OPTICAL MEDIA
There are basically two types of disk storage for computers: magnetic and optical. Magnetic storage is represented by the standard floppy and hard disks installed in most PC systems, where the data is recorded magnetically on rotating disks. Optical disc storage is similar to magnetic disk storage in basic operation, but it reads and records using light (optically) instead of magnetism. Although most magnetic disk storage is fully read and write capable many times over, ma by optical storage media are either read only or write once. Note the convention in which we refer to magnetic as disk and optical as disc. This is not a law or rule but seems to be followed by most in the industry.
Some media combine magnetic and optical techniques, using either an optical guidance system (called a laser servo) to position a magnetic read/write head (as in the LS-120/LS-240 Super Disk floppy drive) or a laser to heat the disk so it can be written magnetically, thus polarizing areas of the track, which can then be read by a lower-powered laser, as in magneto-optical (MO) drives.
The most promising development in the optical area is that CD-RW (compact disc-rewritable) or DVD+RW (DVD + rewritable) drives with Easy Write (Mount Rainier) support are starting to replace the venerable floppy disk as the de facto standard, interchangeable, transportable drive and media of choice for PCs. In fact, some would say that has already happened. Most new systems today include a CD-RW drive, and some also include some type of rewritable DVD drive. Even though a floppy drive is also included with most systems, it is rarely used except for running tests; running diagnostics; or doing basic system maintenance, disk formatting, preparation for OS installation, or configuration.
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Optical technology standards for computers can be divided into two major types:
* CD (CD-ROM, CD-R, CD-RW)
* DVD (DVD-ROM, DVD-RAM, DVD-RW, DVD-R, DVD+RW, DVD+R)
The CD and DVD storage devices are descended from popular entertainment standards; CD-based devices can also play music CDs, and DVD-based devices can play the same DVD videos. However, computer drives that can use these types of media also offer many additional features.
CD-ROM Construction and Technology:
The CD-ROM drives in PCs that read the data discs are almost identical to audio CD players, with the main changes in the circuitry to provide additional error detection and correction. This is to ensure data is read without errors because what would be a minor—if not unnoticeable—glitch in a song would be unacceptable as missing data in a file.
A CD is made of a polycarbonate wafer, 125mm in diameter and 1.2mm thick, with a 15mm hole in the center. This wafer base is stamped or molded with a single physical track in a spiral configuration starting from the inside of the disc and spiraling outward. The track has a pitch, or spiral separation, of 1.6 microns (millionths of a meter, or thousandths of a millimeter).
By comparison, an LP record has a physical track pitch of about 125 microns. When viewed from the reading side (the bottom), the disc rotates counterclockwise. If you examined the spiral track under a microscope, you would see that along the track are raised bumps, called pits, and flat areas between the pits, called lands. It seems strange to call a raised bump a pit, but that is because when the discs are pressed, the stamper works from the top side. So, from that perspective, the pits are actually depressions made in the plastic. The laser used to read the disc would pass right through the clear plastic, so the stamped surface is coated with a reflective layer of metal (usually aluminum) to make it reflective. Then, the aluminum is coated with a thin protective layer of acrylic lacquer, and finally a label or printing is added.
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Figure 1: Compact Disc Figure 2: Spiral Track Figure 3: Cross section of a CD
Pits Lands
Figure 4: Pits and Lands
Mass-Producing CD-ROMs:
Commercial mass-produced CDs are stamped or pressed and not burned by a laser as many people believe. Although a laser is used to etch data onto a glass master disc that has been coated with a photosensitive material, using a laser to directly burn discs would be impractical for the reproduction of hundreds or thousands of copies.
Figure 5: CD manufacture process
The steps in manufacturing CDs are as follows (use figure 5 as a visual):
1. Photoresist Coating: A circular 240 mm diameter piece of polished glass 6mm thick is spin-coated with a Photoresist layer about 150 microns thick and then hardened by baking at 80°C (176°F) for 30 minutes.
2. Laser Recording: A laser Beam Recorder (LBR) fires pulses of blue/violet laser light to expose and soften portions of the photo resist layer on the glass master.
3. Master Development: A sodium hydroxide (NaOH) solution is spun over the exposed glass master, which then dissolves the areas exposed to the laser, thus etching pits in the photo resist.
4. Electroforming: The developed master is then coated with a layer of nickel alloy through a process called electroforming. This creates a metal master called a father.
5. Master Separation: The metal master father is then separated from the glass master. The father is a metal master that can be used to stamp discs, and for short runs, it may in fact be used that way. However, because the glass master is damaged when the father is separated, and because a stamper can produce only a limited number of discs before it wears out, the father often is electroformed to create several reverse image mothers. These mothers are then subsequently electroformed to create the actual stampers. This enables many more discs to be stamped without ever having to go through the glass mastering process again.
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6. Disc Stamping Operation: A metal stamper is used in an injection molding machine to press the data image (pits and lands) into approximately 18 grams of molten (350°C or 662°F) polycarbonate plastic with a force of about 20,000psi. Normally, one disc can be pressed every 2-3 seconds in a modern stamping machine.
7. Metallization: The clear stamped disc base is then sputter-coated with a thin (0.05-0.1 micron) layer of aluminum to make the surface reflective.
8. Protective Coating: The metalized disc is then spin-coated with a thin (6-7 micron) layer of acrylic lacquer, which is then cured with UV (ultraviolet) light. This protects the aluminum from oxidation.
9. Finished Product: Finally, a label is affixed or printing is screen-printed on the disc and cured with UV light.
Pits and Lands:
Reading the information back from a disc is a matter of bouncing a low-powered laser beam off the reflective layer in the disc. The laser shines a focused beam on the underside of the disc, and a photosensitive receptor detects when the light is reflected back. When the light hits a land (flat spot) on the track, the light is reflected back; however, when the light hits a pit (raised bump), no light is reflected back. As the disc rotates over the laser and receptor, the laser shines continuously while the receptor sees, what essentially a pattern of flashing light is as the laser passes over pits and lands. Each time the laser passes over the edge of a pit, the light seen by the receptor changes in state from being reflected to not reflected or vice versa. Each change in state of reflection caused by crossing the edge of a pit is translated into a 1 bit digitally. Microprocessors in the drive translate the light/dark and dark/light (pit edge) transitions into 1 bit, translate areas with no transitions into 0 bits, and then translate the bit patterns into actual data or sound. The individual pits on a CD are 0.125 microns deep and 0.6 microns wide. Both the pits and lands vary in length from about 0.9 microns at their shortest to about 3.3 microns at their longest. The track is a spiral with 1.6 microns between adjacent turns (see Figure 6).
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Figure 6: Pit, land and track geometry on a CD
The height of the pits above the land is especially critical as it relates to the wavelength of the laser light used when reading the disc. The pit (bump) height is exactly 1/4 of the wavelength of the laser light used to read the disc. Therefore, the light striking a land travels 1/2 of a wavelength of light farther than light striking the top of a pit (1/4 + 1/4 = 1/2).
This means the light reflected from a pit is 1/2 wavelength out of phase with the rest of the light being reflected from the disc. The out-of-phase waves cancel each other out; dramatically reducing the light that is reflected back and making the pit appear dark even though it is coated with the same reflective aluminum as the lands. The read laser in a CD drive is a 780nm (nanometer) wavelength laser of about 1 milli watt in power. The polycarbonate plastic used in the disc has a refractive index of 1.55, so light travels through the plastic 1.55 times more slowly than through the air around it. Because the frequency of the light passing through the plastic remains the same, this has the effect of shortening the wavelength inside the plastic by the same factor. Therefore, the 780nm light waves are now compressed to 780/1.55 =500nm. One quarter of 500nm is 125nm, which is 0.125 microns—the specified height of the pit.
Tracks and Sectors:
The pits are stamped into a single spiral track with a spacing of 1.6 microns between turns, corresponding to a track density of 625 turns per millimeter, or 15,875 turns per inch. This equates to a total of 22,188 turns for a typical 74-minute (650MiB) disc. The disc is divided into six main areas
* Hub clamping area (HCA): The hub clamp area is just that a part of the disc where the hub mechanism in the drive can grip the disc. No data or information is stored in that area.
* Power calibration area (PCA): This is found only on writable (CD-R/RW) discs and is used only by recordable drives to determine the laser power necessary to perform an optimum burn. A single CD-R or CD-RW disc can be tested this way up to 99 times.
* Program memory area (PMA): This is found only on writable (CD-R/RW) discs and is the area where the TOC (table of contents) is temporarily written until a recording session is closed. After the session is closed, the TOC information is written to the lead-in area.
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* Lead in: The lead-in area contains the disc (or session) TOC in the Q sub code channel. The TOC contains the start addresses and lengths of all tracks, the total length of the program area, and information about the individual recorded sessions. A single lead-in area exists on disc recorded all at once (Disc At Once or DAO mode), or a lead-in area starts each session on a multisession disc. The lead-in takes up 4500 sectors on the disc (1 minute if measured in time or about 9.2MB worth of data).
The lead-in also indicates whether the disc is multisession and what the next writable address on the disc is (if the disc isn’t closed).
* Program area: This area of the disc starts at a radius of 25mm from the center.
* Lead out: The lead-out marks the end of the program area or the end of the recording session on a multisession disc. No actual data is written in the lead-out; it is simply a marker. The first lead-out on a disc is 6750 sectors long. If the disc is a multisession disc, any subsequent lead-outs are 2250 sectors long.
The hub clamp, lead-in, program, and lead-out areas are found on all CDs, whereas only recordable CDs (such as CD-Rs and CD-RWs) have the additional power calibration area and program memory.
Figure 7: Areas on a CD
Officially, the spiral track of a standard CD-DA or CD-ROM disc starts with the lead-in area and ends at the finish of the lead-out area, which is 58.5mm from the center of the disc, or 1.5mm from the outer edge. This single spiral track is about 5.77 kilometers, or 3.59 miles, long. An interesting fact is that in a 56x CAV (constant angular velocity) drive, when reading the outer part of the track, the data moves at an actual speed of 162.8 miles per hour (262km/h) past the laser. What is more amazing is that even when the data is traveling at that speed, the laser pickup can accurately read bits (pit/land transitions) spaced as little as only 0.9 microns (or 35.4 millionths of an inch) apart! Table-1 shows some of the basic information about the two main CD capacities, which are 74-minute and 80-minute. The CD standard originally was created around the 74-minute disc; the 80-minute versions were added later and basically stretch the standard by tightening up the track spacing within the limitations of the original specification. A poorly performing or worn out drive can have trouble reading the 80-minute discs.
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| | |
Advertised CD length (minutes) | 74 | 80 |
Advertised CD capacity (MB) | 650 | 700 |
1x read speed (m/sec) | 1.3 | 1.3 |
Laser wavelength (nm) | 780 | 780 |
Numerical aperture (lens) | 0.45 | 0.45 |
Media refractive index | 1.55 | 1.55 |
Track spacing (µm) | 1.6 | 1.48 |
Turns per mm | 625 | 676 |
Turns per inch | 15875 | 17162 |
Total track length (m) | 5772 | 6240 |
Total track length (feet) | 18937 | 20472 |
Total track length (miles) | 3.59 | 3.88 |
Pit width (µm) | 0.6 | 0.6 |
Pit depth (µm) | 0.125 | 0.125 |
Min. nominal pit length (µm) | 0.90 | 0.90 |
Max. nominal pit length (µm) | 3.31 | 3.31 |
Lead-in inner radius (mm) | 23 | 23 |
Data zone inner radius (mm) | 25 | 25 |
Data zone outer radius (mm) | 58 | 58 |
Lead-out outer radius (mm) | 58.5 | 58.5 |
Data zone width (mm) | 33 | 33 |
Total track area width (mm) | 35.5 | 35.5 |
Max. rotating speed 1x CLV (rpm) | 540 | 540 |
Min. rotating speed 1x CLV (rpm) | 212 | 212 |
Track revolutions (data zone) | 20625 | 22279 |
Track revolutions (total) | 22188 | 23986 |
Table 1: CD-ROM technical parameters
The spiral track is divided into sectors that are stored at the rate of 75 sectors per second. On a disc that can hold a total of 74 minutes of information, that results in a maximum of 333,000 sectors. Each sector is then divided into 98 individual frames of information. Each frame contains 33 bytes: 24 bytes are audio data, 1 byte contains sub code information, and 8 bytes are used for parity/ECC (error correction code) information. Table 2 shows the sector, frame, and audio data calculations.
| | |
Advertised CD length (minutes) | 74 | 80 |
Sectors/second | 75 | 75 |
Frames/sectors | 98 | 98 |
Number of sectors | 333000 | 360000 |
Sector length (mm) | 17.33 | 17.33 |
Byte length (µm) | 5.36 | 5.36 |
Bit length (µm) | 0.67 | 0.67 |
Sub code bytes | 1 | 1 |
Data bytes | 24 | 24 |
Q+P parity bytes | 8 | 8 |
Total bytes/frame | 33 | 33 |
Audio sampling rate (Hz) | 44100 | 44100 |
Samples per Hz (stereo) | 2 | 2 |
Sample size (bytes) | 2 | 2 |
Audio bytes per second | 176400 | 176400 |
Sectors per second | 75 | 75 |
Audio bytes per sector | 2352 | 2352 |
Q+P parity bytes | 784 | 784 |
Sub code bytes | 98 | 98 |
Audio data bytes | 2352 | 2352 |
Bytes/sector RAW (un encoded) | 3234 | 3234 |
Table 2: CD-ROM sector, frame and audio data information
Sampling:
When music is recorded on a CD, it is sampled at a rate of 44,100 times per second (Hz).
Each music sample has a separate left and right channel (stereo) component, and each channel component is digitally converted into a 16-bit number. This allows for a resolution of 65,536 possible values, which represents the amplitude of the sound wave for that channel at that moment. The sampling rate determines the range of audio frequencies that can be represented in the digital recording. The more samples of a wave that are taken per second, the closer the sampled result will be to the original. The Nyquist theorem (originally published by American physicist Harry Nyquist in 1928) states that the sampling rate must be at least twice the highest frequency present in the sample to reconstruct the original signal accurately. That explains why Philips and Sony intentionally chose the 44,100Hz sampling rate when developing the CD—that rate could be used to accurately reproduce sounds of up to 20,000Hz, which is the upper limit of human hearing. So, you can see that audio sectors combine 98 frames of 33 bytes each, which results in a total of 3,234 bytes per sector, of which only 2,352 bytes are actual audio data. Besides the 98 sub code bytes per frame, the other 784 bytes are used for parity and error correction.
Subcode:
Subcode bytes enable the drive to find data (which are confusingly also called tracks) along the spiral track and also contain or convey additional information about the disc in general. The Subcode bytes are stored as 1 byte per frame, which results in 98 Subcode bytes for each sector. Two of these bytes are used as start block and end block markers, leaving 96 bytes of Subcode information. These are then divided into eight 12-byte Subcode blocks, each of which is assigned a letter designation P–W. Each Subcode channel can hold about 31.97MB of data across the disc, which is about 4% of the capacity of an audio disc. The interesting thing about the Subcodes is that the data is woven continuously throughout the disc; in other words, Subcode data is contained piecemeal in every sector on the disc. The P and Q Subcode blocks are used on all discs, and the R–W Subcodes are used only on CD+G (graphics) or CD TEXT–type discs. The P Subcode is used to identify the start of the tracks on the CD. The Q Subcode contains a multitude of information, including
* Whether the sector data is audio (CD-DA) or data (CD-ROM).
This prevents most players from trying to play CD-ROM data discs, which might damage speakers due to the resulting noise that would occur.
* Whether the audio data is two or four channel. Four channels are rarely if ever used.
* Whether digital copying is permitted. PC-based CD-R and RW drives ignore this; it was instituted to prevent copying to DAT (digital audio tape) or home audio CD-R/RW drives.
* Whether the music is recorded with pre-emphasis. This is a hiss or noise reduction technique.
* The track layout on the disc.
* The track number.
* The minute, seconds, and frame number from the start of the track.
* A countdown during an intertrack pause.
* The minutes, seconds and frames from the start of the first track.
* The barcode of the CD.
* The ISRC (International Standard Recording Code).
This is unique to each track on the disc.
The R-W subcodes are used on CD+G (graphics) discs to contain graphics and text. This enables a limited amount of graphics and text to be displayed while the music is being played. These same Subcodes are used on CD TEXT discs to store disc- and track-related information that is added to standard audio CDs for playback on compatible CD audio players. The CD TEXT information is stored as ASCII characters in the R–W channels in the lead-in and program areas of a CD. On a CD TEXT disc, the lead-in area subcodes contain text information about the entire disc, such as the album, track (song) titles, and artist names. The program area subcodes, on the other hand, contain text information for the current track (song), including track title, composer, performers, and so on. The CD TEXT data is repeated throughout each track to reduce the delay in retrieving the data. CD TEXT–compatible players typically have a text display to show this information, ranging from a simple one- or two-line, 20-character display such as on many newer RBDS (radio broadcast data system) automobile radio/CD players to up to 21 lines of 40-color, alphanumeric or graphics characters on home- or computer-based players. The specification also allows for future additional data, such as Joint Photographic Expert Group (JPEG) images. Interactive menus also can be used for the selection of text for display.
Handling Read Errors:
Handling errors when reading a disc was a big part of the original Red Book CD standard. CDs use parity and interleaving techniques called cross-interleave Reed-Solomon code (CIRC) to minimize the effects of errors on the disk. This works at the frame level. When being stored, the 24 data bytes in each frame are first run through a Reed-Solomon encoder to produce a 4-byte parity code called “Q” parity, which then is added to the 24 data bytes. The resulting 28 bytes are then run though another encoder that uses a different scheme to produce an additional 4-byte parity value called “P” parity. These are added to the 28 bytes from the previous encoding, resulting in 32 bytes (24 of the original data plus the Q and P parity bytes).
An additional byte of subcode (tracking) information is then added, resulting in 33 bytes total for each frame.
To minimize the effects of a scratch or physical defect that would damage adjacent frames, several interleaves are added before the frames are actually written. Parts of 109 frames are cross-interleaved (stored in different frames and sectors) using delay lines. This scrambling decreases the likelihood of a scratch or defect affecting adjacent data because the data is actually written out of sequence.
With audio CDs and CD-ROMs, the CIRC scheme can correct errors up to 3,874 bits long (which would be 2.6mm in track length).
In addition, for audio CDs, only the CIRC can also conceal (through interpolation) errors up to 13,282 bits long (8.9mm in track length).
Interpolation is the process in which the data is estimated or averaged to restore what is missing. That would of course be unacceptable on a CD-ROM data disc, so this applies only to audio discs. The Red Book CD standard defines the block error rate (BLER) as the number of frames (98 per sector) per second that have any bad bits (averaged over 10 seconds) and requires that this be less than 220. This allows a maximum of up to about3% of the frames to have errors, and yet the disc will still be functional.
In a CD-ROM on which data is stored instead of audio information, additional information is added to each sector to detect and correct errors as well as to identify the location of data sectors more accurately. To accomplish this, 304 bytes are taken from the 2,352 that originally were used for audio data and are instead used for sync (synchronizing bits), ID (identification bits), ECC, and EDC information. This leaves 2,048 bytes for actual user data in each sector. Just as when reading an audio CD, on a 1x (standard speed) CD-ROM, sectors are read at a constant speed of 75 per second. This results in a standard CD-ROM transfer rate of 2,048 × 75 = 153,600 bytes per second, which is expressed as either 153.6KBps.
Data Encoding on the disc:
The final part of how data is actually written to the CD is very interesting. After all 98 frames are composed for a sector (whether audio or data), the information is then run through a final encoding process called EFM (eight to fourteen modulation).
This scheme takes each byte (8 bits) and converts it into a 14-bit value for storage. The 14-bit conversion codes are designed so that there are never fewer than 2 or more than 10 adjacent 0 bits. This is a form of Run Length Limited (RLL) encoding called RLL 2, 10 (RLL x, y where x = the minimum and y = the maximum run of 0s).
This is designed to prevent long strings of 0s, which could more easily be misread, as well as to limit the minimum and maximum frequency of transitions actually placed on the recording media. With as few as 2 or as many as 10 0 bits separating 1 bits in the recording, the minimum distance between 1s is 3 bit time intervals (usually referred to as 3T) and the maximum spacing between 1s is 11 time intervals (11T).
Because some of the EFM codes start and end with a 1 or more than five 0s, three additional bits called merge bits are added between each 14-bit EFM value written to the disc. The merge bits usually are 0s but might contain a 1 if necessary to break a long string of adjacent 0s formed by the adjacent 14-bit EFM values. In addition to the now 17-bits created for each byte (EFM plus merge bits), a 24-bit sync word (plus 3 more merge bits) is added to the beginning of each frame. This results in a total of 588 bits (73.5 bytes) which is actually being stored on the disc for each frame. Multiply this for 98 frames per sector and you have 7,203 bytes actually being stored on the disc to represent each sector. A 74-minute disc, therefore, really has something like 2.4GB of actual data being written, which after being fully decoded and stripped of error correcting codes and other information, results in about 682MB (650MiB) of actual user data.
| | |
EFM-Encoded Frames | 74-Minute | 80-Minute |
Sync word bits | 24 | 24 |
Subcode bits | 14 | 14 |
Data bits | 336 | 336 |
Q+P parity bits | 112 | 112 |
Merge bits | 102 | 102 |
EFM bits per frame | 588 | 588 |
EFM bits per sector | 57624 | 57624 |
EFM bytes per sector | 7203 | 7203 |
Total EFM data on disc (MB) | 2399 | 2593 |
Table 3: EFM encoded data calculation
To put this into perspective, see Table 13.5 for an example of how familiar data would actually be encoded when written to a CD. As an example, I’ll use the letters “N” and “O” as they would be written on the disk. Table 13.5 shows the digitally encoded representations of these letters.
| | |
Character | “N” | “O” |
ASCII decimal code | 78 | 79 |
ASCII hexadecimal code | 4E | 4F |
ASCII binary code | 01001110 | 01001111 |
EFM code | 00010001000100 | 00100001000100 |
Table 4: EFM data encoding on a CD
Figure 8: EFM data physically represented as pits and lands on a CD
The edges of the pits are translated into the binary 1 bit. As you can see, each 14-bit grouping is used to represent a byte of actual EFM encoded data on the disc, and each 14-bit EFM code is separated by three merge bits (all 0s in this example).
The three pits produced by this example are 4T (4 transitions), 8T, and 4T long. The string of 1s and 0s on the top of the figure represent how the actual data would be read; note that a 1 is read wherever a pit-to-land transition occurs. It is interesting to note that this drawing is actually to scale, meaning the pits (raised bumps) would be about that long and wide relative to each other. If you could use a microscope to view the disc, this is what the word “NO” would look like as actually recorded.
CD-ROM Mechanic:
At the center of the drive is a cast aluminum or rigid stainless steel frame assembly. As with other drives, the frame is single primary structure for mounting the drives mechanical and electronic components. The front bezel, lid, volume control, and eject button attach to the frame, providing the drive with its clean cosmetic appearance, and offering a fixed reference slot for CD insertion and removal. Keep in mind that many drives use a sliding tray, so the front bezel (and the way it is attached) will not be the same for every drive.
Although the laser type and drive electronics are somewhat different, the physical descriptions and electronic details for CD-ROM drives are also generally true for CD-R and CD-RW drives. The drives electronics package has been split into several PC board assemblies: the main PCB (Printed Circuit Board) which handles drive control and interfacing, and the headphone PCB, which simply provides an audio amplifier and jack for the headphones. The bulk of the drives actual physical work, however, is performed by a main CD subassembly called a drive engine, which is often manufactured by only a few companies. As a result, many of the diverse CD-ROM drives on the market actually use identical “engines” to hold/eject, spin, and read the disk. This interchangeability is part of the genius of CD-ROM drives- a single subassembly performs 80 percent of the work. Sony, Philips and Toshiba are the major manufacturers of CD-ROM engines, but other companies such as IBM and IKKA have also been known to produce engines.
The upper view of the engine features a series of mechanisms that accept, clamp and eject the disk. The foundation of this engine is the BC-7C assembly. It acts as a sub frame on which everything else is mounted. Notice that the sub frame is shock-mounted with four rubber feet to cushion the engine from minor bumps and ordinary handling. Even with such mounting, a CD-ROM drive is a fragile mechanism. The slider assembly, loading chassis assembly, and the cover shield provide the mechanical action needed to accept the disk and clamp it into place over the drive spindle, as well as free the disk and eject it on demand. A number of levers and oil dampers serve to provide a slow, smooth mechanical action when motion takes place. A motor/gear assembly drives the load/unload mechanics.
The serious work of spinning and reading a disk is handled under the engine. A spindle motor is mounted on the sub frame and connected to a spindle motor PC board. A thrust retainer helps keep the spindle motor turning smoothly. The most critical part of the CD engine is the optical device containing the 780nm (nanometer) 0.6mW gallium arsenide (GaAlAs) laser diode and detector, along with the optical focus and detector, along with the optical focus and tracking components. The optical device slides along two guide rails and shines through an exposed hole in the sub frame. This combination of device mounting and guide rails is called a sled. CD-R and CD-RW drives will typically use lasers with different characteristics, though you may not be able to distinguish between CD-ROM, CD-R, or CD-RW engine at first glance.
Sled
Spindle Motor
Optical Head
BC7C Assembly
Linear motor rail
A sled must be made to follow the spiral data track along the disc. While floppy disks (using clearly defined concentric tracks) can easily make use of a steeping motor to position the head assembly a CD drive ideally requires a linear motor to act much like the voice coil motor used to position hard drive RAW heads. By altering the signal that drives a sled motor and constantly measuring and adjusting the sled’s position, a sled can be made to track very smoothly along a disc free from the sudden, jerky motion of stepping motors. Some CD drives still use stepping motors with an extremely fine-pitch lead screw to position the sled. The drive’s main PC board is responsible for managing these operations.
CD-ROM Electronics:
The electronics package used in a typical CD-ROM drive is illustrated in Figure9. The electronics package can be divided into two major areas: the controller section and the drive section. The controller section is dedicated to the peripherals interface its connection to the drive controller board. Most CD-ROM drive today offer a UDMA or EIDE drive interface that will support a CD-ROM right along with your existing hard drive(s).
This allows the unit’s “intelligence” to be located right in the drive itself. You need only connect the drive to a system level interface board such as a SCSI host adapter or IDE type drive controller (such as an IDE, EIDE or UDMA controller) and set the drive’s device identification to establish a working system.
The drive section’s electronics will manage the CD-ROM’s physical operations (load/unload, spin the disc, move the sled etc.), as well as data decoding (EFM) and error correction. Drive circuitry converts and analog output from the laser diode into an EFM signal, which is, in turn, decoded into binary data and CIRC (Cross Interleaved Reed Solomon Code) information. A drive controller IC and servo processor IC are responsible for directing laser.
