Compact Disc Technology: In-Depth Technical Analysis
Linear and Angular Speed
The linear speed of the disc must be very precise. To start reading in the center of the disk, the angular speed is 500 rpm. As the beam moves outside the disc, the shaft speed decreases to 200 rpm, as the linear speed remains constant.
The Pickup
Compact disc reading requires optoelectronic elements that enable the broadcasting of the light source, the fitness, and focus that focus on the pit and reflected light detection, which allows reading and correction of errors. It consists of a laser diode (1), lenses (2), a diode detector (3), a focusing lens system, and a monitor photodiode called APC (5).
Servo System
A servo circuit is a turn or control of a movement. A CD player has five servo systems:
- Sled Servo: Controls the horizontal or radial displacement of the disc.
- Focus Servo: Controls the vertical displacement of the laser beam.
- Tracking Servo: Follows the track, preventing the laser beam from deviating.
- Spindle Servo: Controls the rotation of the disc.
- Tray Servo: Controls the loading and ejecting of the disc carrier.
The Laser Diode
The properties of light consist of a series of waves of the same frequency and phase, called coherent light. Laser light intensity is much greater because it concentrates all the energy emitted at a certain frequency. A commercial laser diode incorporates a monitor photodiode, called APC, which controls the laser light emitted, as possible temperature variations or current can destroy the chip.
Lenses and Optical Assembly
The light emitted by the laser diode must be treated with different lenses to reach the disk surface with great precision and perfectly detect the reflections of the lands and pits. The reflected signal is the RF signal, which is formed by nine sinusoidal signals. The amplitude varies between +1.3 Vpp and -1.3 Vpp, and these nine signals are between 196 and 720 kHz. The RF signal goes to the photodiode pickup. The light reflected from the disc again passes through the focusing lens and the collimating lens, and the prism is diverted to the photodetector. When light strikes a land, it perfectly reflects, and the photodetector receives the maximum light intensity.
Three-Beam System
This system consists of a diffraction grating producing three beams (one main and two secondary) and a collimating lens that reduces the divergence of the beam. The main beam is the most intense and is used for reading the data produced by the pits. The two side beams read the same track, in front and behind the main beam.
Single-Beam System
This system removes the diffraction lens and collimating lens, as it does not need the three beams. This system uses a single beam with a converging lens and focuses it to track the data. It uses the shadow cast by the pit, where data is posted.
Compact Disc Player
The lenses and photodiodes collect reflected light and convert it into digital electrical signals. The signal is amplified and converted into digital information by a modulator. Decoding is done by a microprocessor, which separates the audio information from the error-correcting digital reading. The signal is then converted into an analog signal. The two stereo channels are alternately recorded, thus separating the digital processing information on each channel, so the left channel does not mix with the right channel.
Magneto-Optical Disc
The advantages of magneto-optical discs are that they are rewritable and have little physical wear. They use the same format as hard drives, structured by tracks and sectors. They have a higher capacity due to a very fine laser beam. This laser avoids contact with the playhead, giving more durability to the disc. The MO disc consists of particles oriented in one direction, and the applied magnetic field can change when heated to 200° by the laser. Thus, etching is done with 1 where the laser touched and 0 where it did not. The word is read by turning off the magnetic field and having a lower intensity laser on the particles. Erasing is done by applying the magnetic field in reverse so that when the laser enters, the particles are restored to their original orientation.