HIGH DATA RATE CD-ROM
Original Publication Date: 1999-Sep-01
Included in the Prior Art Database: 2003-Jun-18
Published is a system that provides high data rate and low access time for the writing and reading of a CD-ROM. This publication addresses the issue of how to design a CD-ROM optical drive with significantly higher performance and/or modify a WORM or erasable drive to accommodate CD disks. In doing so it provides high access performance to the data stored in a large and rapidly growing CD-ROM data base and reduces the need for a new lower-density standard that can be run at constant rpm, i.e. OD-ROM. Advantages are: 1)CD-ROM capacity, not OD-ROM capacity; 2) faster access times; 3) higher data rate and 4) compatibility within one drive of CD-ROM, WORM, AND Erasable media true multifunctionality. Alternatively, it can be used solely as a stand alone drive. The invented system consists of 1) a constant rpm motor, 2) a high performance actuator and 3) an asynchronous channel ffl 1". An optical head provides focus, track, data readback and writing capability in the case of writeable CD and/or WORM and Erasable. In a stand alone configuration, a high performance rotary actuator could be used since there are no constraints imposed by a cartridge and window orientation. Key to the invention is the incorporation of an asynchronous data channel that will accommodate the variable data rates experienced with a fixed-rpm motor and a CD disk. To assist the channels ability to rapidly lock onto data at the end of a seek, the target track location is used to generate an approximate data frequency by either table look up or calculation. This look-ahead procedure allows the asynchronous channel to sync with the target data faster. Alternatively, the channel frequency could be continuously updated by using a position signal derived from an encoder on the course actuator. Many systems today employ such an encoder typically a linear grating. In either case, the channel is set to a frequency that is close to the frequency of the desired track. The asynchronous channel would use this clock as its initial basis for synching to the data. The asynchronous channel is described separately ffl 1". It eliminates the need to perform filter changes and is compatible with many codes. It works by using a clock derived from the data as opposed to using a fixed clock. As the data rate changes, the frequency response of the channel is changed. No adjustment of the filters, such as required for analog channels by banded disks, is required. The entire circuitry could be placed on a 5mm x 5mm silicon chip so in large volume production, there is no reason that the cost would not be low.