROMs and CD-R. Write and re-write on special disks (CD-RW).

DVD RAM Digital Versatile Disk Random Access Memory Reads all CD formats. Reads DVD ROM. Reads and writes DVD disks

Let us start by look at the CD-ROM construction. To facilitate understanding, it will be easiest to compare it with other disk types, especially the hard disk. The CD-ROM is a plastic disk of 4.6" diameter.

It is placed in a CD-ROM drive, which is like a drawer in the PC cabinet:

When the CD-ROM disk is placed in the drive, it starts to spin the disk. It reaches operating speed in one to two seconds. Then the drive is ready to read from the disk.

Drives and operating system

The drive must be assigned a drive letter. That is a task for the operating system, which must be able to recognize the CD-ROM drive. That is usually no problem in Windows 95/98. However, the alphabet can be quite messy, if there are many different drives attached. Each drive must have its own letter. They are assigned on a first come first-serve-basis.

The CD-ROM drive usually gets the first vacant letter after other existing drives, typically D, E, or F. But the letter can be changed.

Once the CD-ROM spins and the operating system (DOS or Windows) has "found" the CD-ROM drive, data can be read for processing. Now the CD-ROM works like any other drive. Only, it is Read Only Memory!

The CD-ROM holds its own file system called ISO 9660. It is not using FAT!

About Optic Data Storage

The CD-ROM can be compared to a floppy drive, because the disks are removable. It can also be compared with a hard drive, because of similar data storage capacity. Actually, a CD-ROM disk can hold up to 650 MB of data. This equals the capacity of 470 floppy disks. However, the CD-ROM is neither a floppy nor a hard disk!

While floppy and hard disks are magnetic media, the CD-ROM is an optic media. The magnetic media work in principle like an audio cassette tape player. They have a read/write head, which reads or writes magnetic impressions on the disk. The magnetic media contains myriads of microscopic magnets, which can be polarized to represent a zero or numeral one (one bit).

In the optic readable CD-ROM, the data storage consists of millions of indentations burnt into the lacquer coated, light reflecting silver surface. The burnt dents reflect less light than the shiny surface. A weak laser beam is sent to the disk through a two-way mirror and the sensor registers the difference in light reflection from the burnt and shiny areas as zeros and ones.

Tracks

Our data consist of bits, each of which is a burnt dent or a shiny spot on the CD-ROM disk. Music CDs are designed much in the same manner. The bits are not splashed across the disk, but arranged in a pattern along the track. Without that organization, you could not read the data.

The platters in hard disks and floppies are organized in concentric tracks. There can be hundreds of those from center to periphery:

The CD-ROM is designed differently. It has only one track, a spiral winding its way from the center to the outer edge:

This 5 km long spiral track holds up to 650 MB data in about 5.5 billion dots (each is one bit).

Data read from CD-ROM

Data is read from the CD-ROM at a certain speed. There are two principles used reading from a CD-ROM:

CLV

Constant Linear Velocity was used in the early generations of CD-ROM drives. It implies that the data track must pass under the read head at the same rate, whether in inner or outer parts of the track. This is accomplished by varying the disk rotation speed, based on the read head's position. The closer to the center of the disk the faster the rotation speed to deliver the same constant stream of data.

In the music CD, data are read sequentially. Therefore, rotation speed variation is not necessary.

CAV

Constant Angular Velocity. It is not very smart to change the rotational speed of a CD-ROM all the time, as the CLV drives do. Therefore, in more modern and speedy drives, the CD-ROM rotates at a constant number of rounds per minute. This implies that the data transfer varies; data read from the outer parts of the CD-ROM are read at very high bit rates. Data from the inner parts are read slower.

Let us look at a modern 40X CAV drive. It rotates constantly with a whopping 8900 RPM. This drive will deliver 6 MB per second when reading from the outer tracks. Reading from the inner tracks it only delivers 2.6 MB per second. An average will be 4.5 MB/sec.

Problematic readings

The CD-ROM disk has to read in random pattern. The read head must jump frequently to different parts of the disk. You can feel that. It causes pauses in the read function. That is a disadvantage of the CD-ROM media. Also the faster drives can be rather noisy.

Within the next years the CD-ROM and DVD drives will merge into one unified drive type.

Rotation speed and data transmission

There are different generations of CD-ROM drives. Here you see their data.

CD-ROM type Data transfer rate Revolutions per minute outermost - innermost track

1X 150 KB/sec 200 - 530

2X 300 KB/sec 400-1060

4X 600 KB/sec 800 - 2,120

8X 1.2 MB/sec 1,600 - 4,240

40X CAV 2.6 - 6 MB/sec 8,900 (constant)

40X40 multibeam 6 MB/sec 1,400 (constant)

Personally I experience no big difference between the 24X, 32X, and 40X spin drives. However, their speedy rotation of the disk causes many physical problems, and the performance vary from drive to drive and CD-ROM to CD-ROM.

When you see the rotation speeds, you wonder how much further this technology can be advanced. The hard disk can spin at higher speeds, because it operates in a sealed box. The CD-ROM does not, and the high rotation speed causes a lot of practical problems such as noise and vibrations.<

Multi-beam

An interesting development in this field is the multi-beam CD-ROM drives. Instead of one laser beam, you put up seven of the kind (however, only six of them are used for data read). This TrueX/Multibeam technology from Zen Research gives 36X performance from a steady 6X CLV speed rotation.

See www.hival.com. They produce a so-called 40X40-drives with 7 (6 data + 1 error correcting) laser beams, which read simultaneously. That yields genuine 40X performance with a transfer rate of up to 6MB per second, while the CD-ROM disk only rotates like a old 8X drive. Compaq also produces a drive on this basis.

Another interesting development in the CD-ROM world is the use of blue laser for reading the CD. It has been reported that with the blue laser, a typical CD can hold up to 15GB of data. That's more storage space than most hard drives! Though sadly this technology won't be ready for the home user for at least five years.

Music from the CD-ROM

The PC CD-ROM drive can play regular music CDs. That is a smart "bonus". It requires three things:

You must have a sound card in your PC

The CD-ROM drive must match the MPC-3 multimedia standard (all modern CD-ROM drives do)

You must connect the CD-ROM drive to the sound card with the short special cable, which comes with the drive.

The CD-ROM can easily hold sound data, which can be played directly through the sound card - without use of the short cable I mentioned. It only becomes necessary, when you want to play quality sound music. Certain games (such as Tuneland) contain both types of sound.

S/PDIF outputs

Some CD-ROM drives feature a S/PDIF output that can send a purely digital signal to a soundcard. This gives better sound performance and opens for new interconnectivity.

CD-R and CD-RW

In 1990, the CD-ROM technique was advanced to include personal burning. You could buy your own burner.

A burner is also a drive

To make your own CD-ROMs, you use a drive, which can write on special CD-ROM disks.

These disks have a temperature sensing layer, which can be changed by writing. You can only write on any given part of these disks once. This CD-R disk is also called a WORM disk (Write Once Read Many). Once the CD-R is burnt, it can be read in most newer CD drive for sound or data.p

Steady data streaming

Burning a CD-ROM requires a very steady data streaming. Therefore the drives typically have a 2 MB cache onboard for buffer. If the buffer runs out of data during the writing process, the CD ends up unusable.

On EIDE interface

The best interface for CD-R and CD-RW drives is or used to be SCSI, but many vendors supply the cheaper and very reliable EIDE units. The leading company HP makes no SCSI-based CD-RWs at all.

If you use an EIDE burner, the best is to connect it as a master unit on one of the two EIDE channels with the hard disk and CD-ROM drive on the other channel. However this is not good for the harddisk performance, so I use this setup myself:

It works fine, I can "burn" what I want at 4X speed.

Synchronize data transfer

One detail. Check your for Settings on both CD-ROM drives. You should enable Synchronize data transfer as here:

Please do not enable DMA on your CD-ROM drives. It probably causes troubles.

CD-RW

There is also a type called CD-ReWritable (CD-RW), where you can write multiple times on the same disk surface. However, not all CD drives can read these CDs. New drives, which can adjust the laser beam to match the current media and hopefully read the CD-RW disks, are called multiread.

To work with CD-RW you need special software. I like Adaptecs very much. It comes with the HP 8100 drive:

You need to format the CD-RW disk before use:

DVD

The next optic drives we will see in the next few years is the DVD drive. They are being developed by several companies (Philips, Sony, and others) and represent a promising technology. DVD stands for Digital Versatile Disk.

They are thought of as a future all-round disk, which will replace CD-ROM and laser disks. In the future, DVD might also replace VHS tapes for videos.

Certain DVD drives can both read and write the disks. There are also read only, designed for playing videos.

The DVD is a flat disk, the size of a CD - 4.7 inches diameter and .05 inches thick. Data are stored in a small indentation in a spiral track, just like in the CD.

DVD disks are read by a laser beam of shorter wave-length than used by the CD-ROM drives. This allows for smaller indentations and increased storage capacity.

The data layer is only half as thick as in the CD-ROM. This opens the possibility to write data in two layers. The outer gold layer is semi transparent, to allow reading of the underlying silver layer. The laser beam is set to two different intensities, strongest for reading the underlying silver layer. Here you see a common type DVD ROM drive:

Various DVD types

We have the following DVD versions:

DVD-ROM is for read-only, like the CD-ROM. This medium is usable for distribution of software, but especially for multimedia products, like movies. The outer layers can hold 4.7 GB, the underlying 3.8 GB. The largest version can hold a total of 17 GB.

DVD-R (recordable) are write once only like CD-R. This disk can hold 3.9 GB per side .

DVD RAM can be written and read like a hard disk. Three competing formats exists.

DVD RAM

Three rewritable technologies are present at the market:

DVD-RAM, from Hitachi and Matsushiti.

Pioneer has a DVD-R technology which is a third version.

DVD+RW, supported by HP, Sony, Philips, Yamaha, Ricoh and Mitsubishi.

None of the three products are compatible. However, the companies behind DVD+RW control 75% of the market, so I think this will become the new standard.

It appears that the DVD-RAM disks are extremely sensitive to greasy fingers and other contaminants. Therefore they must be handled in special cassettes, which do not fit into ordinary DVD players.

HP DVD 3100i

HP has a DVD+RW drive. It reads and writes 3 GB disks at 1,7 MB/sec. The DVD+RW disks should be readable to orfinary DVD drives. The disks cost (July 1999) around $25 a piece.

Movies and multimedia

A 4.7 GB side can hold 135 minutes top quality video with 6 track stereo. This requires a transmission rate of 4692 bits per second. The 17 GB disk holds 30 hours top quality music recording.

The following transmission rates are used in the various media:

Music on CD requires 150 KB per second. That corresponds to the transmission rate in the first 1X CD-ROM drives.

Playing of movies (with a new MPEG-2 compression) requires a transmission rate of about 600 KB per second. That corresponds to the 4X CD-ROM drives.

Multimedia programmer is expected to provide 1.3 MB per second, like a current 10X CD-ROM drive.

The DVD drives come in EIDE and SCSI editions and in 5X, etc. versions, like do the CD-ROMs.

The DVD drives will not replace the magnetic hard disks. The hard disks are being improved as rapidly as DVD, and they definitely offer the fastest seek time and transmission rate (currently 5-10 MB/second). No optic media can keep up with this. But the DVD will undoubtedly gain a place as the successor to the CD-ROM. An the new drives will read both CD-ROMs and DVDs.

Codes

DVD movies are made in several "codes." Region one is USA and Canada, while Europe and Asia is region two. Region 4 is Australia, New Zealand & South America. I'm not sure on 3, 5, and 6. When you play movies, your hardware (MPEG decoder) must match the DVD region. The movies are made in separate formats, each with their own coding. The DVD drives has to be set to one of the codes, and this setting can be changed perhaps five times.

Some DVD drive use Dolby AC-3 sound standard. That is a sound system with five full range speakers to surround you with sound, plus a supplementary low frequency special effect channel. To get the full use of the movie sound tracks, you need a AC-3 compatible stereo set up.

MPEG-2 video

The video format on DVD disks is in MPEG-2 coding. That is a compression technology, which requires lots of processor power. The data stream from the DVD must be decoded on a running basis during the play (real time decoding). That can be done in two ways:

Software based decoding. Special programs are read in, which utilize the CPU's power for the decoding.
Hardware based decoding. The PC is equipped with a special chip (such as an add-on card) which only has to decode the MPEG data stream.

The advantage of the hardware based decoding is that the result does not depend on the CPU in your PC. The disadvantage is that you often need to install an extra card in the PC to enable seeing DVD films.

Soft or hard decoding?

The software based decoding does not require any new hardware, but it requires a very powerful CPU. If there is not sufficient processor power, you will see a loss of some individual images. The movie gets "choppy."

In the same way the MPEG cards are not always powerful enough either, according to the reviews. All in all, I recommend that you wait for further DVD and MPEG developments. In a couple of years the MPEG decoding will surely become a standard task (included in graphics chip sets), which all PC�s can perform without problems.

HD-ROM

Futuristic technology. The well known optical drive types function using a laser beam. Norsam Technology has developed another technology where the controlling beam is made of charged gallium ion particles. This gives a very high density, since the beam is very narrow. The new drive technology is called HD-ROM.

Media Beam, width Capacity per disk

CD-ROM 800 nano meter 0.65 GB

DVD 350 nano meter 4.7 GB

HD-ROM 50 nano meter 165 GB