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September 9, 1997

HITACHI DEVELOPS 4.7 GB DVD-RAM TECHNOLOGY

DVD is the acknowledged image and information storage media of the next generation multimedia systems for video, music, computers and a broad range of other applications. Spurred by the introduction of DVD-Video for image reproduction and DVD-ROM (read-only memory) for computers, the market for DVDs has already taken off, and the DVD family is being looked to as a united infrastructure for image and information. Thus compatibility among DVD formats is also essential for the large capacity DVD-RAM.

Hitachi has been very active in the DVD business, introducing the world's first DVD products. Hitachi has been shipping single-speed DVD-ROM sample drives since June 1996, double-speed DVD-ROM sample drives since March 1997, and DVD-RAM sample drives since June 1997. Furthermore, Hitachi, the chair of the DVD-RAM working group in the DVD forum, has greatly contributed to the completion of the unified DVD-RAM format that takes users' opinions into consideration.

The key concept for the 4.7 GB DVD-RAM technology is large capacity without burden on the readout system. The physical format (specifications for allocation of information on the disc) is based on that of the 2.6 GB DVD-RAM; with a reduced track pitch and bit pitch (*1). The bit pitch, which has a major bearing on the readout signal quality, was reduced to the same level as the DVD-ROM. The track pitch was set narrower than that of the DVD-ROM by making good use of the land and groove structure.

The main features of the developed technology are as follows.

(1)High-contrast media
High-contrast media greatly improves the readout signal to noise ratio, resulting in high data reliability for high density recording. Both high readout SNR and constant thermal absorption are realized, which are important for data overwriting. This resulted from the introduction of a contrast enhancement layer (*2) and a thermal buffer layer (*3).

(2)Adaptive recording waveform control
An adaptive recording waveform control method enabled the accurate formation of recording marks much smaller than optical spots.

The technologies developed made possible a 4.7 GB DVD-RAM which is readable by any drive of the DVD family. Through the developed technology, Hitachi will be contributing to 4.7 GB DVD-RAM format development activities, which start this fall in the DVD forum.

Notes:

(*1) General Specifications for the DVD-RAM

Items	2.6 GB DVD-RAM	4.7 GB DVD-RAM (This technology)	4.7 GB DVD-ROM
Disc diameter	120 mm	<-	<-
Substrate thickness	0.6 mm x 2	<-	<-
Recording layer	Phase-change	<-	-
Laser wavelength	650 nm	<-	650/ 635 nm
Lens numerical aperture	0.6	<-	<-
Bit length	0.41 to 0.43 micrometers	0.28 to 0.30 micrometers	0.27 micrometers
Track pitch	0.74 micrometers	0.59 micrometers	0.74 micrometers
Track format	Wobbled Land & Groove	<-	A series of pits
Physical address	Embossed pits	<-	<-
Recording code	8 to 16, RLL(2,10)	<-	<-
Sector length	2048 bytes	<-	<-
Error correction code (ECC)	Reed-Solomon product code	<-	<-
ECC block length	16 sectors	<-	<-
Sector allocation	ZCLV	<-	CLV

(*2) Contrast enhancement layer
A contrast enhancement layer improves the contrast (difference between the reflectivity of recorded regions and unrecorded regions) from 50% to 80 %, which is higher than that of a DVD-ROM. This results in the high SNR readout.
(*3) Thermal buffer layer
This layer keeps the thermal absorption in the recording layer constant, independent of the recording marks before the overwriting process, resulting in the accurate mark formation. In phase change recording, the thermal control on the media is a key to overwriting, since data is recorded by utilizing the difference in cooling rates.
(*4) Adaptive recording waveform control
A method of controlling the form of the recording laser pulse, the power and width of which are dynamically controlled adaptive to the lengths of the recording mark and the preceding and following spaces. This results in a uniform thermal profile during the cooling process.

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