Monday, January 22, 2001		On Hardware

DVD ROM scores over CD ROM

THE world of computer hardware is amazing. Each day, new technology is developed and added to this vast ocean of technology. This leads to a lot of confusion in the minds of the end-users. However, new technologies offer better products and are more futuristic. Older and bigger floppy drives gave way to the current 1.44 MB floppy drives. Today these are also on the verge of extinction.

Similar is the case with CD ROM drives. When CD ROM drives were introduced, it was termed as a revolution in the world of PCs. Indeed, it proved to be a revolution and lived up to its expectation, and during this time CD ROMs saw many generations of changes. Initially the speed of CD ROM was 2X. After several stages of development, it reached a maximum speed of 52 X.

Now CD ROMs also are becoming passé and DVD is now in. Internationally DVD, which was introduces in late 1997, has taken over CD ROM as a part of the standard equipment and comes fitted with most of the computer. However, in India, CD ROMs are still in vogue and continue to remain standard equipment.

What’s the difference between CD ROM and DVD ROM? Basically, there is a world of difference between these. The basic differences between CD ROM and DVD ROM can be summed up in three heads.

1) Storage capacity: Going by the storage capacity, the DVD specifications have four disk configurations, ranging from 4.7 GB to 17 GB. It ranges from single-sided, single-layer disks that are much like traditional CD ROM disks to double-sided double-layered DVDs. Normally a DVD ROM with a minimum storage capacity can store up to the data of 7 CD ROMs. Normally the capacity of a CD ROM is only 650 MB. The capacity of DVD ROM starts from 4.4 GB, which is 7 times more than a normal CD ROM. However, this storage capacity of DVD ROM relates to single-layer storage. But with double-layer storage, the storage capacity of DVD ROM can go up to 8 GB, which is 12.5 times higher than a normal CD ROM. Furthermore, double-sided, DVD ROM drives can store 8.8 GB, 14 times greater than a CD ROM’s capacity. Double-sided, dual-layer DVD-ROM drives store 15.9 GB, 25 times greater than a CD ROMs capacity.

To put it differently a CD ROM can hold the entire encyclopaedia, but a DVD ROM can hold the encyclopaedia, the dictionary, the thesaurus, and the phone number/address of everyone in whole India with still a lot of space left! With the storage of a DVD ROM, you can see interactive multimedia programs that use hours of full-screen video to help you see movies, play games or even learn your courses.

The basic reason why a DVD can store such huge quantities of data is due to its design and also the laser. Because the DVD ROM drive uses a shorter-wavelength light than the standard CD ROM drive, it can read smaller data pits on a spiral track whose turns are closer together than on a CD. And the data layer is only half as thick as on a conventional CD, allowing manufacturers to bond two layers onto a double-sided DVD disk of the same thickness as a CD (1.2 mm). This design provides tremendous storage space, although you need to turn the disk over to access the data on the other side.

Furthermore, with advancement in technologies even more data was squeezed onto one side of a DVD. By using a semi-transparent gold layer atop the reflective silver-coloured layer, the DVD can store two layers of data on one side. Using a lower-power beam, the laser can read the data from the gold layer; then, with an increase of power, it can access and read the silver layer. This method results in slightly less than double the capacity of a single layer, but that is still an impressive 8.5 GB per side. Moreover by combining the two dual-layer platters into a double-sided DVD a whopping capacity of 17GB can be worked out on a single DVD, which is roughly enough for an 8-hour movie.

Thus, overall there is so much technological development in the DVD that it simply outclasses the CD in storage capacity.

2) Speed of data transfer: DVD ROM scores over the CD ROM. Going by figures the minimum transfer rate of CD ROM drives is 150 KB per second. However, the minimum transfer rate of DVD ROM drives is 11.08 MB per second. The data transfer rate of DVD ROM is approximately equivalent to a 92X CD ROM.

3) Reliability: Individually though CD ROMs are very reliable and have a long shelf life yet if compared to the DVD, the DVD scores over the CDs in terms of reliability. Since DVD ROM discs are made of plastics bonded together, the discs are more rigid than CD ROM discs. Though the DVDs can also catch scratch just like CDs yet the scratches in the DVD normally do not matter as they are usually out of focus of the laser and therefore it does not affect the readability of the DVDs. In case of CDs, scratches could affect the data and could also make it useless if the scratches destroy critical tracks. Besides scratches, other factors like error correction also makes DVD much better and reliable in comparison to the CDs. Normally, in case of the DVD, the error correction is 10 times more effective than the CD error correction. Moreover the CDs normally follow RS-CIRC error correction mode, whereas DVD have RS-PC error correction. Likewise the error correction overhead in case of CD ROM is very high at 34 per cent, whereas it is only 13 per cent in case of DVD drive.

Besides these major differences between the DVD and CD ROM other differences also clearly give the verdict in favour of DVD ROM. For example, the speed of the DVD drive, which is 11.1 MBPS, is roughly 9 times faster than the fastest CD ROM. Another difference between DVD ROM and CD ROM is that DVD is backward compatible and can read the data from a CD. It can read the following types of CDs viz. CD DA, CD ROM, CD ROM XA, Video CD (VCD), Enhanced CD, CD Audio, CD R, CD RW, Photo CD, CD G, CD Plus etc. However a CD ROM is not forward compatible and can not read data from a DVD.

— VV