Asynchronous Gain Adjustment for PRML Disk Drive Systems

1. Field of the Invention

The invention relates generally to a direct access storage device (DASD) of the type utilizing partial-response maximum-likelihood (PRML) detection, and more particularly to a method and apparatus for asynchronous gain adjustment for PRML data detection.

2. Description of the Prior Art

Computers often include auxiliary memory storage units having media on which data can be written and from which data can be read for later use. Disk drive units incorporating stacked, commonly rotated rigid magnetic disks are used for storage of data in magnetic form on the disk surfaces. Data is recorded in concentric, radially spaced data information tracks arrayed on the surfaces of the disks. Transducer heads driven in a path toward and away from the drive axis write data to the disks and read data from the disks.

In a PRML data channel, a normalized readback signal amplitude is required for proper data detection. A variable gain amplifier (VGA) typically is used in the analog signal path for scaling of the readback signal. Known PRML channels require an analog envelope detector circuit to sense the amplitude of the incoming readback signal in order to provide gain corrections to the VGA in the idle mode before the PRML channel begins a read or write operation. An acceptable tolerance on the signal amplitude must be maintained during the idle mode in order to guarantee fast convergence at the start of a read operation. Disadvantages of using an analog envelope detector circuit include the usually poor amplitude tolerance without on-chip adjustments because the circuit is analog. Also the analog envelope detector circuit is difficult to build for high speed signals without increasing the circuit power significantly. In addition, recovery from a gain being too low is typically very poor.

A principal object of the present invention is to provide improved methods for gain adjustment for PRML data detection in a disk drive data storage system. Other objects are to provide such improved gain adjustment methods substantially without negative effects, further that eliminates the need for an analog envelope detector circuit, and that overcome many of the disadvantages of prior art arrangements.

In brief, the objects and advantages of the present invention are achieved by apparatus and method for gain adjustment are provided for data detection in a partial-response maximum-likelihood (PRML) data channel. The PRML data channel includes an analog to digital converter (ADC) having a normal operating range and a variable gain amplifier (VGA) coupled to the ADC. A plurality of samples are detected from the ADC. Each of the detected samples are sequentially compared with predetermined threshold values. The predetermined threshold values include a zero value, and a minimum value and a maximum value of the normal operating range of the ADC. An absolute value of each of the detected samples are sequentially compared with a forth predetermined threshold value. A gain adjustment correction value is determined utilizing the sequentially compared values.

The present invention together with the above and other objects and advantages may best be understood from the following detailed description of the embodiment of the invention illustrated in the drawings, wherein:

FIG. 1 is a schematic and block diagram of a data storage disk file embodying the present invention;

FIG. 2 is a diagram showing the accessing mechanism for a single disk surface of the apparatus of FIG. 1;

FIG. 3 is a diagram illustrating apparatus a PRML data channel for carrying out gain adjustment methods according to the present invention in the data storage disk file of FIG. 1;

FIGS. 4 is a logic block flow diagram illustrating a gain adjustment method according to the present invention in the data storage disk file of FIG. 1; and

FIGS. 5 and 6 are graphs to illustrate the operation of asynchronous and synchronous sampling for the gain adjustment methods of the present invention.

In FIG. 1 there is shown a partly schematic block diagram of parts of a data storage disk file 10 including a data storage medium generally designated as 12 and an interface control unit generally designated as 14. In the preferred embodiment of this invention, the data storage medium 12 is embodied in a rigid magnetic disk drive unit 12, although other mechanically moving memory configurations may be used. Unit 12 is illustrated in simplified form sufficient for an understanding of the present invention because the utility of the present invention is not limited to the details of a particular drive unit construction.

Referring now to FIGS. 1 and 2 of the drawings, disk drive unit 12 includes a stack 16 of disks 18 having at least one magnetic surface 20. The disks 18 are mounted in parallel for simultaneous rotation on and by an integrated spindle and motor assembly 26. Data information on each disk 18 are read and/or written to by a corresponding transducer head 28 movable across the disk surface 20.

Transducer heads 28 are mounted on flexure springs 30 carried by arms 32 ganged together for simultaneous pivotal movement about a support spindle 34. One of the arms 32 includes an extension 36 driven in a pivotal motion by a head drive motor 38. Although several drive arrangements are commonly used, the motor 38 can include a voice coil motor 40 cooperating with a magnet and core assembly (not seen) operatively controlled for moving the transducer heads 28 in synchronism in a radial direction in order to position the heads in registration with data cylinders to be followed. The VCM is movable within a fixed magnetic field, and the direction and velocity of the coil movement is controlled by the current supplied. The various components of the disk file 10 are controlled in operation by signals generated by control unit 14 such as motor control signals on line 26A and position control signals on line 38A.

Referring now to FIG. 3, there is shown a block diagram of the PRML data channel 40 for carrying out gain adjustment methods of the invention. The PRML recording channel uses class IV partial-response (PR) signals accomplished by a PR-IV filter function. Data to be written is applied to an encoder 42 for providing a modulation coded output having predefined run length constraints, such as for the minimum and maximum number of consecutive zeros and the maximum run length of zeros in the even and odd recorded sequences in the overall recorded sequence. A precoder 44 follows the encoder 42 described by a 1/(1-D.sup.2) operation where D is a unit delay operator. A PRML precomp 46 coupled to the precoder 44 provides a modulated binary pulse signal applied to a write circuit 48 that provides the modulated write current for writing to the disk surface. An analog read signal is obtained at head and disk block 50 described by the (1-D.sup.2) operation. The read signal is applied to a variable gain amplifier (VGA) 52. The amplified read signal is applied to a lowpass filter 54. The filtered read signal is converted to digital form by an analog to digital converter (ADC) 56 that provides, for example, 64 possible 6-bit sampled values.

The samples of the ADC 56 are applied to a digital filter 58, such as a 10 tap finite impulse response (FIR) digital filter, and are applied to a gain and timing control 60. The gain and timing control 60 provides a gain and ac coupling pole control signal to the VGA 52 and provides a timing control signal to the ADC 56 via a voltage controlled oscillator 62. The filtered signal from the digital filter 58 is applied to a Viterbi decoder 64 coupled to a decoder 66 to complete the maximum-likelihood (ML) detection process for data read back.

In accordance with a feature of the present invention, gain adjustment is an integrated function of the partial-response maximum-likelihood (PRML) channel 40 available in the disk file 10.

Referring to FIG. 4, there is shown a flow chart illustrating the sequential operations of the asynchronous gain adjustment algorithm (AGAA).

The AGAA uses the ADC samples in the PRML channel. By asynchronously sampling the sync field pattern (1/4T), the AGAA guarantees a normalized amplitude of 1.05.+-.5% without requiring any trims or external components. Because the ADC samples are used, all tolerances in the ADC 56 are fully accommodated. On random data patterns, the AGAA is comparable in performance to the conventional analog envelope detector.

A current sample is read from the ADC 56 as indicated at a block 402 labelled X.sub.n =NEW ADC SAMPLE, where ADC range is between +4.0 to -4.0. During the idle mode, the analog-to-digital converter provides samples with the sampling being asynchronous to the data on the disk although the frequency of the sampling is roughly correct. In the channel IDLE mode, the ADC samples are asynchronous with respect the read-back signal, for example, where frequency is .+-.1% and the phase is indeterminate.

As indicated at a decision block 404 an absolute value of the sample is compared with a maximum threshold value. If .vertline.X.sub.n .vertline.=4.0, indicating that the samples saturate the ADC 56, then a saturation indication is set as Sat.sub.n =TRUE as indicated at a block 406 or else Sat.sub.n =FALSE as indicated at a block 408.

Next as indicated at a decision block 410 an absolute value of the sample is compared with a predetermined value to detect abnormally low sample values. If .vertline.X.sub.n .vertline.<1.0 then a null signal indication is set as Zero.sub.n =TRUE as indicated at a block 412 or else Zero.sub.n =FALSE as indicated at a block 414.

Then as indicated at a decision block 416, if X.sub.n >0 then the ideal sample value is set as Xn=+2.0 as indicated at a block 418. Else the ideal sample value is set as X.sub.n =-2.0 as indicated at a block 420. A value used for error correction is calculated as indicated at a block 422 labelled E.sub.n =(sign of X.sub.n).multidot.(X.sub.n -X.sub.n). Then the calculated value E.sub.n is threshold compared as indicated at a decision block 422 labelled E.sub.n >1.0. If E.sub.n >1.0, then E.sub.n =1.0 as indicated at a block 426.

Next a plurality of samples from Zero.sub.n through Zero.sub.n-Q, where Q is selected in a range between 10-30 are checked for too low gain. When samples Zero.sub.n through Zero.sub.n-Q are identified to be TRUE at a decision block 428, then a correction value is set C.sub.n =MAXIMUM POSITIVE as indicated at a block 430. Else if the two consecutive samples Zero.sub.n and Zero.sub.n-1 are identified to be TRUE at a decision block 432, the correction value is set C.sub.n =NONE as indicated at a block 434 so that no correction is provided over short areas of null signal. Else if the two consecutive samples Sat.sub.n or Sat.sub.n-1 are identified to be TRUE at a decision block 436, then the correction value is set C.sub.n =MAXIMUM NEGATIVE as indicated at a block 438. Otherwise a linear correction is provided with the correction value C.sub.n =(E.sub.n +E.sub.n-1).div.2 using two consecutive calculated values as indicated at a block 440.

In FIG. 5, the operation of the gain adjustment algorithm is illustrated. FIG. 5 shows the sync field pattern being sampled both asynchronously and synchronously with various amplitudes. Since synchronous sampling is a degenerate case of asynchronous sampling, the AGAA can provide gain corrections during the acquisition read mode using synchronous sampling of the sync field pattern. Thus, the AGAA can be used for dual purposes eliminating an asynchronous envelope detector in an idle mode, and providing a synchronous gain setting algorithm in an acquisition read mode.

An expanded view of the operation of the algorithm using the sync field pattern at correct amplitude and sliding phase is shown by FIG. 6.

In brief summary because the invention is implemented in digital logic, abnormally low or high signal amplitudes are easily detected. The present invention guarantees proper operation using asynchronous samples. The AGAA provides for maximum positive gain correction when a series of samples appear low in amplitude. Also, maximum negative gain correction is provided when samples saturate the ADC converter. Also the AGAA can be implemented without requiring any additional analog circuits beyond what is already present in the PRML channel.