Magneto-optical disk ups camera storage capacity

Digital still cameras (DSCs) that have more than 3 mega-pixels CCD have been increasing. Most DSCs record not only still images but also moving pictures. Typically, DSCs use a flash memory card such as smart media and compact flash. However, flash memory cards don't have enough capacity to record moving pictures. Also, memory cards that have a large capacity are expensive. In many cases, the user of DSC transfers the image data recorded on the flash memory card into the storage device on a personal computer. Flash memory card is used as temporary storage media.

The iD PHOTO disk is a media that provides large capacity for storage of DSCs. This media is a magneto-optical (MO) disk, which has been standardized as an "iD format" with a diameter of 50 millimeters and a recording capacity of 730 Mbytes. It is possible to store data for an extended time, and rewrite data over 1 million times. Therefore, it is ideal for mobile data storage usage for devices such as digital still cameras.

We have developed a mobile storage device with the media for backup of image data that has been stored in memory cards. The disk diameter is 50.8 millimeters and its thickness is 0.6mm. The cartridge size is 59.5 x 56.5 x 4.8 mm3. The objective lens numerical aperture (NA) is 0.6, and the wavelength of the laser diode is 650 nanometers. The iD PHOTO disk is recorded on land and groove of the media. The track pitch is 0.6 micrometers and the minimum mark length is 0.235 micrometers.

The error correct code (ECC) block size is 32 kbytes for user data, the cyclic redundancy check (CRC), the ECC and the redundant bits for control of the digital sum variation (DSV). Each ECC block consists of 16 frames (2 kbytes sector). Each frame comprises an address segment and 38 data segments. The address segment and data segment have a length of 532 data channel bits respectively.

The logical format of the iD PHOTO disk has been designed for versatile purposes so its file structure complies with UDF and DCF specifications. A high recording density of 4.6 Gbits/inch2, is about 10 times as high as a CD.

The iD PHOTO disk is based on a center aperture detection (CAD) type magnetic super resolution (MSR) MO disk. In this scheme, a readout layer, which has special magnetic properties, is laminated over the recording layer on the disk. In the readout process, a higher temperature part of the readout layer, which is the center part of the laser beam spot, becomes a magnetic aperture. Hence, very small recording marks, which are about a quarter of laser beam spot size, can be resolved through this aperture.

The recording method is laser-pumped magnetic field modulation (MFM). In this method, an external magnetic field is applied through a magnetic head by pulsed laser beam irradiation. The external field is modulated according to the recording data like MD. Since recording marks are defined by the field switching depended on the pulsed laser, very small marks of 0.235 micrometers can be recorded accurately.

The non-return to zero inverted plus (NRZI+) modulation can suppress the low-frequency components of the data stream effectively. Since it is based on the method of transforming n data words into n+1 words, it has the advantage of lower redundancy, where n is the integer, for example, n=91. First, the data words are convoluted with different types of additional redundant data respectively. Next, the variation of DC component, that is, DSV of each convoluted data is calculated, and then the convoluted data, which has the least value in each DSV, is selected for recording.

Partial Response Maximum Likelihood (PRML) is a significant technology for reproducing a high-density recording data. In the iD PHOTO system, the reproducing signal is equalized to a partial response (PR) waveform. Recording a signal after NRZI+ modulation involves 1T signal, where T is the minimum duration of transition of modulated data. It is possible to reproduce signal independent of amplitude of 1T signal by adopting a PR detection method that has three signal levels and viterbi decoding.

The iD PHOTO system with a high-density disk employs the optimum laser power calibration and the phase adjustment of phase locked loop (PLL) clock. The system works under external clocking generated from the embossed pits of fine clock marks (FCMs) on the media. The reproducing signal from FCMs is applied to the PLL circuit. The system clock for read/write is obtained using a frequency multiplier of 532 times to generate the same number of clocks as the number of data channel bits between each FCM.

In a disk realized by the MSR, the read laser power calibration is necessary to obtain a reproduced signal without cross-talk from adjacent tracks because the magnetic aperture size is changed according to temperature of the laser beam spot. Based on this idea, the laser power is changed according to the variation of environmental temperature.

In the external clock generating, reproducing signal involves a clock jitter. The interval between embossed FCM and recorded data varies with the recording condition such as laser power for recording, linear velocity of disk and temperature of layer on disk. The phase compensation of readout clock adjusts the phase of PLL clock using the short mark fields that consist of 2T repeated data included in the header area of each frame.

The iD PHOTO system enables Zoned Constant Linear Velocity (ZCLV) and Zoned Constant Angular Velocity (ZCAV) disk rotation. The iD PHOTO disk is divided into 12 zones of ZCLV and ZCAV. In ZCLV, a disk rotation speed is changed at every zone so that the linear velocity is about the same speed as other zones. When the iD PHOTO system is operating with ZCLV servo, it realizes a data-transfer rate of 20Mbits/second. With regard to being a data storage device, a faster data-transfer rate is desired. The ZCAV reproducing technique has been developed for a high data-transfer rate without changing the recording rate.

In ZCAV reproducing, the disk rotation speed is constant, and the linear velocity is different for each zone. The optimum laser power for readout becomes higher in proportion to the linear velocity. So, the read laser power is calibrated by a factor defined according to the reproducing zone. As a result, a channel bit rate of 31.1Mbit/sec is achieved in the outer area of the disk.

The mobile storage device can copy image data from flash memory cards to iD PHOTO disk. It may be powered with four AA sized batteries, allowing for simple data backup without use of a computer at anytime and anywhere. This product is the second application of the iD PHOTO system, following the digital disk camera called the iDshot.

The mobile storage device is equipped with a dual interface IEEE1394 and USB making it user-friendly as an external drive to copy computer data. For that purpose it provides a multi-drive function through having all three types of slots — the iD PHOTO disk, compact flash, and smart media — built into the computer. We expect that using the Magnetic Amplifying Magneto-Optical System (MAMMOS), the storage capacity will be doubled. It also will be possible to increase memory capacity more than six times higher in the future by employing a blue laser. Several applications using the iD PHOTO disk will be developed.

Other contributors to this article include T. Hamaguchi, T. Hisamitsu, M. Takuma and A. Yamada, Sanyo Electric Co., Ltd.

This article will be presented at ICCE in a paper titled "A Mobile Storage Device with Magneto-Optical Disk of a 50mm Diameter."