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Basic information and parameters of CD-ROM and DVD-ROM drives

CD-ROM

• The CD-ROM cannot be thought apart from computer. Most of recent programs, games and multimedia are stored on CD. Choosy users can buy the CD recorder drive and create CDs also at home. The prise for an MB is several times lower than on another media (1MB prize on different data medium)

• Working with CD on PC is nearly equal to working with a diskette - it differs in amount of data and impossibility of CD recording using a common CD drive. 

• You can save the data from up to 450 common diskettes on a CD. Total capacity of the CD is then 650MB.

• Reading is executed by help of a laser beam and a reader (čtecí zařízení) for the ray reflected from the media.

• Data record is conducted in a spiral (from the middle to the outer part). On a diskette and on the hard disk it is in round (circle) tracks.

• CD players with support of MP3 files are coming to the music industry, then a CD can contain up to several hours of music (approximately 100 - 200 songs). Suitable for portable players - then a CD will suffice instead of several ones.

DVD Disk - Potential substitute for CD

• DVD (Digital Video Disk) - The disk looks like a CD at the first glance, but in fact it has a higher capacity. One DVD disk can hold 4.7GB of data. There also exists a DVD on which the data are recorded onto the both sides and its capacity is 9,4GB. At present you can also see disks capacity of 18 GB, the data can be stored in two plies and on both sides (In case of recording on one of the sides of the two-ply media the capacity reaches 8,5 GB)

• First it was designed for movies that could be played also in the desk DVD players, now you can see some computer items on DVD, too.

• To play movies on PC you need a good computer with quality graphic card supporting MPEG-2 formats. Some DVD drives are equipped with an additional card for MPEG-2 support.

• DVD capacity for movies enables to insert concurrently 2 hours of image, 8 sound tracks and 32 subtitles on a disk, so the language barrier is greatly overcome and the DVD movie can be sold in several lands without the necessity of special adjustements for particular land. Though, some limitations exist. The world is divided into 6 regions. The devices are adjusted so that they are able to read the media in the only region where they were sold (in the same way the DVD movies are denoted). It means that in a player bought in Europe (region no. 2) you cannot play a movie from the USA (region no.1).

• A lot of people do not agree with this partition of the world, so there exist also players that are unofficially adjusted and then they are able to play any region.

• But the DVD movie makers have another problems, too. The ciphering for DVD titles protection was broken by a clever 14 years old boy. Great amount of people use different ways to transfer movie to a digital format and then to save it on a disk or a common and cheap CD. The Divx technology (also connected with Mpeg4 term) enables to have an 1,5 hour movie with a high quality picture and sound on one CD. 

• The raw speed is 1350KB/S (1x) - it is 9x more than on classical CDs. The most frequent are between 4x and 10x DVD

• Most of the DVD drives can also read classical CDs. Some drives are able not only to read the CD media but also to record them. 

• There also exist DVD drives for recording on DVD-R media. Because of their high price for the drive and also for the media they are not so much widespread. There is also not too much standards for the rewritable DVDs.

Types by the way of connection

• EIDE (ATAPI) - Cheap. Lower support than for SCSI devices. Most of the recording programs recognize most of the SCSI drives, but only a limited amount of EIDE drives. 

• SCSI - Fast, higher quality and more expensive than EIDE. A fast controller for PCI slot is recommended. 

• External - Classical connection using parallel port, slower transmission rate (baud rate), high price. Suitable only for portable computers or very frequent transmission. Limited support of recording programs. 

By speed

• In contrast to the harddisks the speed is not measured in turns per minute, but in the maximum speed of reading. The raw speed called also 1x speed means 150KB/S. 1x speed is used for common audio devices to read music CDs. For faster drives the speed is determined in multiples. This speed is not sufficient for the data transmission. At the time I am writing this text the 48x speed drives appear mostly.

• Higher speed does not mean higher quality drive. Some drives have problems with this speed and get easier worn out

• The access time reaches in the latest drives 100ms and more, it means more than 10x higher than in a hards disk.

3 technologies influencing the speed of reading and the access time 

• CLV (Constant Linear Velocity) - For data transmission the constant speed of data transmission with different speed of turns in the middle and at the edge of the CD is kept. This technology comes from the transmission of audio - it was necessary to keep continuous speed of data and sound; for modern drives it means a lot of problems with fluctuation of speed of turns and with the drive performance.

• CAV (Constant Angular Velocity) - For data transmission the non-constant transmission speed with constant speed of turns in the middle and at the edge of the CD is kept. Very good for higher performance of modern CD drives.

• Multibeam - We cannot raise the turns per minute interminably - it affects wear and tear of the drive and causes more problems than the drives with lower speed of turns do. There is coming a new technology called Multibeam that uses lower speed of turns, but more beams for data reading.

Example of speed for a drive as for reading and recording CD and CD/RW

• (example for a recording drive that is able to work with RW. 48x/12x/8x media)

• 48x for reading

• 12x for recording of classical media

• 8x for recording of rewritable media

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