Abstract:
Methods and systems for improving the storage capacity and data throughput of a digital mass storage device. The novel optimizations can be applied to a disk drive, either a hard disk drive or a disk drive having removable media, such as magnetic and optical disk drive technologies. The present invention provides at least two disk drive heads for reading and writing information from two different spinning media surfaces, e.g., platters or disks. If the disk is a read-only device, then the heads only perform the read function. A constant angular velocity drive mechanism is used meaning the rotational speed of the media is constant regardless of the head&#39;s position with respect to the media. In operation, during a media transfer, the first head accesses data by starting at the outside regions (“high track rates”) of the disk media and traversing inward towards the inner regions (“low track rates”). The second head, moving the opposite direction but in synchronization with the first head, starts accessing data in the inner regions of its disk media and traverses towards the outer regions. A data file is portioned and its portions are accessed using both heads combined to provide a uniform and high data throughput. The sectors are specially mapped and the data is interleaved to provide a maximum throughput. Capacity is increased because much, if not all, of the inner regions of the disk media are used in the present invention while still maintaining a high and uniform data throughput.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of mass storage technology. More specifically, the present invention relates to methods and systems for improving the storage capacity and throughput of a digital mass storage device, such as a disk drive. 
     2. Related Art 
     Digital information can be stored magnetically and optically on rotating storage media often called “disk media” or platters. The digital storage media can be removable, as in the case of a floppy disk or optical compact disk (CD) or the platters can be non-removable such as in a hard disk. The digital information maintained in this disk storage media is stored in annular shaped tracks that are positioned at different radii from the center of the disk media. Several arc-shaped sectors can reside within each track. The digital information is read from the disk media by a head disk assembly (HDA) which contains a head that is positioned over the tracks of the rotating disk media. Disk drives that are used to store and retrieve audio/video digital information are called AV drives. 
     Data can be recorded using a constant linear velocity (CLV) technique in which the data density recorded per linear inch of the disk media is constant but the media spins at different rates depending on the radius of the head. The second technique is the constant angular velocity (CAV) technique. In both techniques, the density (e.g., bits/inch) of the information stored within the tracks is constant. In CLV technique, as the head of the disk drive moves across the tracks of the disk media, the rotation speed of the disk media is varied by the disk drive to maintain a constant linear velocity of the head with respect to the recorded information. As the head moves toward the center, the rotation speed is increased because the circumferences of the inner tracks are smaller than the circumferences of the outer tracks. By increasing the rotation speed, the linear velocity, and therefore the data rate, is held constant in the CLV technique. Conversely, in the CAV technique, the rotation speed of the disk media is held constant regardless of the track position of the head. Therefore, in the CAV technique, the data rate of digital information accessed by the head decreases as the read head travels from the outer tracks of the disk media to the inner tracks. 
     The CLV reading technique is not particularly advantageous because each time the head is moved from one position to another, e.g., in response to a “seek” command, the rotation speed of the disk media needs to be adjusted for the new position. During a “seek” command, the head of the disk drive is instructed to move from its current position to a new position in order to access digital information from the disk media. In the CLV technique, the rotation speed of the disk media must be adjusted based on the new track position of the head. While the head can be rapidly positioned from its current position to the new position, it takes longer for the spindle motor of the disk drive to reach the target rotation speed for the new head position. Therefore, reading/writing random accessed information from and to a disk media using the CLV technique requires many adjustments to the rotation speed of the disk media and this may severely reduce data throughput. 
     As result, the CLV technique is disadvantageous for recorded computer information because this information may require many seek commands during retrieval; for each seek, the access time is increased due to the additional time required for the spindle motor to reach its target rotation speed. Moreover, because the rotation speed of the disk media varies based on the head position, the disk drive using the CLV technique requires an advanced speed controllable spindle motor with costly associated control circuitry. Lastly, because the hard drive using the CLV technique is constantly accelerating its spindle motor, torque is required using the CLV technique, which increases the overall power consumption of the disk drive. This makes the CLV technique disadvantageous for battery operated (e.g., portable laptop) computer systems that have limited power stores and are extremely heat sensitive. 
     However, current CAV techniques also have disadvantages. Because the CAV disk drives operate at a constant angular velocity, the data throughput rate changes depending on the region of the disk media at which the head is positioned. The data throughput rate at the inner tracks of the disk media is significantly lower than the data throughput rate achievable at the outer tracks of the platter. The data throughput rate difference is typically large, with the inner tracks&#39; data rate frequency representing only half of the outer tracks&#39; data rate. As a result, CAV disk drives often operate at a reduced capacity in order to meet certain data throughput rate requirements of some audio/video applications that require high sustained data throughput for normal operation. 
     For instance, if a particular application demands a high data throughput that cannot be met or maintained using the inner track regions of the disk media, then the disk drive maker will not allow data storage on the inner track regions. Although the data throughput rate is increased, this limitation will inherently reduce the storage capacity of the disk drive because a large potion of the available disk media will not be used by the disk drive. In effect, hardware manufacturers normally forgo the use of the inner surfaces of the media, thereby reducing the potential capacity of the drives, in an effort to guarantee acceptably high data throughput of their drivers. 
     SUMMARY OF THE INVENTION 
     Accordingly, what is needed is a disk drive having a high and maintained data throughput rate that also makes use of the potential storage capacity of the disk media in that the inner track regions as well as the outer track regions are used to store data thereon. That is needed further is a disk drive having improved storage capacity and high uniform data throughput rate that also uses constant angular velocity (CAV) techniques. What is also needed is a disk drive having the above characteristics that is used for the storage and retrieval of audio/visual digital information. What is yet needed is a disk drive having the above characteristics that can be implemented within a computer system. As described herein, the present invention offers these advantages and others not recited above but made clear within discussions of the present invention to follow. 
     Methods and systems are described herein for improving the storage capacity and data throughput of a digital mass storage device, such as a disk drive. The novel optimizations can be applied to a disk drive, either a hard disk drive or a disk drive having removable media, such as magnetic and optical disk drive technologies. The present invention provides at least two disk drive heads for reading and writing information from two different spinning disk media surfaces, e.g., platters or disks. If the disk drive is a read-only device, then the heads only perform the read function. The drive mechanism is a constant angular velocity drive meaning the rotational speed of the disk media is constant regardless of the head&#39;s position with respect to the disk media. In operation, during a media transfer, the first head accesses data by starting at the outside regions (“high track rates”) of the circular (“disk”) media and traversing inward towards the inner regions (“low track rates”). The second head, traversing in the opposite direction, but in synchronization with the first head, starts accessing data at the inner regions of its disk media and traverses towards the outer regions. 
     Although the data throughput rate of each head varies as the head moves across the tracks, the combined data throughput rate of the two heads remains substantially constant in accordance with the present invention. A data file is portioned and its portions are accessed using both heads combined to provide a uniform and high data throughput rate. The sectors are specially mapped and the data is interleaved to provide a maximum throughput rate. Storage capacity is increased because much, if not all, of the inner regions of the disk media are used in the present invention while still maintaining a high and uniform data throughput rate. 
     More specifically, an embodiment of the present invention includes a digital storage device comprising: a first head for writing digital data on tracks of a first disk media, the first head moving across the tracks of the first disk media in a first direction; a second head for writing digital data on tracks of a second disk media, the second head moving across the tracks of the second disk media in a second direction opposite to the first direction; and a multiplexer circuit for receiving a digital data stream and for supplying first portions of the digital data stream to the first head and second portions of the digital data stream to the second head, wherein respective size ratios of respective first and second portions are based on corresponding track position ratios of the first and second heads. Embodiments include the above and wherein the first head and the second head write digital data with a substantially constant combined data throughput rate as a result of the respective size ratios and wherein the first direction the first head is from outer tracks to inner tracks of the first disk media and wherein the second direction of the second head is from inner tracks to outer tracks of the second disk media. Embodiments include the above and further comprising a mechanism for spinning the first and second disk media at a constant angular velocity. 
     Embodiments of the present invention also include a digital storage device comprising: a first set of heads coupled to a first actuator and for reading digital data off of tracks of a first disk media, wherein the first set of heads moves in a direction from outer tracks to inner tracks of the first disk media; a second set of heads coupled to a second actuator and for reading digital data off of tracks of a second disk media, wherein the second set of heads moves from inner tracks to outer tracks of the second disk media in opposite direction to the first set of heads; and a demultiplexer circuit for receiving the digital data from the first set of heads and from the second set of heads and for supplying a single digital data stream therefrom wherein digital data is supplied from the first and second set of heads at a combined throughput rate that is substantially constant. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a number of tracks having different radii from the center of a disk media in accordance with the present invention. 
     FIG. 2A is a diagram of on e embodiment of the present invention having dual disk heads with opposite position with respect to two disk platters and each head moving in opposite direction but synchronized in motion. 
     FIG. 2B is a diagram of the embodiment of the present invention of FIG. 2A with the dual disk heads in reverse position with respect to their positions in FIG.  2 A. 
     FIG. 2C is a diagram of the embodiment of the present invention of FIG. 2A with the dual disk heads in middle position. 
     FIG. 2D is a graph illustrating exemplary data throughputs for the first head, the second head and their combination for an exemplary configuration in accordance with the present invention. 
     FIG. 3A is a side view of the embodiment of the present invention as shown in FIG.  2 A. 
     FIG. 3B is an embodiment of the present invention having dual sets of disk heads with each head set moving in opposite direction but synchronized in motion. 
     FIG.  4 A and FIG. 4B illustrate one track numbering scheme used in an embodiment of the present invention for the tracks on a first and a second disk media. 
     FIG.  5 A and FIG. 5B illustrate a track interleaved numbering scheme used in an embodiment of the present invention for the tracks on a first and a second disk media. 
     FIG. 6A illustrates steps used in one embodiment of the present invention for writing a digital data stream to the dual head disk mechanism of the present invention. 
     FIG. 6B illustrates steps used in one embodiment of the present invention for reading a digital data stream from the dual head disk mechanism of the present invention. 
     FIG. 7 illustrates a block diagram of components of the dual head drive mechanism of the present invention for a disk drive device. 
     FIG. 8 is a block diagram of a computer system employing the dual head mechanism of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description of the present invention, a dual head disk accessing method and device for uniform high data throughput and increased storage capacity, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be recognized by one skilled in the art that the present invention may be practiced without these specific details or with equivalents thereof. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present invention. 
     Dual Head Storage Device of the Present Invention 
     FIG. 1 illustrates a top view of a disk media  10  used in accordance with the present invention. The disk media  10 , either magnetic or optical, is annular in shape and contains circular tracks, e.g.,  16  and  14 . The magnetic disk can be non-removable (e.g., a platter of a hard disk) or it can be removable (e.g., a floppy or cartridge). The disk media  10  is spun at a constant angular velocity and the bit density (e.g., bits per linear inch) stored on the surface of the disk media  10  is generally constant. In the disk mechanism, a read/write head  20  is positioned over the tracks for data access (read/write) operations. The head  20  is moved by an arm  22  which is coupled to an actuator  24 . Given the above environment, data is accessed at higher rates at the outer tracks  14  when compared to the data access rate of the inner tracks  16 . This is because the circumference of the outer tracks  14  is larger than the circumference of the inner tracks  16 . Each disk rotation takes the same amount of time in a constant angular velocity drive, therefore, per unit time, more data can be accessed from the outer tracks  14  than from the inner tracks  16 . More data can be accessed because, during data access operations, the head  20  passes over more media surface per unit time for the outer tracks  14  than for the inner tracks  16 . 
     FIG. 2A illustrates one configuration  210  of the present invention dual head access mechanism. This embodiment utilizes two read/write heads  240  and  250  which interface with two respective disk media surfaces  220  and  230 . Therefore, each read/write head (“head”)  240  and  250  accesses different magnetic platter surfaces in the drive. The first head  240  accesses data in a conventional manner by starting at the outside track regions  202   a  of its platter surface  220  and traversing across the tracks toward the inner track regions  204   a . Arm  242  and actuator  244  are used to position the first head  240 . Head movement for the first head  240 , during data access operations, is therefore from the outer tracks toward the inner tracks. Because the platters  220  and  230  rotate at a constant angular velocity, the outside track regions  202   a  are called high data rate tracks while the inside track regions  204   a  are called low data rate tracks, as described with respect to FIG.  1 . 
     The second head  250  of FIG. 2A accesses data in the reverse direction, going in the opposite direction as the first head  240 , by starting out at the inner track regions  204   b  of the platter surface  230  and traversing toward the external or high rate track regions  202   b . Head movement for the second head  250 , during data access operations, is therefore from the inner tracks toward the outer tracks. Again, because the platters  220  and  230  rotate at a constant angular velocity, the outside track regions  202   b  are called high data rate tracks while the inside track regions  204   b  are called low data rate tracks, as described with respect to FIG.  1 . Arm  252  and actuator  254  are used to position the second head  250 . The mapping of logical sectors on the media  220  and  230  is done such that it allows for the access operation to the media to be done simultaneously by the two heads  240  and  250 , while the throughput data rate are combined by suitably interleaving the data on the two media surfaces. 
     As shown in FIG. 2A, the first head  240  and the second head  250  are shown in their initial positions. In these positions, the first head  240  is accessing data at a high data rate while the second head  250  is accessing data at a low data rate. It is appreciated that the first and second heads move in opposite directions with respect to each other and with respect to the their associated tracks, but the first and second heads move in synchronization with each other. Therefore, as one head moves, the other head moves in synchronization but in the reverse direction across its associated tracks. In the present invention, the data accessed from the first and second heads are combined together to provide a substantially constant and high data throughput, regardless of the heads&#39; positions. 
     As the first head  240  traverses to relatively lower data rate tracks, the second head  250  simultaneously traverses to relatively higher data tracks, with the combination data rate of the two data rates remaining substantially constant and high. In this fashion, most, if not all, of the inner track regions  204   a - 204   b  can effectively be used for storing data thereon and the disk drive still maintains a high data throughput rate to support high data throughput rate applications, such as audio/video applications. Therefore, the mixing of high and low data throughput rates internal to the drive of the present invention results in a stable average throughput rate that is high enough to satisfy audio/video (AV) application requirements, while allowing for the maximum utilization of available magnetic media for data storage because the inner track regions  204   a - 204   b  are used to store data thereon in accordance with the present invention. 
     It is appreciated that while FIG. 2A illustrates the first and second heads accessing different platters, the present invention is well suited for implementations where the first and second head access different surfaces of the same disk platter. In this case, the first and second disk media represent a first and a second side of the same platter. 
     FIG. 2B illustrates the configuration  210  of the present invention with the first head  240  and the second head  250  in the reverse positions to those shown in FIG.  2 A. As shown in FIG. 2B, the first head  240  and the second head  250  are shown in their final positions. In these positions, the first head  240  is accessing data at a low data rate at the inner track regions  204   a  while the second head  250  is accessing data at a high data rate at the outer track regions  202   b . In the present invention, the data accessed from the first and second heads are combined together to provide a substantially constant and high data throughput, regardless of the heads&#39; positions. As the first head  240  traverses to relatively lower data rate tracks, the second head  250  simultaneously traverses to relatively higher data tracks, with the combination data rate of the two data rates remaining substantially constant and high. 
     FIG. 2C illustrates the configuration  210  of the present invention with the first head  240  and the second head  250  in middle positions to those shown in FIG.  2 A. As shown in FIG. 2C, the first head  240  and the second head  250  are shown in roughly middle positions. In these positions, the first head  240  is accessing data at a high to medium data rate at its track regions while the second head  250  is accessing data at a low to medium data rate at its track regions. In the present invention, the data accessed from the first and second heads are combined together to provide a substantially constant and high data throughput, regardless of the heads&#39; positions. As the first head  240  traverses to relatively lower data rate tracks, the second head  250  simultaneously traverses to relatively higher data tracks, with the combination data rate of the two data rates remaining substantially constant and high. 
     Table I below illustrates the data throughput rates for each head and the output combination data throughput in accordance with the present invention for the example configuration of FIG. 2A, FIG.  2 B and FIG.  2 C. 
     
       
         
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                 First Head 240 
                 Second Head 250 
                 Combined 
               
               
                 Data Throughput 
                 Data Throughput 
                 Data Throughput 
               
               
                   
               
             
             
               
                 4x 
                 2x 
                 6x 
               
               
                 2x 
                 4x 
                 6x 
               
               
                      3.2x 
                      2.8x 
                 6x 
               
               
                   
               
             
          
         
       
     
     FIG. 2D is a graph illustrating exemplary data throughputs for the first head  240 , the second head  250  and the combination for an exemplary configuration in accordance with the dual head design of the present invention. Exemplary data rate values are shown along the vertical axis  350 . Head position for the first head  240  is shown along horizontal  370  while head position for the second head  250  is shown along the horizontal  360 . As shown, the first head moves in opposite direction with respect to the second head but in synchronization with the second head. Exemplary positions of 7.0 to 10 are shown as example track radii only. The data throughput rate corresponding to the first head  240  is the graph shown as curve  320  which is exponential and varies as the square of the head&#39;s radius (e.g., track position or track radius). The data throughput rate corresponding to the second head  250  is the graph shown as curve  330  which is exponential and varies as the square of the head radius (e.g., track position). 
     As shown in FIG. 2D, curve  320  starts high (at the left) and as the first head  240  traverses toward the inner track regions, curve  320  drops in data throughput rate. Correspondingly, curve  330  starts low (at the left) and as the second head  250  traverses toward the outer track regions, curve  330  gains in data throughput rate. The curves  320  and  330  are not linear, but they are very close to linear and therefore their sum is roughly a sustained and high data throughput, as shown by curve  390 . Curve  390  represents the total data throughput rate of the dual head disk embodiment of the present invention. In average, curve  390  represents a substantially constant and high data throughput rate suitable for supporting a number of well known high data throughput applications, such as audio/video (AV) applications. 
     FIG. 3A is a side view of the dual head embodiment of the present invention f or a magnetic hard drive implementation. A spindle motor  264  is mechanically coupled to an axis  260  which secures recording platters  220  and  230 . In this embodiments, platters  220  and  230  are defined as first and second disk media. Spindle motor  254  rotates platters  220  and  230  at a constant angular velocity. Head  240  is shown coupled to arm  242  which is coupled to actuator  244 . Head  250  is shown coupled to arm  252  which is coupled to actuator  254 . Head  240  interfaces with the top surface of platter  220  while head  250  interfaces with the top surface of platter  230 . Actuator  254  is separate from actuator  244 . Any of a number of well known systems can be used as arms and actuators, and control logic for same, for moving heads  240  and  250 . In one embodiment, actuator  244  is coupled to a well known magnetic voice coil system for controlled movement. It is appreciated that in an alternative embodiment of the present invention, head  250  can interface with the bottom surface of platter  220 . In this alternative embodiment, the top and bottom surfaces of the same platter are defined as first and second disk media, as described above. 
     FIG. 3B illustrates a dual head set embodiment  310  of the present invention. In this embodiment, sets of heads are controlled uniformly to increase data throughput. In the implementation shown in FIG. 3B, each head set contains two heads, however, this is an exemplary configuration only and a head set can include any number of heads (e.g., more than two) within the scope of the present invention. In this embodiment  310 , a first head set contains heads  240   a  and  240   b . These heads  240   a - 240   b  are coupled, respectively, to arms  242   a  and  242   b  which are both coupled to a common actuator  244 ′. In this embodiment  310 , a second head set contains heads  250   a  and  250   b . These heads  250   a - 250   b  are coupled, respectively, to arms  252   a  and  252   b  which are both coupled to a common actuator  254 ′. Common actuators  244 ′ and  254 ′ can be controlled using a number of well known techniques. 
     Heads  240   a  and  250   a  move, respectively, in the same manner as described with respect to heads  240  and  250 . All other heads in the first head set track the movement of head  240   a . All other heads in the second head set track the movement of head  250   a . In the configuration of FIG. 3B, the heads of the first head set interface with the top surface of platters  220  and  230  and the heads of the second head set interface with the bottom surface of platters  220  and  230 . In an alternative configuration, the heads of the first and second set could be interleaved with respect to the surfaces of the platters. For instance, in this alternative embodiment, heads  240   a  and  240   b  would interface with the top and bottom surfaces, respectively, of platter  220  while heads  250   a  and  250   b  would interface with the top and bottom surfaces, respectively, of platter  230 . 
     FIG.  4 A and FIG. 4B illustrate exemplary track assignments for the dual head embodiment  210  and the dual head set embodiment  310  of the present invention. In these exemplary track assignments, the top surface of platter  220  is accessed by the first head and therefore its tracks are numbered from 1 to N starting from the outer track regions  202   a  and ending in the inner track regions  204   a  as shown in FIG.  4 A. The tracks are numbered consecutively in these examples. Multiple sectors can reside within a track. The sectors are numbered in the order of and according to their track number with outer track sectors numbered lower and inner track sectors numbered higher. In one implementation, a sector contains a predefined number of bytes each, e.g., 512 bytes, for instance. 
     With respect to FIG. 4B, the top surface of platter  230  is accessed by the second head and therefore its tracks are numbered from N to 1 starting from the outer track regions  202   a  (“N”) and ending in the inner track regions  204   a  (“1”) as shown in FIG.  4 B. Multiple sectors can reside within a track. The sectors are numbered in the order of and according to their track number with outer track sectors numbered higher and inner track sectors numbered lower. In one implementation, a sector contains a predefined number of bytes each, e.g., 512 bytes, for instance. In operation, data is interleaved on the tracks between the disk medias  220  and  230 . 
     FIG.  5 A and FIG. 5B illustrate that within the present invention, track assignments for the disk media can be ordered differently based on the data rates of the track positions and interleaved across the first and second disk media. Therefore, FIG.  5 A and FIG. 5B illustrate track number interleaving. As shown, in this example, tracks  1  through track x are located at the inner region  316  of the second disk media  230 . When the second head  250  is within this region  316 , the first head  240  is within the outer region  302  of the first disk media  220  and therefore these tracks are numbered (x+1) to y. Moreover, tracks (y+1) through track z are located at the middle region  314  of the second disk media  230 . When the second head  250  is within this region  314 , the first head  240  is within the middle track region  304  of the first disk media  220  and therefore these tracks are numbered track (z+1) to track a. Lastly, tracks (a+1) through track b are located at the outer region  312  of the second disk media  230 . When the second head  250  is within this region  312 , the first head  240  is within the inner track region  306  of the first disk media  220  and therefore these tracks are numbered track (b+1) to track c. 
     It is appreciated that the three track groupings shown per disk media are exemplary only and that more or fewer track groupings can be used per disk media. Also, the actual number of tracks per grouping can vary depending on the average radius of the tracks of the grouping. 
     It is also appreciated that the within the embodiments of the present invention, whether or not the track numbers are ordered consecutively or interleaved, as described above, digital data is still accessed in an interleaved fashion between the first and second heads (and as between the first and second head sets). Because the data rates between the first and second heads are not always constant, the amount of data interleaved between the first and second heads varies depending on the expected track position of the heads when the data is to be written. For instance, when the first head is located on the outer track regions and the second head is therefore located on the inner track regions, the first head can have a data rate that is twice the second head. In this case, the present invention will supply twice the amount of data to the first head as to the second head, per unit time. In this fashion, the maximum combined data rate can be maintained and no head is waiting for the other head to complete. 
     Alternatively, when the first head is located on the inner track regions and the second head is therefore located on the outer track regions, the first head can have a data rate that is half the second head. In this case, the present invention will supply twice the amount of data to the second head as to the first head, per unit time. In this fashion, the maximum combined data rate is again maintained and no head is waiting for the other head to complete. When the first head is located on the middle track regions and the second head is therefore located on the middle track regions, the first head can have a data rate that is roughly equal to the second head. In this case, the present invention will supply equal amounts of data to the second head and to the first head per unit time. 
     As shown by the above examples, the data rate of a particular head, and therefore the amount of data supplied to that head per unit time, is based on the expected track position of the head during data access operations. The present invention therefore routes data to the heads (during writing operations) and receives data from the heads (during reading operations) in amounts that depend on their respective track positions during the data accessing operations. Because the track position of the first head is dependent on the track position of the second head, and vice-versa, the present invention can effectively deal with head track position ratios between the first and second heads. These track position ratios can be translated into data rate ratios which can then be used to determine the data throughput amounts for each head depending on their corresponding track position ratios. As described further below, the correspondence between track position ratios and data rate ratios can be maintained by a look-up table stored in a memory unit. 
     FIG. 6A illustrates steps used in a process  400  of the present invention for performing write operations using the dual head disk drive embodiments of the present invention. At step  410 , a request is received to store a digital data stream that corresponds to a particular file. In one embodiment, the digital data stream represents AV content to be stored on disk media. At step  420 , the present invention determines which tracks are available for storage of the digital data stream. A number of well known techniques can be used to determine which tracks of a disk media are available for receiving new digital information. The available tracks that are determined at step  420  reside on the first  220  and second  230  disk media. 
     It is appreciated that once the available tracks are determined at step  420 , the present invention then determines the data rate of the first head and the data rate of the second head for those determined tracks. This information can readily be determined because the track number indicates the radius position along the disk media and the radius position can be translated into a data rate for the corresponding head. Step  420  then returns a data rate ratio between the data throughput rate of the first head  240  over the data throughput rate of the second head  250 . In one embodiment, a look-up table is stored in memory and is referenced to produce the data rate ratio. The look-up table corresponds respective track position ratios (e.g., the ratio of the track position for the first head over the track position for the second head) with their corresponding data rate ratios. The data for the look-up table can be determined empirically by performing disk calibration at the factory, or it can be dynamically determined and updated as the disk drive is being used. The important feature of the memory is that it will produce a data throughput value for any head track position or range of positions. These values can then be translated into data rate ratios. 
     At step  430  of FIG. 6A, the present invention then determines sizes of data portions to forward to the first head and to forward to the second head. At step  430 , the present invention utilizes the data throughput ratios obtained in step  420  to determine the proper data portion sizes for a given unit of time. The ratio of the sizes of the data portions corresponds to the data rate ratio obtained from step  420 . The data portion forwarded to the first head  240  is I bytes in size and the data portion forwarded to the second head  250  is J bytes in size. Therefore, I/J represents the data ratio determined at step  420 . Using these data portion sizes and the corresponding track positions of the first and second heads, a constant and high data throughput rate can be maintained (as shown in FIG. 2D) by the disk drive of the present invention when writing the I+J bytes. 
     At step  440  of FIG. 6A, the next I bytes of the digital data stream are forwarded to the first head  240  for storage thereby onto the first disk media  220 . Simultaneously with the forwarding of the I bytes, the next J bytes of the digital data stream are forwarded to the second head  250  for storage thereby onto the second disk media  230  at the determined tracks. It is appreciated that the digital data stream can be buffered so that simultaneous forwarding of data to the first and second heads can be readily accomplished. At step  440 , the digital data can be directly supplied to the first and second heads or can optionally be buffered into queues, one queue for each head. The allocated data is then stored at the tracks determined at step  420  when the data reaches the output point of the queue. 
     At step  450 , the present checks if more of the digital data stream remains to be stored. If so, then at step  460  a next portion of the digital data stream is to be processed and step  420  is entered again. At step  450 , if the entire digital data stream has been processed, then the write process  400  terminates. 
     FIG. 6B illustrates steps of process  500  for retrieving a stored digital data stream from the dual head disk drive embodiment of the present invention. At step  510 , the present invention receives a request to obtain data stored on the disk media. At step  510 , the present invention is informed of the tracks on which the data is stored. The tracks correspond to tracks on the first  220  and second  230  disk media. At step  520 , the present invention instructs the disk servo mechanisms for the first head to obtain I bytes from the given tracks of the first disk media. The present invention also simultaneously instructs the disk servo mechanisms for the second head to obtain J bytes from the given tracks of the second disk media. Using these data portion sizes and the corresponding track positions of the first and second heads, a constant and high data throughput rate can be maintained (as shown in FIG. 2D) by the disk drive of the present invention for obtaining the I+J bytes of the digital data stream. 
     At step  530  of FIG. 6B, the present invention combines the I and J bytes to produce a segment of the digital data stream requested. At step  540 , the combined digital data stream is supplied as an output for use by another electronic device, e.g., by a host computer system. The output can optionally be buffered. At step  550 , a check is made if the entire data stream was processed yet. If not, then at step  560  the present invention obtains track information for the next stored portion of the digital data stream and step  520  is entered again. At step  550 , if the entire digital data stream was retrieved already, then process  500  terminates. 
     FIG. 7 illustrates a block diagram of components of a dual head disk drive  104  implemented in accordance with the present invention. Disk drive  104  contains a buffer  610  for receiving an input digital data stream to be stored onto disk media and for supplying an output digital data stream that was retrieved from disk media. The buffer  610  is coupled to supply data to a multiplexer/demultiplexer (mux/demux) circuit  620  and to receive data from the mux/demux circuit  620 . The mux/demux circuit  620  is responsible for routing data portions of the digital data stream to the first head  240  and to the second head  250  during write operations to maintain a substantially uniform and high data throughput rate. The mux/demux circuit  620  is also responsible for combining data portions retrieved from the first head  240  and from the second head  250  to form a segment of the digital data stream during read operations to maintain a substantially uniform and high data throughput rate. 
     The mux/demux circuit  620  is coupled to supply data to and receive data from a first head buffer (“first buffer”)  650   a . The mux/demux circuit  620  is also coupled to supply data to and receive data from a second head buffer (“second buffer”)  650   b . The first buffer  650   a  supplies data to the first head  240  during write operations and also receives data from the first head  240  during read operations. The second buffer  650   b  supplies data to the second head  250  during write operations and also receives data from the second head  250  during read operations. A first servo mechanism  670   a  is coupled to the actuator  244  of the first head  240  and functions to position the first head  240  during read and write operations. A second servo mechanism  670   b  is coupled to the actuator  254  of the second head  250  and functions to position the second head  250  during read and write operations. Any of a number of well known devices can be used as the servo mechanisms  670   a  and  670   b . Head position control logic  660  is coupled to control the servo mechanisms  670   a  and  670   b  and is responsive to track numbers received over line  655 . 
     Each servo mechanism  670   a  and  670   b  of FIG. 7 supplies the current head position back to a data rate controller circuit  630  for use in read/write operations and for calibration of a look-up table information. Data rate controller circuit  630  determines available tracks of the disk media for receiving digital data stream information during a write operation and also controls the control logic  660  when retrieving stored information from the disk media. Data rate controller circuit  630  also contains a memory unit  640  which has a look-up table (LUT) stored thereon. The LUT correlates track position ratios with data rate ratios as described with respect to FIG.  6 A and FIG.  6 B. Data rate ratio information is used to control the mux/demux circuit  620  using control line  665 . 
     During a write operation, data rate controller  630  determines the available tracks for storage of new data of buffer  610 . This track position information (of the first and second heads) is translated into a track position ratio which is then input as an index to the memory stored LUT  640 . The LUT  640  then supplies a corresponding data rate ratio over line  665  which instructs the mux/demux circuit to route I bytes of the input data stream to buffer  650   a  and J bytes of the input data stream to buffer  650   b . By using the LUT  640  to determine values of J and I based on the track position, the present invention is able to sustain a maximum throughput data rate regardless of the track position ratio of the first and second heads. The actual track numbers for storing the I and J bytes are simultaneously forwarded to the control logic  660  (“firmware”) via line  655 . The control logic  660  controls servo mechanisms  670   a - 670   b  which properly position heads  240  and  250 , respectively, to store the bytes with a substantially uniform and high data throughput rate. This process is repeated for each segment of the input data stream as described with respect to FIG.  6 A. 
     The information in the LUT  640  of FIG. 7 can be obtained empirically through device calibration at the factory during the manufacturing process. It can also be obtained dynamically by the drive  104  and re-calibrated periodically. By providing feedback of the heads&#39; track positions from the servo mechanisms  670   a - 670   b  to the data rate controller  630 , the drive  104  can scan across all tracks and record the corresponding data rate throughput at each track. From this information, the track position ratios and their corresponding data rate ratios can be determined and the contents of the LUT  640  can be updated periodically with this information. In an alternative embodiment, the LUT  640  can reference track ranges to data rate ratios where the input index is a track number and the output is a data rate ratio. 
     During read operations, the data rate controller  630  contains the tracks on which the desired data resides. The data rate controller  630  supplies this track information to the control logic  660  which instructs the servo mechanisms to position the first and second heads to obtain the data. The retrieved data is stored in buffers  650   a  and  650   b . The data rate controller  630  also supplies data rate ratio information to mux/demux circuit  620  which uses this information to obtain I bytes from buffer  650   a  and J bytes from buffer  650   b  during the read operation. By using the LUT  640  to determine values of J and I based on the track position, the present invention is able to sustain a maximum throughput data rate regardless of the track position ratio of the first and second heads. The mux/demux circuit  620  then combines the received data and outputs a segment of the digital data stream to buffer  610 . This process is repeated for each segment of the input data stream as described with respect to FIG.  6 B. 
     It is appreciated that buffers  650   a  and  650   b  are optional and function to group the associated data and perform data distribution so that precise synchronization between the head positions and data availability is not always required. With the buffers, data streaming can generally continue uninterrupted even in the face of bad tracks that require remapping operations, etc. It is also appreciated that the average maximum throughput data rate is generally set to be slightly higher than required by the application program requesting the data or supplying the data. In cases when the average maximum throughput is faster, the additional data is buffered in buffer  610  and the drive may be forced to wait for the application program to catch up with the data supply. In cases when the data rate decreases temporarily, e.g., as a result of bad sector remapping, etc., the buffer contents are then used to ensure that an uninterrupted supply of data is available. The high sustained data throughput and buffering features make the present invention particularly useful for supplying AV data to AV application programs. 
     Computer System  112  of the Present Invention 
     The novel dual head disk drive device of the present invention can be implemented within a computer system  112  as shown in FIG.  8 . The computer system  112  acts as a platform for application programs which supply data to the disk drive  104  and/or request data from the disk drive  104 . The computer system  112  can be integrated within a portable electronic device or system, e.g., a personal digital assistant, a portable computer system (e.g., a laptop, a palm sized device), or a portable consumer based electronic device. Although a variety of different computer systems can be used with the present invention, an exemplary general purpose computer system  112  is shown in FIG. 8 having the dual head disk drive of the present invention as storage device  104 . 
     In general, computer system  112  of FIG. 8 includes an address/data bus  100  for communicating information, a central processor  101  coupled with the bus for processing information and instructions, a volatile memory  102  (e.g., random access memory RAM) coupled with the bus  100  for storing information and instructions for the central processor  101  and a non-volatile memory  103  (e.g., read only memory ROM) coupled with the bus  100  for storing static information and instructions for the processor  101 . Computer system  1   12  also includes the data storage device  104  (“dual head disk subsystem”) coupled with the bus  100  for storing information and instructions and a display device  105  coupled to the bus  100  for displaying information to the computer user. System  112  can also be referred to as an embedded system. 
     Also included in computer system  112  of FIG. 8 is an optional alphanumeric input device  106  including alphanumeric and function keys coupled to the bus  100  for communicating information and command selections to the central processor  101 . System  112  also includes an optional cursor control or directing device  107  coupled to the bus for communicating user input information and command selections to the central processor  101 . The cursor directing device  107  can be implemented using a number of well known devices such as a mouse, a track ball, a track pad, an electronic pad and stylus, an optical tracking device, a touch screen etc. Computer system  112  can also include an optional signal generating device  108  coupled to the bus  100  for interfacing with other networked computer systems. The display device  105  utilized with the computer system  112  is optional and may be a liquid crystal device, cathode ray tube (CRT), light emitting diode (LED), field emission device (FED, also called flat panel CRT) or other display device suitable for creating graphic images and alphanumeric characters recognizable to the user. 
     The preferred embodiment of the present invention, a dual head disk accessing method and device for uniform high data throughput and increased storage capacity, is thus described. While the present invention has been described in particular embodiments, it should be appreciated that the present invention should not be construed as limited by such embodiments, but rather construed according to the below claims.