CD-ROM drive
A CD-ROM drive (including CD drive is called) a device for reading CDs and especially CD-ROMs . The CD rotates therein with a speed from 200 to 10000 / min and is driven by a laser ( infrared - laser diode , a few milliwatts) scanned, which moves radially on a carriage. This enables random access to any point on the CD. The first CD-ROM drives hit the market around 1985. Since the 2010s, they have largely been replaced by DVD and CD / DVD combination drives.
Designs
format
There are three standardized versions: 5.25 ″ (e.g. for PCs), 5.25 ″ SlimLine (e.g. for notebooks) and, since 2013, 5.25 ″ UltraSlim. The specification 5.25 inches (≈ 133 mm) comes from the diameter of the floppy disks, whose drives have the same form factor .
The dimensions of the drives in mm are usually:
| Form factor | width | height | depth |
|---|---|---|---|
| 5.25 "PC drive | 146 | 41 | 165-200 |
| 5.25 "SlimLine drive | 128 | 12.7 | 124-130 |
| 5.25 "UltraSlim drive | 128 | 9.5 | 124-130 |
Inserting the optical medium
There are several ways in which the optical media can get in and out of the drive.
- Drawer drives have a drawer that can be retracted and extended using a button on the front of the drive or software-controlled. The optical medium is placed in the drawer. Nowadays this is the most common type. The first types were only suitable for horizontal installation. Later, drives with brackets also appeared, with which the optical medium could also be inserted into a vertically installed drive without falling out. Example: Toshiba SD-L902A.
- Slot-in drives only have a narrow slot into which the optical medium is inserted. The ejection also takes place at the push of a button or via software. By doing away with the drawer, there are fewer mechanical parts that could possibly break. Example: Panasonic CW-8124B
- In the case of caddy drives , the optical medium is placed in a special case, the caddy. Then the caddy is pushed into the drive. The advantage of this technology: Each disc can remain in its own caddy, where it is protected from damage and dirt. Disadvantage, the caddies cost extra money. This technique is no longer in use today. Example: Plextor 4Plex.
- Top loaders are drives that have a flap on the top. The flap is opened for insertion and removal. This variant is not suitable for internal drives. Today it is still used occasionally for externally connected drives. Plextor PX-608CU.
- However, there was a special form that was also suitable for installation: the combination of top loader and drawer drive . The drive itself (with the exception of a holder frame and the rear connections) forms the drawer. The drive extends when you press the front of the drive. A flap can then be opened. This type of drive is no longer produced today. Example: Mitsumi LU005S.
- Several CDs / DVDs can be accommodated in CD / DVD changers so that there is no need to change them, especially with frequently used data carriers. Drives of this type are particularly useful when different CDs / DVDs are to be made available in a local network or via the Internet. In the era of terabyte hard drives, however, such changers have lost their importance, as CDs and DVDs can be transferred to hard drives in large numbers. The read speed of hard disks is also significantly faster than that of CD and DVD drives, and there is no need to change. Example: Pioneer DRM-6324X
Disassembled CD-ROM drive
CD-ROM reading unit
Movable carriage for random access to the CD
Emergency ejection
With drawer drives, slot-in drives and caddy drives, there is usually a so-called emergency eject at the front . This allows the data carrier to be ejected even when the drive has no power (e.g. when the computer is switched off). It is a 1–2 mm wide, round opening into which a long, thin object, usually a straightened paper clip , can be inserted. Pressing the mechanism hidden behind it ejects the drawer, CD / DVD or caddy.
Interfaces
Controller interface
The most commonly used interface is SATA . There are also SCSI and ATA for internal drives as well as USB and Firewire for external drives. Some of the first CD-ROM drives (Sony, Panasonic, Mitsumi) have proprietary interfaces for which the corresponding controller cards (legacy ISA cards) are available. Some sound cards (legacy ISA cards) of this time also have corresponding connections. In addition, legacy ISA cards, which provide a single ATA interface, were also sold as “CD-ROM controllers”.
Controller card for proprietary Mitsumi drives
Sound card with connections for proprietary CD-ROM drives: Sony (SONY), Mitsumi (MIT), Panasonic (PAN)
IDE CD-ROM drive
Audio outputs
CD-ROM drives usually have an analog and a digital audio output.
The pins for the analog output are standardized as follows:
Nase Rechts Masse Masse Links
if you look at the pins.
In the case of drives in the 5.25 ″ format, there is also usually a line-out output (usually in combination with a volume control ) on the front . B. headphones can be connected.
Power supply
Internal CD-ROM drives have a 4-pin Molex connector or a more modern SATA 15-pin power connector for power supply . External drives either have a built-in power supply unit , a connection for an external power supply unit or are supplied with power via USB, provided the drive does not require more than 500 mA of current. If the drive is connected to a hub together with other devices, this can be a problem, as the drive then has to share the 500 mA with other devices - unless the hub has its own power supply.
Optical scanning
CD-ROM drives, like other optical drives, have laser diodes for scanning (or writing). CD uses 780 nm (infrared), DVD 650 nm (red) and Blu Ray Disc 405 nm (violet) vacuum wavelength. The numerical apertures of the focusing lenses also increased in the course of the further developments CD / DVD / BR, which explains the strongly increased data densities with the further development: the smaller the wavelength and the larger the aperture, the smaller the focus diameter.
The laser beam is focused on the data carrier layer for scanning. For this purpose, the focusing lens is readjusted laterally (track position) and vertically (focus) on the basis of the laser beam reflected from the data layer and guided via a beam splitter to a photo receiver. For this purpose, it is mounted in such a way that it can be moved in the corresponding planes and moved via an electrodynamic drive. Corresponding beam guidance and detection of the reflected beam with several photodiodes can be used to generate not only the useful signal but also the manipulated variables for driving the track position and the focus from the detected signals . Different methods are used for this; the two most common are called single-beam pickup and three-beam pickup or three-spot system . In the latter, a receiver with four photodiodes arranged in a square is used for focusing, the sum and difference signals of which are evaluated. A cylindrical lens is used to create an orthogonally oriented ellipse before and after the focal plane (see article Image errors ). If the track position drive cannot cover the entire CD on its own (like a galvanometer drive), a motor-driven scanning head carriage is used, which is controlled depending on the tracking control.
The data rate of the useful signal in comparison with a crystal oscillator reference serves as a control variable for the speed of the CD drive motor.
speed
The music information on the audio CD , which was developed earlier, is organized in 75 data blocks per second. If a CD-ROM drive (like the first CD drives from 1985) works at the same speed, the result is a data rate of 153.6 kB / s (since a data block on the CD-ROM comprises 2048 bytes) , which is known as single speed (1 ×) is called. (Details on this can be found in the section Data rate of a CD in the article Compact Disc .) The speed was soon increased further and further (in the mid-1990s there were terms such as double-speed or quad-speed drive for drives with two or four times the speed) until a limit has been reached at 60 times the speed. If this limit is exceeded, there is a risk that the CD will be destroyed by the centrifugal forces acting on it .
In order to further increase the reading speed, drives were developed whose lasers can read several tracks at the same time (multibeam). With the TrueX technology used by Kenwood , the drive reads seven tracks simultaneously. Such a device with actually 72-fold reading speed allows the CD to rotate at a maximum of 2700 / min and thus works much quieter than a 52-fold drive without a multibeam. However, due to the high purchase price, it never caught on.
Further developments
CD burner
For writing (colloquially “burning”) CD-R and CD-RW , special drives with powerful lasers have been developed, known as CD burners .
DVD drives
The further development of the CD-ROM drives are the DVD drives and DVD burners , which can read and write CDs as well as DVDs. Since all of these drives are based on reading out the data by a laser, they are also referred to collectively as optical drives .
DVD successor
In the meantime there are also successors to the DVD with a multiple of storage capacity. These include Blu-ray Disc , HD DVD , VMD, and HVD .
Individual evidence
- ↑ http://www.computer-masters.de/laufwerksgroessen.php
- ↑ a b c d Glenn Baddeley: Compact Disc - The Inside Story / Part 5 - Laser Tracking ( Memento of the original from July 19, 2018 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ John A Cope: The physics of the compact disc - The pickup
