HARD DISC :-

 A hard disk is a sealed unit that a PC uses for nonvolatile data storage.  The hard drive is used to store crucial programming and data.  A contains rigid, disk-shaped platters, usually constructed of aluminum or glass.

 A hard disk uses round, flat disks called platters, coated on both sides with a special media material designed to store information in the form of magnetic patterns.  The platters are mounted by cutting a hole in the center and stacking them onto a spindle.  The platters rotate at high speed, driven by a special spindle motor connected to the spindle.  Special electromagnetic read/write devices called heads are mounted onto sliders and used to either record information onto the disk or read information from it. The sliders are mounted onto arms, all of which are mechanically connected into a single assembly and positioned over the surface of the disk by a device called an actuator.  A logic board controls the activity of the other components and communicates with the rest of the PC.  Each surface of each platter on the disk can hold tens of billions of individual bits of data.  Each platter has two heads, one on the top of the platter and one on the bottom, so a hard disk with three platters (normally) has six surfaces and six total heads.  Each platter has its information recorded in concentric circles called tracks.  Each track is further broken down into smaller pieces called sectors, each of which holds 512 of information. Hard Disk – Platters and Media :- • Every hard disk contains one or more flat disks that are used to actually hold the data in the drive. • These disks are called platters (sometimes also "disks" or "discs"). • They are composed of two main substances: – a substrate material that forms the bulk of the platter and gives it structure and rigidity, – a magnetic media coating which actually holds the magnetic impulses that represent the data. • Hard disks get their name from the rigidity of the platters used, as compared to floppy disks and other media which use flexible "platters" (actually, they aren't usually even called platters when the material is flexible.) Hard Disk – Tracks and Sectors :- • Each platter is broken into tracks--tens of thousands of them--which are tightly-packed concentric circles. • Track is one of the many concentric circles that holds data on a disk surface. • A track holds too much information to be suitable as the smallest unit of storage on a disk, so each one is further broken down into sectors. • A sector is normally the smallest individually-addressable unit of information stored on a hard disk, and normally holds 512 bytes of information. – The first PC hard disks typically held 17 sectors per track. – Today's hard disks can have thousands of sectors in a single track, and make use of zoned recording to allow more sectors on the larger outer tracks of the disk. A platter from a 5.25" hard disk, with 20 concentric tracks drawn over the surface. This is far lower than the density of even the oldest hard disks; even if visible, the tracks on a modern hard disk would require high magnification to resolve. Each track is divided into 16 imaginary sectors. Older hard disks had the same number of sectors per track, but new ones use zoned recording with a different number of sectors per track in different zones of tracks

Hard Disk – Read & Write Head :- • The read/write heads of the hard disk are the interface between the magnetic physical media on which the data is stored and the electronic components that make up the rest of the hard disk (and the PC). • The heads do the work of converting bits to magnetic pulses and storing them on the platters, and then reversing the process when the data needs to be read back. Hard Disk Read/Write Operation :- • Older, conventional (ferrite, metal-in-gap and thin film) hard disk heads work by making use of the two main principles of electromagnetic force. – Write : applying an electrical current through a coil produces a magnetic field;. The direction of the magnetic field produced depends on the direction that the current is flowing through the coil. – Read : that applying a magnetic field to a coil will cause an electrical current to flow; this is used when reading back the previously written information. • Newer (MR and GMR) heads don't use the induced current in the coil to read back the information; they function instead by using the principle of magnetoresistance, where certain materials change their resistance when subjected to different magnetic fields. Hard Disk :- – An MR head employs a special conductive material that changes its resistance in the presence of a magnetic field. As the head passes over the surface of the disk, this material changes resistance as the magnetic fields change corresponding to the stored patterns on the disk. A sensor is used to detect these changes in resistance, which allows the bits on the platter to be read. – MR technology is used for reading the disk only. For writing, a separate standard thin-film head is used. This splitting of chores into one head for reading and another for writing has additional advantages. • Ferrite vs MR Head – The use of MR heads allows much higher areal densities to be used on the platters than is possible with older designs, greatly increasing the storage capacity and (to a lesser extent) the speed of the drive. – allows the use of weaker written signals, which lets the bits be spaced closer together without interfering with each other, improving capacity by a large amount.

Extreme closeup view of a ferrite read/write head. The head is at the end of the slider, wrapped with the coil that magnetizes it for writing, or is magnetized during a read.

Closeup view of an MR head assembly. Note that the separate copper lead wire of older head designs is gone, replaced by thin circuit-board-like traces. The slider is smaller and has a distinctive shape. The actual head is too small to be seen without a microscope. Hard Disk Tracks, Cylinders and Sectors :- All information stored on a hard disk is recorded in tracks, which are concentric circles placed on the surface of each platter, much like the annual rings of a tree. The tracks are numbered, starting from zero, starting at the outside of the platter and increasing as you go in. A modern hard disk has tens of thousands of tracks on each platter. Data is accessed by moving the heads from the inner to the outer part of the disk, driven by the head actuator. This organization of data allows for easy access to any part of the disk, which is why disks are called random access storage devices.Each track can hold many thousands of bytes of data. It would be wasteful to make a track the smallest unit of storage on the disk, since this would mean small files wasted a large amount of space. Therefore, each track is broken into smaller units called sectors. Each sector holds 512 bytes of user data, plus as many as a few dozen additional bytes used for internal drive control and for error detection and correction.

Clusters Each partition on your hard disk is subdivided into clusters. A cluster is the smallest possible unit of storage on a hard disk. The size of a cluster depends on two things: 1. The size of the partition 2. The file system installed on the partition  Regardless of the size of your partition, the maximum number of clusters remains the same. For example, in a standard FAT file system, a partition can contain no more than 65,536 clusters. Because of this limitation, cluster sizes vary depending on partition size. Basically, a small partition uses small cluster sizes while a large partition uses large cluster sizes.  In a world where bigger is better and size does matter, you might assume that a big hard drive with huge cluster sizes is the way to go. However, this isn’t the case. As you’ll recall, clusters are the smallest unit of storage space on a hard disk. This means that you can’t share a cluster among multiple files. If you have a tiny file and a huge cluster, the portion of that cluster unused by the file is wasted.  To understand the above statement, it’s necessary to look at some real numbers. In a standard FAT file system, the smallest size that a cluster can be is 2 KB. Therefore, if you save a 1- file on a hard disk using a 2-KB cluster size, you’ll actually use the entire 2 KB (2,048 bytes). So, you could potentially waste over 2,000 times more space than you’d use if you had a hard disk full of 1-byte files.  What makes this fact even scarier is that, as I mentioned before, 2 KB is the smallest size that a cluster can be. However, don’t forget about that 65,536-cluster limit. This means if you want to use 2-KB clusters, your partition size can be no more than 128 MB (2-KB clusters multiplied by 65,536 total clusters equals 134,217,728 bytes, or 128 MB). If you want to have a FAT partition larger than 128 MB, the cluster size will have to grow.  As the size of your partition increases, the cluster sizes double to accommodate it. Below is a chart that outlines the number of clusters required to accommodate various partition sizes. Remember that the maximum size of a FAT partition is 2 GB. MBR :-  A master boot record (MBR) is a type of popularized by the IBM Personal .[1] It consists of a sequence of 512 byteslocated at the first sector of a data storage device such as a hard disk. MBRs are usually placed on storage devices intended for use withIBM PC-compatible systems.  The MBR may be used for one or more of the following: 1. Holding a partition table, which describes the partitions of a storage device. In this context the boot sector may also be called a partition sector. 2. Bootstrapping an . The BIOS built into a PC-compatible computer loads the MBR from the storage device and passes execution to machine code instructions at the beginning of the MBR. 3. Uniquely identifying individual disk media, with a 32-bit disk signature, even though it may never be used by the operating system.[2][3][4][5]  Because of the broad popularity of PC-compatible , the MBR format is widely used, to the extent of being supported by computer operating systems in addition to other pre-existing or cross-platform standards for bootstrapping and partitioning.[6]

Zone Recording: - Zone Bit Recording (ZBR) is used by disk drives to store more sectors per track on outer tracks than on inner tracks. It is also called ZoneConstant Angular Velocity (Zone CAV or Z-CAV or ZCAV). On a disk consisting of roughly concentric tracks - whether realized as separate circular tracks or as a single spiral track-, the physical track length (circumference) is increased as it gets farther from the center hub. The inner tracks are packed as densely as the particular drive's technology allows, but with a CAV drive the data on the outer tracks are less densely packed. Using ZBR the drive divides all the tracks into a number of zones, and the inner track of each zone is packed as densely as it can, with the other tracks in that same zone recorded with the same read/write rate. This permits the drive to have more bits stored in each track outside of the innermost zone than drives not using this technique. Storing more bits per track equates to achieving a higher total data capacity on the same disk area.[1] On a hard disk using ZBR, the data on the tracks in the outer most zone will have the highest data transfer rate. Since both hard disks and floppy disks typicallly number their tracks beginning at the outer edge and continuing inward, and since operating systems typically fill the lowest-numbered tracks first, this is where the operating system typically stores its own files during its initial installation onto an empty drive. Testing disk drives when they are new or empty after defragmenting them with some benchmarking applications will often show their highest performance. After some time, when more data is stored in the inner tracks, the average data transfer rate will drop, because the transfer rate in the inner zones is slower; often making people think their disk drive is slowing down over time.[1]

Formatting: - is the process of preparing a hard disk drive or flexible disk medium for data storage. In some cases, the formatting operation may also create one or more new file systems. The formatting process that performs basic medium preparation is often referred to as "low-level formatting." The term "high level formatting" most often refers to the process of generating a new file system. In certain operation systems (e.g., Microsoft Windows), the two processes[clarification needed] are combined and the term "format" is understood to mean an operation in which a new disk medium is fully prepared to store files. Illustrated to the right are the prompts and diagnostics printed by MS-DOS's FORMAT.COM utility as a hard drive is being formatted. As a general rule, formatting a disk is "destructive," in that existing data (if any) is lost during the process.

Low Level Formatting :- Hard disk drives prior to the 1990s typically had a separate that defined how data was encoded on the media. With the media, the drive and/or the controller possibly procured from separate vendors, low level formatting was a potential user activity. Separate procurement also had the potential of incompatibility between the separate components such that the subsystem would not reliably store data.[7]  User instigated low-level formatting (LLF) of hard disk drives was common for minicomputer andpersonal computer systems until the 1990s. IBM and other mainframe system vendors typically supplied their hard disk drives (or media in the case of removable media HDDs) with a low-level format. Typically this involved subdividing each track on the disk into one or more blocks which would contain the user data and associated control information. Different computers used different block sizes and IBM notably used variable block sizes but the popularity of the IBM PC caused the industry to adopt a standard of 512 user data bytes per block by the middle 1980s.  Depending upon the system, low-level formatting was generally done by an operating system system utility. IBM compatible PCs used the BIOS which is involved using the MS-DOS debug program to transfer control to a routine hidden at different addresses in different .[8] Low-level format function can also be called as "erase" or "wipe" in different tools. For best results it's highly recommended to use tools created by hard disk's manufacturer. High Level formatting :- High-level formatting is the process of setting up an empty file system on the disk and, for PC's, installing a boot sector. This is a fast operation, and is sometimes referred to as quick formatting.  The entire logical drive or partition may optionally be scanned for defects, which may take considerable time.  In the case of floppy disks, both high- and low-level formatting are customarily performed in one pass by the disk formatting software. In recent years,[when?] most floppies have shipped pre-formatted from the factory as DOS FAT12 floppies.  In current IBM mainframe operating systems derived from OS/360 or DOS/360, this may be done as part of allocating a file, by a utility specific to the file system or, in some older access methods, on the fly as new data are written. Partitioning :- Partitioning is the process of writing information into blocks of a storage device or medium that allows access by an operating system. Some operating systems allow the device (or its medium) to appear as multiple devices; i.e. partitioned into multiple devices.  On PC and UNIX-based operating systems (such as BSD, Linux/GNU, OS X) this is normally done with a partition editor, such as fdisk,parted, and Disk Utility. These operating systems support multiple partitions.  In current IBM mainframe OSs derived from OS/360 and DOS/360, such as z/OS and z/VSE, this is done by the INIT command of the ICKDSF utility.[13] These OSs support only a single partition per device, called a volume. The ICKDSF functions include creating a volume label and writing a Record 0 on every track.  Floppy disks are not partitioned; however depending upon the OS they may require volume information in order to be accessed by the OS.  Partition editors and ICKDSF today do not handle low level functions for HDDs and optical disk drives such as writing timing marks, and they cannot reinitialize a modern disk that has been degaussed or otherwise lost the factory formatting. PATA :- Parallel ATA (PATA), originally AT Attachment, is an interface standard for the connection of storage devices such as hard disks, solid-state drives, floppy drives, and optical disc drives in computers. The standard is maintained by X3/INCITScommittee.[1] It uses the underlying AT Attachment (ATA) and AT Attachment Packet Interface (ATAPI) standards.

 The Parallel ATA standard is the result of a long history of incremental technical development, which began with the original AT Attachment interface, developed for use in early PC AT equipment. The ATA interface itself evolved in several stages from 's original Integrated Drive Electronics (IDE) interface. As a result, many near- synonyms for ATA/ATAPI and its previous incarnations are still in common informal use. After the introduction of Serial ATA in 2003, the original ATA was renamed Parallel ATA, PATA for short.

 Parallel ATA cables have a maximum allowable length of only 18 in (457 mm).[2][3]Because of this limit, the technology normally appears as an internal computer storage interface. For many years ATA provided the most common and the least expensive interface for this application. It has largely been replaced by Serial ATA (SATA) in newer systems. Parallel ATA Interface :-

Parallel ATA cables transfer data 16 bits at a time. The traditional cable uses 40-pin connectors attached to a ribbon cable. Each cable has two or three connectors, one of which plugs into an adapter interfacing with the rest of the computer system. The remaining connector(s) plug into drives.

ATA's cables have had 40 wires for most of its history (44 conductors for the smaller form-factor version used for 2.5" drives — the extra four for power), but an 80-wire version appeared with the introduction of the Ultra DMA/33 (UDMA) mode. All of the additional wires in the new cable are ground wires, interleaved with the previously defined wires to reduce the effects of capacitive coupling between neighboring signal wires, reducing crosstalk. Capacitive coupling is more of a problem at higher transfer rates, and this change was necessary to enable the 66 per second (MB/s) transfer rate of UDMA4 to work reliably. The faster UDMA5 and UDMA6 modes also require 80-conductor cables. SATA :- Serial ATA (SATA or Serial Advanced Technology Attachment) is a computer interface for connecting host bus adapters to mass storage devices such as hard disk drives and optical drives. Serial ATA was designed to replace the older parallel ATA (PATA) standard (often called by the old name IDE), offering several advantages over the older interface: reduced cable size and cost (7 conductors instead of 40), native , faster data transfer through higher signalling rates, and more efficient transfer through an (optional) I/O queuing protocol.

SATA host-adapters and devices communicate via a high-speed serial cable over two pairs of conductors. In contrast, parallel ATA (the redesignation for the legacy ATA specifications) used a 16-bit wide data bus with many additional support and control signals, all operating at much lower frequency. To ensure backward compatibility with legacy ATA software and applications, SATA uses the same basic ATA and ATAPI command-set as legacy ATA devices.

As of 2009, SATA has replaced parallel ATA in most shipping consumer desktop and laptopcomputers, and is expected to eventually replace PATA in embedded applications where space and cost are important factors. SATA's market share in the desktop PC market was 99% in 2008.[2] PATA remains widely used in industrial and embedded applications that useCompactFlash storage, though even there, the next CFast storage standard will be based on SATA.[3][4] Features Hotplug

The Serial ATA Spec includes logic for SATA device hotplugging. Devices and that meet the interoperability specification are capable of hot plugging. Advanced Host Controller Interface Advanced Host Controller Interface (AHCI) is an open host controller interface published and used by , which has become a de facto standard. It allows the use of advanced features of SATA such as hotplug and (NCQ). If AHCI is not enabled by the and , SATA controllers typically operate in "IDE emulation" mode, which does not allow features of devices to be accessed if the ATA/IDE standard does not support them. Windows device drivers that are labeled as SATA are often running in IDE emulation mode unless they explicitly state that they are AHCI mode, in RAID mode, or a mode provided by a proprietary driver and command set that was designed to allow access to SATA's advanced features before AHCI became popular. Modern versions of Microsoft Windows, Mac OS X, FreeBSD, Linux with version 2.6.19 onward,[6] as well as Solaris and OpenSolaris, include support for AHCI, but older OSs such as Windows XP do not. Even in those instances, a proprietary driver may have been created for a specific chipset, such as Intel's

External SATA :- Standardized in 2004, eSATA (e standing for external) provides a variant of SATA meant for external connectivity. It uses a more robust connector, longer shielded cables, and stricter (but backward-compatible) electrical standards. The protocol and logical signaling (link/transport layers and above) are identical to internal SATA. The differences are:

. Minimum transmit amplitude increased: Range is 500–600 mV instead of 400–600 mV. . Minimum receive amplitude decreased: Range is 240–600 mV instead of 325–600 mV. . Maximum cable length increased to 2 metres (6.6 ft) (USB and FireWire allow longer distances.) . The external cable connector is a shielded version of the connector specified in SATA 1.0a with these basic differences: . The external connector has no "L"-shaped key, and the guide features are vertically offset and reduced in size. This prevents the use of unshielded internal cables in external applications and vice-versa. . To prevent ESD damage, the design increased insertion depth from 5 mm to 6.6 mm and the contacts are mounted farther back in both the receptacle and plug. . To provide EMI protection and meet FCC and CE emission requirements, the cable has an extra layer of shielding, and the connectors have metal contact-points. . The connector shield has retention springs in on both the top and bottom surfaces. . The external connector and cable have a design-life of over five thousand insertions and removals, whereas the internal connector is specified to withstand only fifty. Aimed at the consumer market, eSATA enters an external storage market served also by the USB and FireWire interfaces. The SATA interface has certain advantages. Most external hard-disk-drive cases with FireWire or USB interfaces use either PATA or SATA drives and "bridges" to translate between the drives' interfaces and the enclosures' external ports; this bridging incurs some inefficiency. Some single disks can transfer 157 MB/s during real use,[8] about four times the maximum transfer rate of USB 2.0 or FireWire 400 (IEEE 1394a) and almost twice as fast as the maximum transfer rate of FireWire 800. The S3200 FireWire 1394b spec reaches ~400 MB/s (3.2 Gbit/s), and USB 3.0 has a nominal speed of 5 Gbit/s. Some low-level drive features, such asS.M.A.R.T., may not operate through some USB [24] or FireWire or USB+FireWire bridges; eSATA does not suffer from these issues provided that the controller manufacturer (and its drivers) presents eSATA drives as ATA devices, rather than as "SCSI" devices, as has been common withSilicon Image, JMicron, and NVIDIA nForce drivers for Windows Vista. In those cases SATA drives will not have low-level features accessible. Firewire's future 6.4 Gbit/s (768 MB/s) will be faster than eSATA I. The eSATA version of SATA 6G will operate at 6.0 Gbit/s (the term SATA III is being eschewed by the SATA-IO to avoid confusion with SATA II 3.0 Gbit/s, which was colloquially referred to as "SATA 3G" [bps] or "SATA 300" [MB/s] since 1.5 Gbit/s SATA I and 1.5 Gbit/s SATA II were referred to as both "SATA 1.5G" [b/s] or "SATA 150" [MB/s]). Therefore, they will operate with negligible differences between them.[25] Once an interface can transfer data as fast as a drive can handle them, increasing the interface speed does not improve data transfer. Most newer computers, including netbooks/laptops, have external SATA (eSATA) connectors, in addition to USB 2.0 and sometimes USB 3.0 ports, although relatively few have built-in FireWire ports. There are some disadvantages, however, to the eSATA interface. Devices built before the eSATA interface became popular lack external SATA connectors. For small form-factor devices (such as external 2.5-inch (64 mm) disks), a PC-hosted USB or FireWire link can usually supply sufficient power to operate the device. However, eSATA connectors cannot supply power, and require a power supply for the external device. The related eSATAp (but mechanically incompatible, sometimes called eSATA/USB) connector adds power to an external SATA connection, so that an additional power supply is not needed.[26] Older desktop computers without a built-in eSATA interface can install an eSATA host bus adapter (HBA); if the motherboard supports SATA, an externally available eSATA connector can be added. Notebook computers can be upgraded with Cardbus[27] or ExpressCard[28]versions of an eSATA HBA. With passive adapters, the maximum cable length is reduced to 1 metre (3.3 ft) due to the absence of compliant eSATA signal-levels.

Design OF DVD :-

Comparison of several forms of showing tracks (not-to-scale); green denotes start and red denotes end. * Some CD-R(W) and DVD-R(W)/DVD+R(W) recorders operate in ZCLV, CAA or CAV modes, but most work inConstant linear velocity (CLV) mode. As a movie delivery medium DVD was adopted by movie and home entertainment distributors to replace the ubiquitous VHS tape as the primary means of distributing films to consumers in the home entertainment marketplace. DVD was chosen for its superior ability to reproduce moving pictures and sound, for its superior durability, and for its interactivity. Interactivity had proven to be a feature which consumers, especially collectors, favored when the movie studios had released their films on LaserDisc. When the price point for a LaserDisc at approximately $100 per disc moved to $20 per disc at retail, this luxury feature became available for mass consumption. Simultaneously, the movie studios decided to change their home entertainment release model from a rental model to a for purchase model, and large numbers of were sold. At the same time, a demand for interactive design talent and services was created. Movies in the past had uniquely designed title sequences. Suddenly every movie being released required information architecture and interactive design components that matched the film's tone and were at the quality level that Hollywood demanded for its product. As an interactive medium DVD as a format had two qualities at the time that were not available in any other interactive medium: enough capacity and speed to provide high quality, full motion video and sound, and low cost delivery mechanism provided by consumer products retailers. Retailers would quickly move to sell their players for under $200, and eventually for under $50 at retail. In addition, the medium itself was small enough and light enough to mail using general first class postage. Almost overnight, this created a new business opportunity and model for business innovators, such as Netflix, to re-invent the home entertainment distribution model. It also opened up the opportunity for business and product information to be inexpensively provided on full motion video through direct mail. Internal mechanism of a drive

Internal mechanism of a DVD-ROM Drive. This mechanism is shown right side up; the disc would sit on top of it. The laser and optical system scans the underside of the disc. With reference to the photo, just to the right of image center is the disc spin motor, a gray cylinder, with its gray centering hub and black resilient drive ring on top. A clamp (not in the photo, retained in the drive's cover), pulled down by a magnet, clamps the disc when this mechanism rises, after the disc tray stops moving inward. This motor has an external rotor – every visible part of it spins. The gray metal chassis is shock-mounted at its four corners to reduce sensitivity to external shocks, and to reduce drive noise when running fast. The soft shock mount grommets are just below the brass-colored washers at the four corners (the left one is obscured). Running through those grommets are screws to fasten them to the black plastic frame that's underneath. Two parallel precision guide rods that run between upper left and lower right in the photo carry the "sled", the moving optical read-write head. As shown, this "sled" is close to, or at the position where it reads or writes at the edge of the disc. A dark gray disc with two holes on opposite sides has a blue lens surrounded by silver-colored metal. This is the lens that's closest to the disc; it serves to both read and write by focusing the laser light to a very small spot. Under the disc is an ingenious actuator comprising permanent magnets and coils that move the lens up and down to maintain focus on the data layer. As well, the actuator moves the lens slightly toward and away from the spin-motor spindle to keep the spot on track. Both focus and tracking are relatively quite fast and very precise. To select tracks (or files) as well as advancing the "sled" during continuous read or write operations, a stepping motor rotates a coarse-pitch leadscrew to move the "sled" throughout its total travel range. The motor, itself, is the gray cylinder just to the left of the most-distant shock mount; its shaft is parallel to the support rods. The leadscrew, itself, is the rod with evenly-spaced darker details; these are the helical groove that engages a pin on the "sled". The irregular orange material is flexible etched copper foil supported by thin sheet plastic; these are "flexible printed circuits" that connect everything to the electronics (which is not shown). Dual-layer recording

Dual-layer recording (sometimes also known as double-layer recording) allows DVD-R and DVD+R discs to store significantly more data—up to 8.5 per disc, compared with 4.7 gigabytes for single-layer discs. Along with this, DVD-DLs have slower write speeds as compared to ordinary DVDs and when played on a DVD player a slight transition can sometimes be seen between the layers. DVD-R DL was developed for the DVD Forum by Pioneer Corporation; DVD+R DL was developed for the DVD+RW Alliance by Philips and Mitsubishi Kagaku Media (MKM).[26] A dual-layer disc differs from its usual DVD counterpart by employing a second physical layer within the disc itself. The drive with dual-layer capability accesses the second layer by shining the laser through the first semitransparent layer. In some DVD players, the layer change can exhibit a noticeable pause, up to several seconds.[27] This caused some viewers to worry that their dual-layer discs were damaged or defective, with the end result that studios began listing a standard message explaining the dual-layer pausing effect on all dual-layer disc packaging. DVD recordable discs supporting this technology are backward-compatible with some existing DVD players and DVD-ROM drives.[26] Many current DVD recorders support dual-layer technology, and the price is now comparable to that of single-layer drives, although the blank media remain more expensive. The recording speeds reached by dual-layer media are still well below those of single-layer media. A potential reason is how Dual Layer discs are not as well matured compared to the Single Layer discs, as well as consumers, on a whole, have no desire for increased burning speeds. There are two modes for dual-layer orientation. With Parallel Track Path (PTP), used on DVD-ROM, both layers start at the inside diameter (ID) and end at the outside diameter (OD) with the lead-out. With Opposite Track Path (OTP), used on many Digital Video Discs, the lower layer starts at the ID and the upper layer starts at the OD, where the other layer ends; they share one lead-in and one lead-out.

CD-ROM drives

Further information:

CD-ROM discs are read using CD-ROM drives. A CD-ROM drive may be connected to the computer via an IDE (ATA), SCSI, SATA, FireWire, or USB interface or a proprietary interface, such as the CD interface. Virtually all modern CD-ROM drives can also play audio CDs(as well as Video CDs and other data standards) when used in conjunction with the right software.

CD-ROM drive can sometimes be a misnomer for newer drives that are capable for reading and burning DVDs, the CD's successor which is now the standard optical disc drive. Laser and optics CD-ROM drives employ a near-infrared 780 nm laser diode. The laser beam is directed onto the disc via an opto-electronic tracking module, which then detects whether the beam has been reflected or scattered. Transfer rates If a CD-ROM is read at the same rotational speed as an audio CD, the data transfer rate is 150 KiB/s, commonly referred to as "1×". At this data rate, the track moves along under the laser spot at about 1.2 m/s. To maintain this linear velocity as the optical head moves to different positions, the angular velocity is varied from 500 rpm at the inner edge to 200 rpm at the outer edge. By increasing the speed at which the disc is spun, data can be transferred at greater rates. For example, a CD-ROM drive that can read at 8× speed spins the disc at 1600 to 4000 rpm, giving a linear velocity of 9.6 m/s and a transfer rate of 1200 KiB/s. Above 12× speed most drives read at Constant angular velocity (CAV, constant rpm) so that the motor is not made to change from one speed to another as the head seeks from place to place on the disc. In CAV mode the "×" number denotes the transfer rate at the outer edge of the disc, where it is a maximum. 20× was thought to be the maximum speed due to mechanical constraints until introduced the SCR-3230, a 32x CD-ROM drive which uses a ball bearing system to balance the spinning disc in the drive to reduce vibration and noise. As of 2004, the fastest transfer rate commonly available is about 52× or 10,400 rpm and 7.62 MiB/s. Higher spin speeds are limited by the strength of the polycarbonate plastic of which the discs are made. At 52×, the linear velocity of the outermost part of the disk is around 65 m/s. However, improvements can still be obtained by the use of multiple laser pickups as demonstrated by the Kenwood TrueX 72× which uses seven laser beams and a rotation speed of approximately 10×. CD-Recordable drives are often sold with three different speed ratings, one speed for write-once operations, one for re-write operations, and one for read-only operations. The speeds are typically listed in that order; i.e. a 12×/10×/32× CD drive can, CPU and media permitting, write to CD-R discs at 12× speed (1.76 MiB/s), write to CD-RW discs at 10× speed (1.46 MiB/s), and read from CDs at 32× speed (4.69 MiB/s). The 1× speed rating for CD-ROM (150 KiB/s) is different than the 1× speed rating for DVDs (1.32 MiB/s).

CD-ROM DRIVE OPERATION: - Apart from far more sophisticated error-checking techniques, the innards of a CD-ROM drive are pretty much the same as those used in CD audio players. Data is stored in the same way on all CDs. The information is stored in sequential 2KB sectors that form a single spiral track that starts at the centre of the disc and wraps around many times until it reaches the outer edge of the disc. A player reads information from the CD’s spiral track of pits and lands, starting from the centre of the disc and moving to the outer edge. It does this by firing an infrared laser – 780 nano-millimetres wide and generated by a small gallium arsenate semiconductor – through the clear optical grade polycarbonate plastic layer and onto the metallic sheet. Although it’s of very low power, it’s strong enough to damage the eye if shined directly into it. As the disc rotates at between 200 and 500 rpm, the light bounces off the pits and the frequency of the light changes.

The areas around the pits, called lands, also play a part in the process. The reflected light passes through a prism and onto a photosensor, the output of which is proportional to the amount of light it receives. Light reflected from a pit is 180 degrees out of phase with the light from the lands, and the differences in intensity are measured by the photo-electric cells and converted into electrical pulses. The result is that the series of pits and lands of varying lengths stamped into the surface of the disc are interpreted as a series of corresponding 0s and 1s from which the data – or, via a digital-to-analogue converter (DAC), the audio – stored on the disc is recreated. And since just a weak bandwidth laser is the only thing to touch the surface of the CD directly, there’s none of the wear and tear of traditional analogue media to contend with. Things would be relatively simple if CD-ROM discs were perfectly flat and could be spun with no horizontal deviation. In fact, a considerable amount of extra electronics wizardry is needed to ensure that the laser stays in focus on the disc surface and that it follows the track it’s reading. There are various methods for maintaining radial tracking, the most common being the three-beam approach. The laser beam isn’t shone directly onto the disc surface but is emitted from a semiconductor laser unit and passed through a diffraction grating to produce two extra light sources, one each side of the main beam. A collimator lens takes the three beams and makes them parallel, after which they’re passed through a prism called a polarised beam splitter. The beam splitter’s job is to allow the outbound beams to pass through while reflecting the returning beams through 90 degrees down to the photodiode that interprets the signal. The two side beams are measured for intensity, which remains equal as long as they stay on either side of the track. Any sideways movement of the disc will result in an imbalance and a servo motor will reposition the objective lens. Vertical movement is countered by splitting the receptor photodiode into four quadrants and placing it halfway between the horizontal and vertical focal points of the beam. Any deviation of the disc will cause the spot to become elliptical, with a corresponding current imbalance between each opposing pair of quadrants. The objective lens is then moved up or down to ensure the spot remains circular. CD technology has built-in error correction systems which are able to suppress most of the error that arise from physical particles on the surface of a disc. Every CD-ROM drive and CD player in the world uses Cross Interleaved Reed Solomon Code (CIRC) detection and the CD- ROM standard provides a second level of correction via the Layered Error Correction Code algorithm. With CIRC, an encoder adds two dimensional parity information, to correct errors, and also interleaves the data on the disc to protect from burst errors. It is capable of correcting error bursts up to 3,500 bits (2.4 mm in length) and compensates for error bursts up to 12,000 bits (8.5 mm) such as caused by minor scratches.