CD player
A CD player (often CD player ) is a device that plays audio CDs. Other media formats such as DVD or data CD -ROM can not be processed.
They are available in different versions:
- Stationary device for connection to a hi-fi system
- A portable device, such as the CD Walkman (registered trademark of Sony)
- Integrated into other devices ( hi-fi systems or car stereos )
- As a function of a personal computer with built-in CD drive (CD -ROM or DVD burner ), together with a sound card and a media player software.
Basic function
The information on the CD are stored on a line extending spirally from inside to outside data track in the form of pits and lands ( small depressions / elevations ) located in the interior of a transparent CD metal layer. The pits on the CD provide not directly represent the data bits, but are means of 8-14 encoding ( engl. Eight -to- Fourteen Modulation EFM) encoding. Here, a change ( land / pit or pit / land ), a logic 1, no change represents a logical 0 is (see construction of a CD). The data track is scanned by a non-contact infrared laser diode having a wavelength of 780 nm ± 10 nm from the back side of the CD. The reflected light from the CD light is received by an array of a plurality of photo diodes and converted into an electrical signal. Lasers and photodiodes are in the scanning unit, the so-called pickups ( Optical Pickup Unit also OPU called ) are summarized.
Construction
Drive
The drive includes all mechanical and electronic components for loading and scanning the CD. The charging process takes place in stationary devices usually have a motorized drawer. After complete insertion of the drawer, the disc is lowered to the drive plate and the drive plate of the scanning unit is lifted with the CD. For portable devices, the CD is usually placed directly on the drive plate and secured by closing the device lid.
The drive of the disc is accomplished by a DC motor with high-quality devices also by a wear-resistant electronically commutated motor. The speed is controlled by the associated power electronics.
The scanning unit (also called pickup) scans the data track of the disc from the bottom and is moved radially from the inside to the outside. This tracking movement can take place as a pivotal movement like a record player tone arm or linear. The pivoting movement (see picture ) by a coil which is mounted directly to the swing arm and moved in a circular ring segment-shaped magnetic field.
In today's CD players staggered tracking has prevailed with linear actuator and galvanometer due to the lower vibration susceptibility. The scanning unit ( pickup) including galvanometer is moved on two guide rods on a rack or a spindle drive with a direct current motor. This is called a sledge (sled ) motor. Some more expensive CD players, the carriage with a so-called linear motor is driven. However, it alone can not move quickly and accurately enough the focusing lens. The fine positioning of the lens via a galvanometer drive, similar to the moving coil of a loudspeaker.
Also a galvanometer is used to focus the laser beam on the information layer of the CD. Moving the lens up and down, thereby changing the spacing of the CD.
The galvanometer can also compensate concentricity and Planizitätsfehler the CDs to a certain extent due to the low moving mass ( plastic lens ).
Laser scanning system
The scanning system provides for tracking of the focus position and tracking a spring in a galvanometer ( solenoids ) suspended focusing lens ( coated plastic lens ). Furthermore, it is a small, continuously operating semiconductor laser ( laser diode, wavelength 780 nm, power few milliwatts ), a beam splitter ( glass, gold plated ) and comprising a plurality of photodiodes photoreceptor.
The laser beam passes through the beam splitter to the focusing lens, which focuses the reflected from the data layer of the CD light back through the beam splitter to the photodetector. The photodiodes are arranged such that not only the digital data signal, but also the control signals for focus and tracking control can be obtained.
For example a laser scanning system
The laser light is split in the three- beam system through an optical system into three beams. Here, a beam is used to read the data from the CD and for focusing, the other two beams for tracking (tracking). The three-beam scanning system is the most widely used.
Components of the system in Figure 1:
The laser unit is the most sensitive component and therefore causes the greater part of the defects in CD- players. Will not read CDs correctly, is usually cleaned in the repair shops as a first attempt, the focusing lens. Performance is not improved, the pickup is complete ( should one ever be procured ) replaced.
Track sampling at the three-beam system:
Figure 2 shows the location of the laser beams on the track of a CD. The auxiliary beams LB1 and LB3 are used for tracking. These rays share with the CD player of the electronics if the tracking is correct. The main beam LB2 to read the data from the disc 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 in the three- jet system:
The six existing in the three-beam system photodiodes are arranged as shown in Fig 3. The main beam is incident on the four diodes AD, the two auxiliary beams to each one of the diodes D and F. In the uniform illumination of the four diodes of the lens chief ray distance set correctly ( Fig. 3b). In case of incorrect focusing of the beam illuminates two opposite diodes greater (Fig. 3a and 3c). By a differential analysis of the diode currents, a correction signal for the lens distance is obtained:
With proper focus as shown in Figure 3b, the error signal becomes zero. In Figure 3a, it would be positive, in Figure 3c, there would be negative.
Electronics
Power supply
The power supply provides all the details necessary for operation of the device operating voltages. For mains-operated appliances conventional power transformers or switching power supplies are used. The goal is to keep minimal inter alia, the mutual influence of the different areas of consumer electronics. In particular, the analog audio range and its power supply must be done carefully to keep interference of the steep-sided streams of digital modules and the servo control.
Control electronics
In the control electronics contains all functions that have to do with the operation and control of the CD player. These include query the keyboard, the display, if necessary, the reception of remote control signals that control the display (formerly often a LED digital display, nowadays usually a liquid crystal display or a fluorescent display ) and the drive functions. One or more microcontrollers are used.
Servo control
In a CD player, there are three control loops, which independently control the drive speed of the CD, the focusing of the laser ( Focus) and tracking (tracking).
The CD is played from the inside to the outside. The farther the laser unit is moved to the outer edge of the disc, the more slowly rotating the disc so that the peripheral speed with which the data track is detected, and thus the amount of data per unit of time remains constant. The drive speed is regulated so that the data stream is always as high as required by the subsequent signal processing. To compensate for variations in the data is temporarily stored in a buffer. Depending on the position of the sampling unit is the speed between 200 min -1 and 500 min -1.
The focus control circuit receives its input signals from the scanning unit accommodated in the photodiodes. By the astigmatic optics in the scanner to focus error can be detected and lead to a corresponding correction signal to the focus moving coil drive.
The tracking control circuit receives its input signal also from the scanning unit accommodated in the photodiodes. Small tracking errors are first corrected with the radial moving coil drive. The scanning of the spiral data track on the disc leads to steady increase in the tracking error, and a threshold is exceeded, from a radial tracking of the scanning means of the rack and pinion drive and the carriage (sled ) motor is executed by a certain amount. These steps are necessary every few seconds and are controlled with the tracking control loop. In the case of the rotary actuator for tracking the movement of the division is omitted in this coarse and fine movement, here is the tracking of direct drive centrifugal motor is running alone.
Signal processing
The electrical signals from the photodiode ( RF eye pattern signal or Engl. Eye pattern signal) of the scanning unit are first amplified and evaluated for the position control. The digital data stream is buffered in a FIFO buffer, the level of which serves as input signal for the speed control of the CD- drive motor. This is followed by error correction in accordance with the Cross Interleaved Reed-Solomon code and the 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 which have been produced with high boost ( Emphasis), there is a mirror-image reduction in the Hochtonanteils ( de-emphasis ), to restore a flat frequency response. The amount of reduction is automatically turned on by a Subcodebit on the CD, but is rarely used. Then the signal is available in analog form at the output of the CD player.
In addition, the digital audio signal can be provided ( by 75 -ohm coaxial cable with RCA or BNC ) or optical form ( by TOSLINK cable) in the S / PDIF format in electrical form. This digital signal allows lossless transmission to other digital audio devices, especially to external digital - to-analog converters and digital recording devices (eg DAT recorders ). Here, the Serial Copy Management System ( SCMS ) is used to ensure that only one copy generation can be made of copyrighted CDs.
To avoid damage to the sampling theorem, any frequencies may be encoded, which are equal to or greater than half the sampling frequency. In order to an upper cut-off frequency encode the audio frequency range of 20 kHz and can play an extremely high-slope filter is required on the reproduction side, the strongly attenuated all frequencies above 20 kHz. At 22.05 kHz attenuation of approximately 100 dB are already required.
The Fourier transform of the frequency response of this low-pass filter in the time domain shows that high selective pulses can not be accurately reproduced. The analysis and measurement of the step response results in transients. A spike is reproduced with inputs and decay processes, which are not present in the original signal. These inputs and are decaying resonance on the cutoff frequency of the filter. To reduce these unwanted distortions, CD player were developed whose impulse response has been optimized at the expense of a less even- frequency response. The principal problems that caused such a high-slope filter in the signal path, but can also so not be completely eliminated.
Special
The playback speed of CD players is crystal controlled and therefore very consistent. However, there are also CD player for DJs, in which the playback speed control ( " pitching " ) leaves.
Newer CDs containing CD text. This additional information, eg about the artist and the title can be viewed from a specially equipped CD player.
Most CD players can now also play mixed data and music CDs.
Some CD players have problems correctly play back copy-protected CDs, because they do not conform to the CD Red Book standard. This is especially true for recent CD player, based on CD -ROM drives, as in a PC.
Recent developments have led to MP3 and DVD players, they allow in addition to the CD playback and the playback of MP3 and other compressed audio formats.
The DVD player to play DVDs developed are constructed the same principle as CD player, but differ in the density of data, the recording methods and formats.
In late, equipped for the playback of digitally encoded sound laser disc players often the playback function for CDs was integrated ( CD / CDV / LD combi players). Thus one could ( LDs ) play the small discs and the large laser discs with only one device.
Electronic Skip Protection prevents the short OPENING HOURS exposure of CD players in collisions.
Discman
In a Discman is a trademark of Sony for their own CD player, also compare Walkman. The word Discman (as well as Walkman ) borrowed from the English language.