Sony MINIDISC manual Overwrite Technology, Overwrite Mechanism

What it is and How it Works

The same 0.5mW laser is used for the playback of premastered optical discs. The amount of light reflected depends on whether or not a pit exists on the surface of the disc. If there is no pit, a high proportion of the light is reflected back through the beam splitter and analyzer into the photodiodes. If a pit does exist, some of the light is diffracted and less light reaches the photodi- odes. The electrical signals from the photodiodes are added up in this case and depending on the sum, a “1” or “0” is read.

Overwrite Technology

In order to meet the requirements of a compact, lightweight, recordable audio system for perso- nal use, the MD system employs a newly develo- ped Sony magneto-optical (MO) disc that utilizes magnetic field modulation with direct “overwri- te” capability. With MO disc technology, the recording of data requires the use of a laser and a polarizing magnetic field. When the magnetic

layer in the disc is heated by the laser to a tem- perature above the Curie point (approximately 200° C), it temporarily looses its coercive force. As the disc rotates and the irradiated domain returns to normal temperature, its magnetic ori- entation is determined by an externally applied magnetic field. Polarities of “N” and “S” can thus be recorded, corresponding to digital data “1” and “0”.

Unlike conventional MO rewrite mechanisms, the ”Sony MD overwrite system” positions a magne- tic head directly accross from the laser source on the opposite side of the disc. A magnetic field corresponding to the input signal is generated over the laser spot. The rotation of the disc then displaces the area to be recorded, allowing the temperature at the spot to drop back below the Curie point. At that point, the domain takes on the polarity of the applied magnetic field regard- less of the polarity that previously existed.

Overwrite Mechanism

1

0

Head Drive signal

				Recording Head
					Magnetic field of
					Recording Head
						Cross section of
						MO-Layer with
						magnetic pattern
Move direction of Disc		Laser Beam
	New			Old		Top view
N S	N	S	S N	S	N
						of one Track
Frozen magnetic pattern			Heated area by Laser
after cooling below			above Curie Point
Curie Point			(erased magnetic pattern)

4DADC

A U S T R I A