Editing CD player (section)

===Analog signal recovery from the disc===
{{unreferenced section|date=January 2014}}
[[File:Philips RAFOC CD Optical tracking device..png|thumb|Photodiode array on the Philips RAFOC single-beam tracking optical device used in many CDM optical assemblies]]
[[File:BenQ DW1640 - laser unit - lens with coils-0612.jpg|thumb|Movable lens with coils]]
To read the data from the disc, a laser beam shines on the surface of the disc. Surface differences between discs being played, and tiny position differences once loaded, are handled by using a movable lens with a very close focal length to focus the light on the disc. A low-mass lens coupled to an electromagnetic coil is in charge of keeping focused the beam on the 600&nbsp;[[nanometer|nm]] wide data track.

When the player tries to read from a stop, it first does a focus seek program that moves the lens up and down from the surface of the disc until a reflection is detected; when there is a reflection, the servo electronics lock in place keeping the lens in perfect focus while the disc rotates and changes its relative height from the optical block.

Different brands and models of optical assemblies use different methods of focus detection. On most players, the focus position detection is made using the difference in the current output of a block of four photodiodes. The photodiode block and the optics are arranged in such a way that a perfect focus projects a circular pattern on the block while a far or near focus projects an ellipse differing in the position of the long edge in north–south or west-southwest. That difference is the information that the servo amplifier uses to keep the lens at the proper reading distance during the playback operation, even if the disc is warped.<ref>Egon Strauss - Compact Disc, Digital storage medium, Ed. Quark 1998</ref>

Another servo mechanism in the player is in charge of keeping the focused beam centered on the data track.

Two optical pick-up designs exist, the original CDM series from Philips use a magnetic actuator mounted on a swing-arm to do coarse and fine tracking. Using only one laser beam and the 4 photodiode block, the servo knows if the track is centered by measuring side-by-side movement of the light of beam hitting on the block and corrects to keep the light on the center.

The other design by Sony uses a diffraction grating to part the laser light into one main beam and two sub-beams. When focused, the two peripheral beams cover the border of the adjacent tracks a few [[micrometers]] apart from the main beam and reflect back on two photodiodes separated from the main block of four. The servo detects the RF signal being received on the peripheral receivers and the difference in output between these two diodes conform the tracking error signal that the system uses to keep the optics in the proper track. The tracking signal is fed to two systems, one integrated in the focus lens assembly can do fine tracking correction and the other system can move the entire optical assembly side by side to do coarse track jumps.

The sum of the output from the four photodiodes makes the RF or high-frequency signal which is an electronic mirror of the pits and lands recorded on the disc. The RF signal, when observed on an oscilloscope, has a characteristic [[eye pattern]] and its usefulness in servicing the machine is paramount for detecting and diagnosing problems, and calibrating CD players for operation.