KarbosGuide.com Module 4c.. 2
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 650/700 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 canCD-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 in Windows . If you hit Win+Pause, the System box opens. Find your CD-ROM drives like here (The box is Danish, but you'll find it) :
Highlight the drive and choose Properties. Then you can arrange the drive letters:
I like to reserve the letters I: and J: for CD drives.
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 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 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.
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 track1X / 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.
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 (Sony/Philips Digital InterFace) output that can deliver a purely digital signal. This gives better sound performance and opens for new interconnectivity (i.e. with a minidisc-recorder).
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:
· Copying music CDs for personal use
· Backing up data (documents, images, programs)
· 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 multiread.
To work with CD-RW you need special software like Adaptecs. 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.
The CD-R/RW
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 12X speed. I also burn directly from CD-ROM til CD-R without copying to a harddisk. In the last case, it is best to leave the machine undisturbed during the process, even with modern "burn-proof" CD-recorders.
If you need harddisk number two in a configuraton as above, you should not connect it as a slave on the vacant EIDE 2 slave channel. Instead I recommend the inexpensive FastTrack controller for expanding your EIDE system. Or better: Get yourself a 60 or 80 GB harddisk!
Burn speed and buffers
In 2000 the best drives selling were operating at speeds like 40x20x10x. This meens :
· CD-ROM 40X
· CD-R 20X
· CD-RW 10X
Here you see an drive called 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.