Sony MINIDISC What it is and How it works, What it is and How it Works, Recordable MiniDiscs

What it is and How it works

1.2. What it is and How it Works

Two Kinds of MiniDiscs are Available:

Premastered MiniDiscs for

Music Software

Premastered MiniDiscs are used most commonly for music and are sold in record stores just the same as compact cassettes and CDs are.

MiniDiscs, just like CDs, are manufactured in large volumes by high-speed injection molders, and the music signals are recorded during repli- cation in the form of pits.

Forget the times when you would worry about cassette tapes becoming worn out or wrinkled. Moreover, the discs are encased in a cartridge, so there is no worry about their being scratched. The design of the premastered MiniDisc cartridges

is special. Prerecorded music packages require a label, featuring the artist’s picture or other infor- mation – for it is the artwork that most often captures the buyer’s eye. Therefore the top face of the cartridge is left completely free for the label. A window for the laser beam to read the disc is only necessary on the bottom face.

Recordable MiniDiscs

Magneto-optical (MO) technology forms the heart of the recordable MiniDisc technology. Not only are MO discs durable enough to withstand being rerecorded about 1 million times, they also have a life expectancy approaching that of CDs.

When recording the MO discs, a laser beam must be focused onto the magnetic layer from behind while a magnetic field is applied to the front. For this reason, the cartridges for recordable MiniDiscs have a read/write window on both sides.

MiniDisc Pick-up Technology

Optical Pick-up of MD Player:
0	1	0	1	MD
				Recording track
				Objective Lens
				Polarized
				beam splitter
		High intensity	Low intensity		Photo Detector A
					+	MO signal
	Cross sectional view				-
					+
	of recording track
				Laser Beam	+	Pit signal
	Laser	Laser
Recordable disc		Premastered disc		Photo Detector B

The optical pick-up developed for the MD system has the remarkable ability to read both recorda- ble and premastered MiniDiscs. For recordable discs, the pick-up detects the magnetic structure along the track. For premastered optical discs, it detects the geometric structure along the track. The MD system is based on the design of the standard CD pick-up with the addition of a MO signal readout analyzer and two photodiodes. During playback of recordable discs a 0.5mW laser

is focused onto the magnetic layer. The magnetic signal on the disc affects the polarization of the reflected beam. The direction of polarization is converted into light intensity by the MO signal readout analyzer. Depending on the direction of polarization one of the two photodiodes will detect more light. The electrical signals from the photodiodes are subtracted and depending on whether the difference is positive or negative, a “1” or “0” signal is read.

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