CD-ROM & DVD

· 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 is 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 650/700 MB of data, and it is very inexpensive to produce. Today, there are three types of CD drives and DVD drives are on their way:

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

· 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 in Windows.

CD-ROM

· 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 680 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 audiocassette 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 is 5 km long and hold up to 650 MB data in about 5.5 billion dots.

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.

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.

· 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.

They produce so-called 40X40-drives with 7 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.

· 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

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 we 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.

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.

• Most people use CD-Recorders for:

1. Copying music CDs for personal use

2. Backing up data (documents, images, programs)

3. Producing MP3 CDs (with up to 12 hours of music)

· CD-RW:

The CD-ReWritable (CD-RW) is another type of CD, 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 multi-read.

To work with CD-RW you need special software like Adaptec. It comes with the HP 8100 drive:

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

With the falling prices of CD-R medias the CD-RW option has become less interesting. Most people do not use CD-RW very much, from what I hear; they just burn the CD-Rs they need.

· 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 great IDE-based CD-RWs.

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.

This so-called buffer under run happens quite often, especially if you process other disk-intensive work on the PC while burning a CD-ROM in a EIDE-based drive.

If you use an EIDE burner, the best is to connect it as a master unit on the secondary EIDE channel with the hard disk and CD-ROM drive on the other channel. Therefore I use this setup:

It works fine; I can "burn" what I want at 4X speed. I also burn directly from CD-ROM till CD-R without copying to a hard disk. In the last case, it is best to leave the machine undisturbed during the process.

If you need hard disk number two in a configuration as above, you should not connect it as a slave on the vacant EIDE 2 slave channel. Instead I recommend the inexpensive Fast Track controller for expanding your EIDE system. Or better: Get yourself a 40 GB hard disk.

· Burn speed and buffers:

In 2000 the best drives selling were operating at speeds like 32x12x10x. This means:

· CD-ROM 32X

· CD-R 12X

· CD-RW 10X

This goes for a drive like PleXWriter 12/10/32A.

It also includes a new technology called "BURN-Proof". It should enable the recorder to pause if the Buffer-Under-Run-situation occurs. When data start rolling again, the recorder continues writing where it left. I do not know if it works (it probably does), but I do know, that this is a very good drive. It holds 2 MB of cache.

The HP 9310e is another top-drive. It is rated for the speeds 32x10x4x.

The HP 9310e is also a very fine performer. It holds a buffer of 4 MB cache, which is quite a lot.

Personally I do not understand why they do not use 16 or 32 MB for buffer - RAM being so cheep. It would effectively remove the buffer under run problem. The 4 MB of buffer in HP 9310e is good for a 2,3 seconds long delay at 10X speed burning.

· Synchronize data transfer:

One detail. Check yours for Settings on both CD-ROM drives. You should enable Synchronize data transfer.

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

DVD

· An introduction to the DVD:

The DVD is a high-capacity optic media. The DVD standard was developed in the mid 1990ies by leading companies like

Philips and Sony. DVD stands for Digital Versatile Disk. The DVD is an all-round disk, which probably will replace CD-ROM and laser disks. Over a few years DVD should replace VHS tapes for videos as well. Some DVD drives can both read and write the disks. The drives are sold in many versions and with many incompatible sub-standards.

· A CD-like disk:

The DVD is a flat disk of the same size as a CD. It holds a diameter of 4.7 inches (12 cm) and is .05 inches (1.2 mm) thick. Data are stored in a small indentation in a spiral track, just like in the CD; only the tracks are narrower.

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.

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

The DVD drives are often bundled with a MPEG-2 decoder. This is required if you want to replay DVD videodisks at optimal quality. Some graphics cards like Matrox-G400 MAX come with a Cinematic-based software decoder. This works together with the graphics accelerator chip and gives reasonable DVD replay quality.

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 20-30 MB/second). No optic media can keep up with this nor with the speedy seeks we get from the hard disks.

But the DVD will undoubtedly gain a place as the successor to the CD-ROM. New drives will read both CD-ROMs and DVDs.

· Various DVD types:

We have several versions of the DVD:

1. DVD-ROM:

This is the simplest format made for data read-only. It is like a beefed-up CD-ROM. This medium is usable for distribution of software and other data for PC use.

The outer layers can hold 4.7 GB, the underlying 3.8 GB. The largest version can hold a total of 17 GB.

A single layer DVD-5 disk holds 4.7 GB. A dual-layered DVD-9 disk holds 8.5 GB.

The dual-sided DVDs are named DVD-10 (9.4 GB) and DVD-18 (17 GB).

2. DVD Video disk:

This the most important standard. It is a highly sophisticated and very complex format mixing video, sound and data in a very special format.

3. DVD-R:

The Recordable DVD are written once only like CD-R. This disk can hold 3.95 GB per side.

4. DVD RAM/+RW:

There an at least three different and in compatible formats of re-writable DVD.

The disk can be written and read like a hard disk or perhaps more like a CD-RW.

· DVD RAM:

Three writable technologies are present at the market:

1. Pioneer has a DVD-Recordable technology placing 3.95 GB per disk.

2. DVD-RAM is an RW-disk from Hitachi and Matsushiti. The 1st Generation disks hold 3.6 GB, while the 2nd generation holds 4.7 GB. The disks are hold in a special cartridge.

3. The so-called DVD+RW, supported by HP, Sony, Philips, Yamaha, Ricoh and Mitsubishi holds up to 4.7 GB per disk.

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.

· The DVD Video Disk:

The DVD videodisk is the most well known DVD format. It is a 4.7 GB disk, which can hold up to 135 minutes top quality video with 8 digital soundtracks (AC3, Digital Dolby) and sub-titles in 32 languages. Plus special features like interviews and trailers, alternate versions.

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.

The Dolby AC-3 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 an AC-3 compatible stereo set up.

The DVD videodisk is protected against illegal copying.

• MPEG-2 decoding:

The video format on DVD disks is in MPEG-2 coding. That is a compression technology, which requires lots of processor power. When you buy or rent a DVD videodisk, the digital video stream of the movie has been heavily compressed.

Hence, the data stream from the disk has to be decoded when you watch the film.

This has to be done in real-time during the replay (real time decoding).

Soft or hard decoding?

MPEG-2 decoding can be done in two ways:

1. Software based decoding

2. Hardware based decoding

The PCs CPU using special software does the software-based decoding. This is not always very good since it drains the PC. Also the CPU seldom is powerful enough to perform a perfect decoding. Some graphics chips include DVD decoding features, but have to work together with a software decoder as well.

. The software based decoding does not require any new hardware, but it requires a very powerful CPU and/or graphics adapter.

If there is not sufficient processor power, you will see a loss of some individual images. The movie gets "choppy."

The hardware-based decoding is to prefer. Here the PC is equipped with a special chip (on an adapter), which only has to decode the MPEG data stream. The most well known products are based on the called Real Magic Hollywood+ chipset.

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 need to install an extra card in the PC to enable seeing DVD films.

However, MPEG cards are not always powerful enough either, according to the reviews and to what I so far have seen. All in all, I might 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.

· The players:

You can view DVD video several ways:

1. In a PC with a DVD drive as described above.

2. Using a specific DVD-player.

3. Using a Sony PlayStation2 device.

If you use your PC to replay videos, you should have a hardware-based MPEG-2 decoder in the PC. But the best replay comes from a DVD-Video home player.

These units only play DVD video disk and do not interface with the PC.

· Multiplayer:

In 1999 we suddenly saw a series of new products, namely integrated CD, MP3 and DVD players from Taiwan. The devices are designed to fit into the HIFI stereo set.

I have one of those new players myself, and it works fine. It plays so to say any type of optic disc, be it music CD, CD-R with (12 hours of) MP3, DVD or small laserdiscs.

The device is based on a standard PC DVD drive, so it can be pretty noisy. The Video DVD replay is comparable to the one from a PC with DVD drive and MPEG-2 adapter.