Magnetic Recording and DVD Optical Technology Principles

Magnetic Recording Fundamentals

• Helical Recording: Uses an A/D recorder with high head-to-tape speed to record video signals. The tape information is shaped according to the selected format.
• Hysteresis: The impact of an external field depends on the tape's prior magnetic state. The magnetization curve is characterized by reversible and non-reversible areas; linear areas are ideal for registration.
• Polarization: Bias is used to solve non-linear polarization issues. High-frequency audio signals are added to analog signals to utilize the linear portion of the curve.
• Equalizer: Performs amplitude correction using a complementary curve. (Audio: 10 octaves, Video: 18 octaves).
• Azimuth: High-density recording systems use azimuth recording for A/D signals.

Acoustics and Microphone Technology

• dB SPL: Sound pressure per unit surface area of the molecules receiving the pressure.
• Microphone Sensitivity: The relationship between the sound pressure acting on the diaphragm and the generated output voltage.
• Isophonic Curves: Characteristic curves representing sound behavior as perceived by the human ear.
• Hypercardioid Microphone: Features a smaller rear lobe for directional sound pickup.

DVD Player Architecture

The general block diagram of a DVD player consists of an optical reading system and a servo system for position and speed control.

Optical Reading Process

• Laser Diode: Provides a 650nm wavelength signal.
• Diffraction: Light rays pass through a grid to create different patterns.
• Prism: A semi-reflective prism directs the signal toward the DVD or the photodiode.
• Exposure Lock Lenses: Focus the laser range onto the surface of the mobile media.
• Cylindrical Lenses: Concentrate light toward the photosensors.
• Photodiode Matrix: Converts light signals into electricity.

The system utilizes 6 photosensors: the central 4 provide light and sound data as well as focusing information, while the remaining 2 diodes handle track follow-up. The servo system controls the disk's rotational speed via CLV (Constant Linear Velocity).