CD & DVD TECHNOLOGY
INTRODUCTION
Recordable CD & DVD drives have become a must-have for home computers. More and more machines are including CD or DVD writer drives as a standard feature, and fast, affordable drives are easily available for machines without.
However, there's some question about new recordable DVD formats. There are no fewer than seven DVD formats, each with different specs and drive requirements — and not all of them are intended for computing.
However, there's some question about new recordable DVD formats. There are no fewer than seven DVD formats, each with different specs and drive requirements — and not all of them are intended for computing.
DIFFERENCE BTW CD AND DVD
DVD media has at least seven times the capacity of CD media — a feat partially accomplished by using multiple recording layers and recording on both sides of the media. The write method also helps boost the capacity of DVD.
Data pits & lasers Like CDs, DVDs store data in microscopic grooves running in a spiral around the disc.
All DVD drive types use laser beams to scan these grooves: Minuscule reflective bumps (called lands) and nonreflective holes (called pits) aligned along the grooves represent the zeros and ones of digital information.
DVD technology writes in smaller "pits" to the recordable media than CD technology. Smaller pits mean that the drive's laser must produce a smaller spot. DVD technology achieves this by reducing the laser's wavelength from the 780nm infrared light used in standard CD drives to 625nm to 650nm red light.
Smaller data pits allow more pits per data track. The minimum pit length of a single layer DVD-RAM is 0.4 micron as compared to 0.834 micron for a CD. Additionally, DVD tracks are closer together, allowing more tracks per disc. Track pitch-the distance from the center of one part of the spiral information or "track" to the adjacent part of the track-is smaller. On a 3.95GB DVD-R, track pitch is 0.8 microns; CD track pitch is 1.6 microns. On 4.7GB DVD-R media, an even smaller track pitch of 0.74 microns helps boost storage capacity.
These narrow tracks require special lasers for reading and writing — which can't read CD-ROMs, CD-Rs, CD-RWs, or audio CDs. DVD-ROM drive makers solved the problem by putting two lasers in their drives: One for DVDs, the other for CDs.
Layers To facilitate the focusing of the laser on smaller pits, DVD media uses a thinner plastic substrate than do CDs. This reduces the depth of the layer that the laser must pass through to reach the pits to record or read data. This reduction originally resulted in disks that were 0.6mm thick — half the thickness of a CD.
Even single-sided DVDs have two substrates, even though one isn't capable of holding data.
DVD technology writes in smaller "pits" to the recordable media than CD technology. Smaller pits mean that the drive's laser must produce a smaller spot. DVD technology achieves this by reducing the laser's wavelength from the 780nm infrared light used in standard CD drives to 625nm to 650nm red light.
Smaller data pits allow more pits per data track. The minimum pit length of a single layer DVD-RAM is 0.4 micron as compared to 0.834 micron for a CD. Additionally, DVD tracks are closer together, allowing more tracks per disc. Track pitch-the distance from the center of one part of the spiral information or "track" to the adjacent part of the track-is smaller. On a 3.95GB DVD-R, track pitch is 0.8 microns; CD track pitch is 1.6 microns. On 4.7GB DVD-R media, an even smaller track pitch of 0.74 microns helps boost storage capacity.
These narrow tracks require special lasers for reading and writing — which can't read CD-ROMs, CD-Rs, CD-RWs, or audio CDs. DVD-ROM drive makers solved the problem by putting two lasers in their drives: One for DVDs, the other for CDs.
Layers To facilitate the focusing of the laser on smaller pits, DVD media uses a thinner plastic substrate than do CDs. This reduces the depth of the layer that the laser must pass through to reach the pits to record or read data. This reduction originally resulted in disks that were 0.6mm thick — half the thickness of a CD.
Even single-sided DVDs have two substrates, even though one isn't capable of holding data.
Double-sided discs with two data surfaces must be turned over (much like old vinyl records!) to read data on each side.
Data access speeds DVD accesses data faster than CD and uses more robust error correction. In fact, the speed of DVD demands a new unit of measure. CD drive speeds are expressed as multiples of that format's original data transfer rate "X," or 150KB per second. A 32X CD-ROM drive reads data at 32 times 150KBps or 4MBps. DVD's 1X is a blistering 1.38MBps. That's faster than an 8x CD drive.
Data access speeds DVD accesses data faster than CD and uses more robust error correction. In fact, the speed of DVD demands a new unit of measure. CD drive speeds are expressed as multiples of that format's original data transfer rate "X," or 150KB per second. A 32X CD-ROM drive reads data at 32 times 150KBps or 4MBps. DVD's 1X is a blistering 1.38MBps. That's faster than an 8x CD drive.
Universal Data Format (UDF) Another big difference between DVD recording and CD recording is the recording format.
DVD recording uses UDF — Universal Data Format. UDF makes it possible to store data, video, audio, or a mix of all three within a single physical file structure. This file structure ensures that any file can be accessed by any drive, computer, or consumer video. UDF includes the CD-standard ISO 9660 compatibility, but CDs do not comply with UDF.
Updated December 10, 2002
Updated December 10, 2002
SUMMARY
Compact Disc (CD): Red Book, 5.1 Music Disc, SACD, PhotoCD, CD-R, CD-ROM, CD-RW, CD Video (CDV), Video CD (VCD), SVCD, CD+G, CD-Text, CD-ROM XA, CD-i
MiniDisc (MD) (Hi-MD)
DVD: DVD-R, DVD+R, DVD-R DL, DVD+R DL, DVD-RW, DVD+RW, DVD-RW DL, DVD+RW DL, DVD-RAM, DVD-D
Ultra Density Optical (UDO)
Universal Media Disc (UMD)
HD DVD: HD DVD-R, HD DVD-RW, HD DVD-RAM, HD DVD-ROM Blu-ray Disc (BD): BD-R, BD-RE
Ultra Density Optical (UDO)
Universal Media Disc (UMD)
HD DVD: HD DVD-R, HD DVD-RW, HD DVD-RAM, HD DVD-ROM Blu-ray Disc (BD): BD-R, BD-RE
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