Optical Storage Technologies

1. Compact Disc-Read Only Memory (CD-ROM)

2. Compact Disc- Record (CD-R Write Once Read Many times – WORM)

3. Compact Disc- Re-Writable (CD-RW)

 

Compact Disc Read Only Memory (CD-ROM)

A CD-ROM has several advantages over other forms of data storage, and a few disadvantages. It can hold nearly 700 megabytes (MB) of data, the equivalent of nearly 500 high-density floppy disks. The data on a CD-ROM can be accessed much faster than a tape, but CD-ROMs are slower than hard discs.

While audio CDs can be played at only one speed, CD-ROM drives exist with a range of speed options up to 50 times normal speed or even more. As the speed increases the access time also decreases. CD-ROM discs differ from CD audio discs in two important ways:

1. The data on a CD-ROM disc are divided into sectors, which contain both user data and other data for control and error protection.

2. The data on a CD-ROM are contained in files. All CD-ROMs therefore need a file structure to enable the computers to access the required file easily and quickly.

The data on a CD-ROM can be accessed much faster than a tape, particularly using the latest high-speed drives (52x is now common). To reduce the maximum angular velocity these faster drives use CAV (constant angular velocity) rather than CLV (constant linear velocity). Therefore the data rate for data near the inside is less than the data rate at the outside of the disc. For example a 40x drives gives a maximum data rate of between 2.8 and 6 MB/s, depending where on the disc the data is being read. Faster drives can create problems so some drives make use of multiple laser beams to increase the data rate without increasing the angular velocity.

All audio CDs are CLV (Constant Linear Velocity) discs, which means that they are played at a constant velocity of between 1.2 and 1.4 m/s. The rotation speed (rpm) will reduce from the centre to the outside of the disc by a factor of 2.4. This means that the pits retain the same geometry wherever they are on the disc and there will be no change in performance (including error rate) between the centre and the outside of the disc.

CD-ROM drives are designed also to read CD audio discs at the correct speed. Other discs, like Video CDs, which are designed for single speed, are read in bursts to maintain the correct data rate.

The specification of a CD-ROM disc is summarised below:

Data capacity 682 Mbytes Assuming 74 minutes

Raw data bitrate 1.41Mbits/s Includes all bytes in sector

User data rate 150 kB/s At 1x speed

Block (sector) size 2,352 bytes

User data per sector 2048 With full error correction

Sector rate 75 sectors/s At 1x speed reading

Sector Modes 1 or 2

Sector Forms 1 or 2 Mode

 

Laser Technology

Optical discs such as the CD rely on laser technology to read (and write) the data on discs. The word LASER stands for Light Amplification by Stimulated Emission of Radiation. Lasers generate coherent light, ie light comprising photons with the same wavelength and in-phase. This allows the light beam to be focused to a very small spot size similar to the actual wavelength of the light itself. The information is stored in pits (ie minute indentations) in the disc surface. These pits measure about 0.5 microns in width and are arranged in a spiral pattern, like vinyl records. The Compact Disc uses exactly the same method with identical pit sizes and spacing. However, the pits are used to indicate whether a data bit is '0' or '1'. The length of the pits varies for different sequences of 0s and 1s.

CD players use infra red light emitting diode lasers (see diagram below),  to read the data contained in these pits. The laser diode is mounted on a swivel arm, which can be moved radially to follow the pits up and down to keep them in focus.

An objective lens is used to focus the laser beam on the pits. A two-way prism mirror allows the reflected light to pass back to the photo-detector. When the laser beam falls on a pit the light is scattered and very little is reflected. The changing light pattern detected is then converted into a series of zeros and ones, which are then decoded.

 

Sensitive control of the radial position of the laser diode and the vertical position of the objective lens are used to ensure that the laser follows the pits accurately, even if the CD is slightly eccentric, due perhaps to the centre hole being slightly off centre. The beam focus can be moved up and down to compensate for the disc being slightly warped. When the laser beam falls on a pit very little is reflected. The changing light pattern detected is then converted into a series of zeros and ones, which are then decoded into the original computer data signal.

Optical System

· Light source: laser diode (red laser) wavelength (l): 650nm (DVD) – 780nm (CD-ROM) power: 5mW – reading, 50mW - writing

· Laser is split into 3 beams; central beam for reading and writing, others for tracking.

· Collimator lens makes the 3 beams parallel.

· Beam splitter deflects the light through 90 degrees.

· Objective lens focus the laser beam to a diffraction-limited spot on the disk surface (with Numerical Aperture, NA = 0.4 – 0.6).

The tracking laser beam sees the pits as raised areas which are about a quarter-wavelength high for the laser light.

Objective lens is mounted on a platform which moves the lens across the diameter of the disk so the laser beam can access any data track.

Light reflected from disk retraces original path. In the return path, the beam splitter directs the beam to a photo-detector array where the recording information, focusing and tracking-error signals are extracted.

When the focused beam overlaps neighbouring pits, destructive interference occurs in the reflected light causing a reduction in the output signal strength. Light reflected from a pit has travelled a quarter-wavelength further than light reflected from a land  and light reflected from a pit is therefore  out of phase with the incoming light by a half-wavelength and so tends to cancel with the light from the pit area.

Reflected light is extinguished in the beam splitter and not passed to the photo-detector, so there is no signal. Light reflected from land is in phase with incoming light and passed through the beam splitter to the photo-detector.

Tracking

The laser beam in the compact disc player must precisely track a row of pits which encode the binary data on the disc. In the "three-beam" system, a grating is used to produce the first order diffraction maximum to each side of the main beam. Those diffracted beams overlap the track, and the reflected light from the two side beams should be equal, on the average, if the main beam is centered on the track. If they are unequal, then their difference can be used to generate an error voltage to correct the tracking. The illustration of the side beam positions is not to scale, they deviate about 20 micrometers from the main beam.

 

       

The data is detected by a laser beam which tracks the concentric circular lines of pits. The pits are 0.8 to 3 micrometers long and the rows are separated by 1.6 micrometers

 

Focus Error Correction

A cylindrical lens generates a correction signal to position the main focusing lens for the detector in a compact disc player. The combination of a symmetric lens and a cylindrical lens produces a circular beam at only one distance past the cylindrical lens. A segmented photodiode arrangement can detect whether the beam is circular and generate an error voltage to reposition the main lens so that it is. The error voltage drives a coil which can rapidly reposition the lens in response to changes in distance to the CD as it rotates. 

Data Modulation & Error Correction

It is not possible to manufacture CDs where every pit is intact. Small defects in manufacture are permissible and even minor scratches, which can occur with use, do not usually affect the disc's playability. Therefore the CD specification includes CIRC error correction to compensate for these defects followed by EFM.

CIRC (Cross Interleaved Reed-Solomon Code) encoder adds two-dimensional parity information, to correct errors, and also interleaves the data on the disc to protect from burst errors. CIRC corrects 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. The data is distributed over a large physical area of the disc so that if there is a small problem in one spot of the disc it does not corrupt a complete byte. CIRC interleaves the data before recording and de-interleaves during playback. One data block or frame of 24 data bytes is distributed over 109 adjacent blocks, so to destroy one byte needs a lot of the disc to be faulty.

The EFM (Eight to Fourteen) modulation scheme encodes each 8-bit symbol as 14 bits plus 3 merging bits. The EFM data is then used to define the pits on the disc. The merging bits ensure that pit and land lengths are not less than 3 and no more than 11 channel bits. This reduces the effect of jitter and other distortions on the error rate.

 

Compact Disc- Record (CD-R Write Once Read Many – WORM)

A focused laser beam melts a spot on the dye layer and surface tension pulls the molten material aside creating a pit. Replay is via the reflected light from pits and lands –similar to CD-ROM.

 

Compact Disc Re-Writable (CD-RW)

CD-RW media are made from a special alloy that is sputter-deposited on a polycarbonate substrate. The phase change layer is sandwiched between two dielectric layers to protect against enviromental conditions such as oxidation, which prevent phase change material from fluidisation or evaporation during writing and optimises optical contrast during playback. Each disk is heated at the factory to transform the recording layer into its crystalline state (erased).

A high power, focused laser pulse (beam) melts the recording  layer locally. Molten material is rapidly quenched when the laser beam is swiched to a low level, and an amporphous dot is formed in the crystalline layer.

Concentric tracks of amorphous and crystalline regions are created in the disk.The amorphous and crystalline phase have different refractive index, leading to amplitude and/or phase modulation of the reflected laser beam during readback of the output signal. Erasure is achieved by operating the laser at an intermediate continous power (between the read and write powers) changing the amorphous marks back to the crystalline state.