Description
1.Outline
A CD deck can be a type of D/A converter that converts recorded digital music or voice signals on the CD into original analog signals.
Since the digital signals are not degraded in the process of processing the signals, repeating the recording will not make the S/N ratio worse as with analog signals, and will not reduce the dynamic range.
2.CD
“CD” refers to something with the globally unified standard mark shown on the left is attached on. Therefore, a CD or CD deck without this mark is out of standard. A CD is a disc whose shape is 120 mm (or 80 mm) outer diameter and 1.2 mm thick, and is a compact size phonorecord that consists of three layers of clear board (poly-carbonate), aluminium reflection membrane and protective membrane (plastic).
The music data is recorded in a digital signal that is expressed by the presence or absence of pits. The pits are 0.5μm wide, 0.9 to 3.3μm long and 0.11μm deep protrusions and form the track by convoluting counterclockwise from the inside of the disc to the outside. At the starting position of the track (the innermost), the contents (the total number of songs, the total playing time, the position of each song, and so on) that summarizes the music data recorded on the CD are recorded as read-in information. The track number and playing time are displayed and selection and search of the songs are performed according to this information.
Principle and Operation of CD Player
1.Outline
A CD player picks up the signals as electrical signals according to the strength of the reflected light by emitting a laser beam on the pits of digital signals recorded on the CD.
2.Optical pick-up
The optical pick-up is a part that correctly emits a laser at the pits on the CD and picks up the reflected light. When the laser beam strikes a place with no pit, nearly 100% of the beam is reflected and returns to the photodiode. However, when the beam strikes a pit, the resulting diffraction causes only about 30% of its light to return to the photodiode.
The strength of the light received by the photodiode and the generated current is used as audio signals.
3.Optical pick-up tracking servo
The optical pick-up tracking servo is used to track the optical pick-up on the track according to the rotation of the CD and always keeps the focus lens in the correction range of the
4.CLV servo (Constant Linear Velocity)
In a CD, music data is recorded with a constant linear velocity.
Therefore, when reading the signals, it is necessary to control the signals that pass through the optical pick-up at a constant speed by changing the CD’s rotational speed. When the optical pick-up is at the innermost of the CD, the rotational speed is high (500 rpm), and when the optical pick-up is at the outermost of the CD, the rotational speed is low (200 rpm). The CLV servo works so that the linear velocity becomes constant by the synchronized signal recorded on the CD.
5.Signal processing
The CD uses a special error correction circuit to correct or compensate and converts to analog signal when performing the playback even if a tiny scratch or foreign matter exist on the CD.
CD auto changer
A CD auto changer automatically changes the discs and plays back the digital sound by setting the magazine into which multiple CDs can be inserted. The CD auto changer consists of the optical pick-up that reads the pit signals on the CD and changer mechanism that automatically changes the CD in the cartridge, etc.
There are two types of CD changer: a type separated from the radio and a type combined with a radio and to become a unit.
The latter is called the In-dash CD auto changer.