Editing CD player (section)

==Pickup mechanisms==
[[File:Optical Deck Bottom.jpg|thumb|left|Underside view showing a radial-type tracking mechanism with screw drive]]
[[File:Philips CDM4 Swing Arm CD mechanism.jpg|thumb|Philips swing arm optical mechanism]]
[[File:CD Player Photodetector Chip.jpg|thumb|The optical chip extracted from a CD player. The three dark rectangles are photosensitive, read the data from the disk and keep the beam focused. Electronic tracking, aided with the two photodiodes at the sides, keeps the laser beam centered on the middle of the data track.]]
Two types of optical tracking mechanisms exist:
* The '''swing-arm mechanism''', originally designed by Philips<ref>{{cite web|url=http://siber-sonic.com/audio/swingarm.html |title=Swing-arm mechanism description |publisher=Siber-sonic.com |access-date=2012-05-06}}</ref>&nbsp;– the lens moves at the end of an arm, in a manner similar to the tone arm assembly of a [[record player]]. Used in earliest Philips CD players and later replaced with cheaper radial mechanisms.
* The '''radial mechanism''', designed by Sony, which is the one used in most CD players in the 2000s – the lens moves on a radial rail being driven by a rotating gear from a motor or a linear magnetic assembly. The motor or linear magnetic assembly consists of a solenoid mounted to the moving laser assembly, wound over a permanent magnetic field attached to the base of the mechanism. It is also known as three-beam linear tracking.
[[File:Philips 1 beam tracking Laser Optical Assembly.jpg|thumb|left|Philips one-beam laser assembly]]
The swing-arm mechanism has a distinctive advantage over the other in that it does not skip when the rail becomes dirty. The swing arm mechanisms tend to have a much longer life than their radial counterparts.{{Citation needed|date=May 2014}} The main difference between the two mechanisms is the way they read the data from the disc. The swing-arm mechanism uses a magnetic coil wound over a permanent magnet to provide the tracking movement to the laser assembly in a similar way a [[hard drive]] moves its head across the data tracks. It also uses another magnetic movement mechanism attached to the focusing lens to focus the laser beam on the disc surface. By operating the tracking or the focus actuators, the laser beam can be positioned on any part of the disc.
This mechanism employs a single laser beam and a set of four photodiodes to read, focus and keep track of the data coming from the disc.<ref>Philips CD100 Service Manual</ref>

[[File:CD Player focusing lens assembly.jpg|thumb|left|Sharp laser optical assembly. All six focusing and tracking coils can be seen.]]
The linear tracking mechanism uses a motor and reduction gears to move the laser assembly radially across the tracks of the disc and it also has a set of six coils mounted in the focusing lens over a permanent magnetic field. One set of two coils moves the lens closer to the disc surface, providing the focusing motion, and the other set of coils moves the lens radially, providing a finer tracking motion. This mechanism uses the three-beam tracking method in which a main laser beam is used to read and focus the data track of the disc using three or four photodiodes, depending on the focus method, and two smaller beams read the adjacent tracks at each side to help the servo keep the tracking using two more helper [[photodiode]]s.<ref>Sony CDP-101 Service Manual</ref>