1. Field of the Invention
This invention relates to the field of magnetic storage read/write apparatus employing magnetic disks and in particular to method and apparatus for recording information on such disks at a constant density.
2. Background Art
Magnetic disk storage media and apparatus are often utilized in conjunction with computer systems as a means for providing a large amount of accessible and manipulable storage area for the computer. The computer industry has adopted a number of standard sizes for magnetic media storage disks to be used in such systems. For example, 51/4" hard disks are typically employed in Winchester type hard disk drives. It is often desired to provide additional storage capabilities for disk drive systems without increasing the number of disks required or the size of the disks to be used.
In a typical disk storage system, the disk is rotated at a constant frequency. A read/write head flies over the disk surface creating and/or reading magnetic flux reversals in the surface of the rotating disk. Data is stored in a series of concentric tracks on the disk, with the tracks being of increasing diameter away from the center of the disk. If each magnetic flux reversal is considered to be a "bit", then the bit density at an outer data track on the disk will be less than the bit density on an inner track. If the bit density at the inner track is the maximum bit density which may be maintained on the disk, each track away from this inner track will have a bit density below the maximum achievable density. Therefore, if each track could have the same bit density, and that bit density was the maximum achievable, the maximum storage capability of the disk could be achieved. The concept of providing equal bit density in each track of a disk is known as constant density recording.
One prior art method for implementing constant density recording is by means of spindle motor speed variations in direct proportion to head radius, head radius corresponding to the radius of the data track currently being accessed. This method has the disadvantage of the necessity to rapidly alter the speed of the spindle and consequently rotation of the disk, which is incompatible with fast access times.
A second method of constant density recording requires coupling of a potentiometer to the access mechanism of a disk drive in order to define a control voltage proportional to head radius. This voltage is then used to control the recording electronics. However, the attachment of the potentiometer to the access mechanism is undesirable from a cost, size, and reliability stand point.
A third prior art method of constant density recording is the defintion of a plurality of constant density "zones" with a number of redundant recording electronic circuits such that one set of electronics is provided for each recording zone. This method suffers the disadvantage of cost, reliability and size. Further, in addition to the duplicate and redundant channels, switching means must be provided for switching between the channels.
Therefore, it is an object of the present invention to provide a method and apparatus for single channel constant density recording which may be reliably implemented without increasing the size and cost a disk drive system. It is another object of the present invention to provide a method and apparatus for providing constant density recording in which net capacity is independent of record length.
It is yet another object of the present invention to provide method and apparatus for providing constant density recording which results in increased storage capacity over fixed frequency methods.