A CD player (Engl. CD player ) is a player for audio CDs . Many more modern devices are also able to play back other formats such as MP3 , AAC and WMA . DVD and Blu-ray players are usually downward compatible and can therefore also be used as CD players.
The information on the CD is stored on a data track running in a spiral from the inside to the outside in the form of pits and lands (small depressions / elevations) of a metal layer located inside the transparent CD. The pits on the CD do not directly represent the data bits, but are encoded using 8-14 coding ( Eight-to-Fourteen modulation EFM). A change (land / pit or pit / land) means a logical 1, no change represents a logical 0 (see structure of a CD ). The data track is scanned without contact by an infrared laser diode with a wavelength of 780 nm ± 10 nm from the back of the CD. The light reflected from the CD is picked up by an array of several photodiodes and converted into an electrical signal. Laser and photodiodes are combined in the scanning unit, the so-called pickup unit (also called Optical Pickup Unit, OPU).
There are different versions of CD players:
- stationary device for connection to a stereo system
- integrated in other devices (hi-fi systems or car radios )
- portable device such as CD Walkman (registered trademark of Sony )
- as a function of a personal computer with a built-in CD-ROM drive (if there is no audio output directly on the drive, then only together with a sound card and a media player ).
- CD changer , a mechanical device that allows multiple CDs to be played in a row. They are available as stationary devices, integrated into other devices (e.g. car radios), but also as pure CD-ROM changers
The drive includes all mechanical and electronic components for loading and scanning the CD. Stationary devices are usually charged via a motor-driven drawer. After the drawer has been fully retracted, the CD is lowered onto the drive plate or the drive plate with scanning unit is lifted under the CD. In the case of portable devices, the CD is usually placed directly on the drive plate and secured by closing the device cover.
The CD is driven by a direct current motor, and in the case of high-quality devices also by a low-wear, electronically commutated motor. The speed is controlled by the associated servo electronics.
The scanning unit (also called pick-up) scans the data track of the CD from the underside and is moved radially from the inside out. This tracking movement can take place as a pivoting movement, as in the case of a turntable tonearm, or linear. The swivel movement (see picture) is carried out by a coil that is mounted directly on the swivel arm and is moved in a magnetic field in the form of a circular ring segment.
With today's CD players, staggered tracking with linear drive and galvanometer drive has become established due to the lower susceptibility to vibrations . The scanning unit (pickup) including galvanometer drive is moved on two guide rods via a rack or spindle drive with a direct current motor. This is called a sled motor. In some more expensive CD players, the carriage is driven by a so-called linear motor. However, he alone cannot position the focusing lens quickly and accurately enough. The lens is precisely positioned using a galvanometer drive, similar to the moving coils of a loudspeaker.
A galvanometer drive is also used to focus the laser beam on the information layer of the CD. He moves the lens up and down, changing its distance from the CD.
Due to the low moving mass (plastic lens), the galvanometer drives can to a certain extent compensate for concentricity and planarity errors in the CDs.
Laser scanning system
The scanning system contains a focusing lens (coated plastic lens) that is resiliently suspended in a galvanometer drive (magnetic coils) to adjust the focus position and tracking. It also consists of a small, continuously operating semiconductor laser ( diode laser , wavelength around 780 nm, power a few milliwatts), a beam splitter (gold-coated glass) and a photo receiver consisting of several photodiodes .
The laser beam passes through the beam splitter to the focusing lens, which also focuses the light reflected from the data layer on the CD back via the beam splitter to the photo receiver. Its photodiodes are arranged in such a way that not only the digital data signal but also the control signals for focus and tracking regulation can be obtained.
Example of a laser scanning system
In the three-beam system, the laser light is split into three beams by an optical system. One beam is used for reading the data from the CD and for focusing, the other two beams for tracking. The three-beam system is the most commonly used scanning system:
The laser unit is the most sensitive component and therefore causes most of the defects in CD players. If CDs are no longer read correctly, the first attempt in repair shops is usually the focusing lens. If this does not improve the situation, the entire pickup (if available) will be replaced.
Track scanning with the three-beam system:
The illustration shows the position of the laser beams on the track of a CD. The auxiliary beams LB1 and LB3 are used for tracking. These beams tell the electronics of the CD player whether the track position is correct. The main beam LB2 for reading the data from the CD and for focusing. The scanning unit is guided radially to the track (red arrows) so that the angle of the laser beams to the track remains constant.
Beam focusing with the three-beam system:
The six photodiodes in the three-beam system are arranged as shown in the illustration. The main beam hits the four diodes A – D, the two auxiliary beams each on one of the diodes E and F. If the four main beam diodes are evenly illuminated, the lens spacing is set correctly (Fig. 3b). If the focus is incorrect, the beam illuminates two diodes opposite each other more intensely (Fig. 3a and 3c). A correction signal for the lens spacing is obtained from a differential consideration of the diode currents:
With correct focusing, as in Fig. 3b, the error signal becomes zero. In Fig. 3a it would be positive, in Fig. 3c it would be negative.
The power supply provides all of the operating voltages required to operate the device. Conventional mains transformers or switched-mode power supplies are used for mains -operated devices . One of the goals is to keep the mutual influence of the individual electronics areas to a minimum. In particular, the analog audio area and its power supply must be carefully designed in order to keep away interference from the steep-edged currents of the digital assemblies and the servo control.
All functions that have to do with the operation and control of the CD player are combined in the control electronics. This includes the query of the keyboard, the display, if necessary the reception of remote control signals, the control of the display (previously often an LED numeric display, today mostly a liquid crystal display or a fluorescent display ) and the drive functions. One or more microcontrollers are used.
There are three control loops in a CD player, which independently regulate the drive speed of the CD, the focusing of the laser (focus) and the tracking (tracking).
The CD is played from the inside out. The further the laser unit moves to the outer edge of the CD, the slower the CD rotates, so that the peripheral speed at which the data track is scanned and thus the amount of data per unit of time remains constant. The drive speed is regulated in such a way that the data stream is always as large as required by the subsequent signal processing. The data is temporarily stored in a buffer to compensate for fluctuations. Depending on the position of the scanning unit, the speed is between 200 min −1 and 500 min −1 .
The focus control circuit gets its input signals from the photodiodes housed in the scanning unit. Due to the astigmatic optics in the scanning unit, focusing errors can be detected and lead to a corresponding correction signal to the focusing moving coil drive.
The tracking control loop also receives its input signals from the photodiodes housed in the scanning unit. Small tracking errors are first corrected with the radial moving coil drive. The scanning of the spiral data track on the CD leads to a steady increase in the tracking error until a threshold is exceeded, from which a radial tracking of the scanning unit by means of the rack and pinion drive and the carriage (sled) motor is carried out by a certain amount. These steps are necessary every few seconds and are controlled by the tracking control loop. In the case of the swivel drive for the tracking movement, there is no division into these coarse and fine movements; the tracking is carried out here by the direct drive radial motor alone.
The electrical signals from the photodiodes (HF eye pattern signal or eye pattern signal) of the scanning unit are first amplified and evaluated with regard to the position control. The digital data stream is temporarily stored in a FIFO buffer , the level of which is used as an input signal for regulating the speed of the CD drive motor. This is followed by error correction according to the Cross Interleaved Reed-Solomon Code and extraction of the audio and subcode data.
This is followed by the oversampling filter , the digital-to-analog conversion and the low-pass filter . In the case of CDs that were produced with treble boost ( emphasis ), the high-frequency component is reduced in mirror image ( deemphasis ) in order to restore a linear frequency response. The treble reduction is switched on automatically by a subcode bit on the CD, but is only rarely used. The signal is then available in analog form at the output of the CD player.
In addition, the digital audio signal in S / PDIF format can be provided in electrical form (using 75-ohm coaxial cable with Cinch or BNC connectors ) or in optical form ( using TOSLINK cable). This digital signal enables loss-free transmission to other digital audio devices, in particular to external digital-to-analog converters and digital recording devices (e.g. DAT recorders). The Serial Copy Management System (SCMS) is used to ensure that only one generation of copies of copyright-protected CDs can be made.
In order not to violate the sampling theorem , no frequencies may be coded that are equal to or greater than half the sampling frequency. In order to be able to code and reproduce the audio frequency range up to an upper limit frequency of 20 kHz, an extremely steep-edged filter is required on the reproduction side, which strongly attenuates all frequencies above 20 kHz. At 22.05 kHz, attenuation of around 100 dB is required.
The Fourier transformation of the frequency response of this low-pass filter in the time domain shows that steep-edged pulses cannot be reproduced exactly. The analysis or measurement of the step response results in transient processes. A needle impulse is reproduced with transients that are not present in the original signal. These transient and decay processes are resonances at the cutoff frequency of the filter. In order to reduce these undesirable falsifications, CD players were developed whose impulse response was optimized at the expense of a less even frequency response. However, the fundamental problems that such a steep-edged filter causes in the signal path cannot be completely eliminated in this way.
The playback speed of CD players is crystal controlled and therefore very constant. However, there are also CD players for DJs with which the playback speed can be controlled (“ pitched ”).
Newer CDs can contain CD-Text . This additional information, for example about the artist and the title, can be displayed by a suitably equipped CD player.
Most CD players can also play mixed data and music CDs.
Some CD players have problems playing copy-protected CDs correctly because they do not conform to the CD Red Book standard. This is especially true for newer CD players that are based on CD-ROM drives , similar to a PC .
More recent developments have led to MP3 and DVD players , which allow not only CD playback but also MP3 and other compressed audio formats.
The DVD players developed for playing DVDs are basically constructed in the same way as CD players, but differ in terms of data density, recording methods and formats.
In late LaserDisc players equipped for playing digitally coded sound, the playback function for CDs was often integrated (CD / CDV / LD combination player). This meant that the small CDs and the large LaserDiscs (LDs) could be played with just one device.
Electronic skip protection prevents CD players from being temporarily exposed in the event of impact.
In a Discman is a trademark of Sony for their own portable CD player, similar to the Walkman . The word Discman (like Walkman) is borrowed from the English language.
- ^ Sony Corporate info home audio. Retrieved August 30, 2014 .