Editing CD player (section)

==Inner workings==
The process of playing an audio CD, touted as a digital audio storage medium, starts with the plastic polycarbonate compact disc, a medium that contains the digitally encoded data. The disc is placed in a tray that either opens up (as with portable CD players) or slides out (the norm with in-home CD players, computer disc drives and game consoles). In some systems, the user slides the disc into a slot (e.g., car stereo CD players). Once the disc is loaded into the tray, the data is read out by a mechanism that scans the circular data tracks using a [[laser]] beam. An electric motor spins the disc. The tracking control is done by analog servo amplifiers and then the high-frequency analog signal read from the disc is digitized, processed and decoded into analog audio and digital control data which is used by the player to position the playback mechanism on the correct track, do the skip and seek functions and display track, time, index and, on newer players in the 2010s, display title and artist information on a display placed in the front panel.<ref>ISO/IEC Standard 60908</ref>

===Analog signal recovery from the disc===
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[[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.

===Digital signal processing===
The first stage in the processing chain for the analog RF signal (from the photoreceptor device) is digitizing it. Using various circuits like a simple comparator or a data slicer, the analog signal becomes a chain of two binary digital values, 1 and 0. This signal carries all the information in a CD and is modulated using a system called [[Eight-to-Fourteen Modulation|EFM]] (Eight-to-fourteen modulation). The second stage is demodulating the EFM signal into a data frame that contains the audio samples, error correction parity bits, according with the [[Cross-interleaved Reed-Solomon coding|CIRC]] error correction code, and control data for the player display and micro-computer.
The EFM demodulator also decodes part of the CD signal and routes it to the proper circuits, separating audio, parity and control (subcode) data.

After demodulating, a CIRC error corrector takes each audio data frame, stores it in a [[Shadow Random Access Memory|SRAM]] memory and verifies that it has been read correctly, if it is not, it takes the parity and correction bits and fixes the data, then it moves it out to a [[Digital-to-analog converter|DAC]] to be converted to an analog audio signal. If the data missing is enough to make recovery impossible, the correction is made by interpolating the data from subsequent frames so the missing part is not noticed. Each player has a different interpolation ability. If too many data frames are missing or unrecoverable, the audio signal may be impossible to fix by interpolation, so an audio mute flag is raised to mute the DAC to avoid invalid data to be played back.

The Redbook standard dictates that, if there is invalid, erroneous or missing audio data, it cannot be output to the speakers as digital noise, it has to be muted.

===Player control===
The Audio CD format requires every player to have enough processing power to decode the CD data; this is normally made by [[application-specific integrated circuit]]s (ASICs). ASICs do not work by themselves, however; they require a main microcomputer or [[microcontroller]] to orchestrate the entire machine. The [[firmware]] of basic CD players typically is a [[real-time operating system]].

Some early optical computer drives are equipped with an audio connector and buttons for standalone CD playback functionality.<ref>{{Cite web |url=https://www.cnet.com/products/nec-multispin-6x-cdr-1350a-cd-rom-drive-ide/ |title=NEC MultiSpin 6X CDR-1350A - CD-ROM drive - IDE - internal Specs - CNET |access-date=2021-07-19 |archive-date=2021-07-19 |archive-url=https://web.archive.org/web/20210719174756/https://www.cnet.com/products/nec-multispin-6x-cdr-1350a-cd-rom-drive-ide/ |url-status=dead }}</ref>