Abstract:
A method and system are disclosed for providing a variable speed digital read channel. The read channel includes an analog portion and a digital portion. The read channel includes an analog-to-digital converter (ADC) for converting an analog read signal into a sampled read signal. The ADC is synchronized to a sample clock. The sample clock has a variable clock rate. The digital portion includes a digital filter for reducing noise. The digital filter can be implemented as a low pass filter (LPF), a high pass filter (HPF), or a bandpass filter (BPF). The filter receives the sampled read signal from the ADC. The digital filter is synchronized to the sample clock. The digital filter&#39;s cutoff frequencies adjust automatically as the variable clock rate is changed, and does not require reprogramming. The digital portion may also include a digital automatic gain control device.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a storage device that includes a digital read channel including digital filtering and digital automatic gain control devices, where the digital read channel is coupled to a variable speed storage device. 
   2. Background of the Invention 
   Magnetic tape is effectively used to store digital data. Data is written onto the magnetic tape by a write head. The write head converts a current signal containing the digital information into flux patterns which are written as field transitions onto the magnetic tape. The data is retrieved when the magnetic tape is passed by a read head. The read head passes over the magnetic medium and transduces the magnetic transitions into pulses in an analog read signal, which are then decoded by read channel circuitry to reproduce the digital sequence. 
   Decoding the pulses into a digital sequence can be performed by a simple pulse detector read channel or, as in more recent designs, by a partial response maximum likelihood (PRML) read channel. The PRML read channel scheme is preferred over the simpler pulse detection scheme because it decreases the necessary bandwidth, thereby allowing more data to be stored on the storage medium. 
     FIG. 1  is a block diagram of a PRML read channel  100  in accordance with the prior art. PRML read channel  100  receives an electronic signal from a read head  102 . Read head  102  is included in a storage device, such as a disk drive or tape drive, and is used to retrieve data stored using the device. The storage device may be capable of operating at a variable speed. Thus, PRML read channel  100  must receive a variable rate clock  104  as an input. 
   PRML read channel  100  includes an analog portion  106  and a digital portion  108 . An analog-to-digital converter (ADC)  110  receives an output from the analog portion  106 , converts the output to a digital data sample signal, and provides the digital data sample signal to digital portion  108 . 
   Analog portion  106  includes a preamplifier  112 , an analog low pass filter  114  and an automatic gain control (AGC) circuit  116 . Read head  102  outputs an analog read signal that is received by preamplifier  112  which amplifies the signal. Next, the amplified signal is input into analog low pass filter  114  which is used to filter noise content from the signal. The filtered signal is then input into AGC  116 . The output of AGC  116  is provided as an input to ADC  110 . 
   The output of ADC  110  is provided to a pulse shaping filter  118 , typically implemented as a finite impulse response filter (FIR). The output of pulse shaping filter  118  is provided as an input to Viterbi detector  120 . 
   Because the storage device is capable of operating at a variable speed, a programming line  122  is used to program analog low pass filter  114  and AGC  116  so that analog low pass filter  114  and AGC  116  will operate at each of the variable speeds. The analog filter and AGC are programmed so that they are optimized for the data transfer rate of the input signal. Thus, when the data transfer rate changes, the analog filter and AGC must be reprogrammed. 
   The variable speed operation of the device will change the frequencies over which filter  114  and AGC  116  must operate. As the range of clock rates increases, the range of programmability must increase. This increases the complexity, difficulty, and cost of analog low pass filter  114  and of AGC  116 . 
   Another solution to providing filtering and gain control over a wide range of frequencies is to provide multiple filters and AGC circuits that must be switched in and out of the read channel. This will also increase the cost of the read channel. 
   SUMMARY OF THE INVENTION 
   A method and system are disclosed for providing a variable speed digital read channel. The read channel includes an analog portion and a digital portion. The read channel includes an analog-to-digital converter (ADC) for converting an analog read signal into a sampled read signal. The ADC is synchronized to a sample clock. The sample clock has a variable clock rate. The digital portion includes a digital filter for reducing noise. The digital filter can be implemented as a low pass filter (LPF), a high pass filter (HPF), or a bandpass filter (BPF). The filter receives the sampled read signal from the ADC. The digital filter is synchronized to the sample clock. The digital filter&#39;s cutoff frequencies adjust automatically as the variable clock rate is changed, and does not require reprogramming as is required by an analog filter. The digital portion may also include a digital automatic gain control device. 
   The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following detailed written description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a block diagram of a read channel in accordance with the prior art; 
       FIG. 2  is a block diagram of a first embodiment of a read channel that includes a digital filter that adjusts automatically as a variable rate sample clock changes in accordance with the present invention; 
       FIG. 3  is block diagram of a second embodiment of a read channel that includes a digital filter that adjusts automatically as a variable rate sample clock changes in accordance with the present invention; and 
       FIG. 4  is block diagram of a third embodiment of a read channel that includes a digital filter that adjusts automatically as a variable rate sample clock changes in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A preferred embodiment of the present invention and its advantages are better understood by referring to the figures, like numerals being used for like and corresponding parts of the accompanying figures. 
   The present invention provides a digital PRML read channel in a variable speed magnetic tape drive. The read channel includes a digital filter in the PRML channel. This reduces or eliminates the need for an analog low pass filter. The digital filter may be implemented utilizing a low pass filter (LPF), a high pass filter (HPF), or a bandpass filter (BPF). 
     FIG. 2  is a block diagram of a first embodiment of a read channel  200  that includes a digital filter that adjusts automatically as a variable rate sample clock changes in accordance with the present invention. PRML read channel  200  receives an electronic signal from a read head  202 . Read head  202  is included in a storage device, such as a disk drive or tape drive, and is used to retrieve data stored using the device. The storage device is capable of operating at a variable speed. Thus, PRML read channel  200  must receive a variable rate clock  204  as an input. 
   PRML read channel  200  includes an analog portion  206  and a digital portion  208 . An analog-to-digital converter (ADC)  210  receives an output from the analog portion  206 , converts the output to a digital data sample signal, and provides the digital data sample signal to digital portion  208 . Analog portion  206  includes a preamplifier  212 . 
   Read head  202  outputs an analog read signal that is received by preamplifier  212  which amplifies the signal. Next, the amplified signal is input into ADC  210  which is subjected to sampling and quantization within ADC  210  which, when synchronized to variable rate clock  204 , generates raw digital data samples as the output of ADC  210 . The digital signal output from ADC  210  is then input into digital portion  208 . 
   Digital portion  208  includes a digital filter  214 , preferably implemented as a finite impulse response (FIR) filter, for filtering noise from the received digital signal. Digital FIR  214  receives and filters the signal output from ADC  210 . FIR  214  then outputs a sampled signal levels signal  216 . Signal  216  is received within a digital pulse shaping FIR filter  218  which shapes the received signal and generates a shaped signal output. The shaped signal is then received by a detector, preferably implemented as a Viterbi detector  220 . Viterbi detector  220  detects the digital sequence from the sampled data input into the detector based upon the Viterbi maximum likelihood algorithm. 
   A digital automatic gain control device  220  receives the sampled signal levels signal  216 , generates a gain control signal, and provides the gain control signal as an input into digital FIR  214 . Automatic gain control(AGC)  222  generates its gain control values from the conditioned data samples included in sample signal levels signal  216  output by the digital FIR filter  214 . AGC  222  provides its output as an input into digital  214 . Alternatively, AGC  222  could instead provide its output as an input into pulse shaping FIR  218 . 
   AGC  222  is a digital device. As with filtering, the automatic gain control function has traditionally been done in the analog domain. However, as with filtering, the time constants associated with the AGC control need to change as the tape speed changes. The digital AGC adjust automatically as the rate of variable rate clock  204  is changed. 
   ADC  210  and digital portion  208  receive and are synchronized to variable rate clock  204 . Digital FIR filter  214  employs a single set of filter coefficients for filtering and conditioning the input data samples in order to produce filtered and conditioned output data samples. The coefficients do not change in response to a change in the rate of variable rate clock  204 . The coefficients can be reprogrammed to change the desired conditioning, but do not change as a result of the variable rate clock. 
   By providing a digital filter  214 , an analog low pass filter is not required, but may still be used. Digital filter  214  adjusts automatically as the rate of variable rate clock  204  is changed. The analog LPF required reprogramming of the cutoff as the rate changed. The digital filter does not require reprogramming since the coefficients used do not change. This makes the operation of the filter automatic and seamless. The digital filter adds little or no cost to implement since the read channel is already digital and the digital filter is a very tiny and very simple function to add. In addition, the digital filter does not suffer from the tolerance problems inherent in making a programmable analog filter. Thus, the spectral response of a digitized signal produced when passed through an FIR, is the same from device to device, whereas the spectral response produced when an analog LPF is used will vary considerably from device to device. 
     FIG. 3  is block diagram of a second embodiment of a read channel that includes a digital filter that adjusts automatically as a variable rate sample clock changes in accordance with the present invention. PRML read channel  300  receives an electronic signal from a read head  302 . Read head  302  is included in a storage device, such as a disk drive or tape drive, and is used to retrieve data stored using the device. The storage device is capable of operating at a variable speed. Thus, PRML read channel  300  must receive a variable rate clock  304  as an input. 
   PRML read channel  300  includes an analog portion  306  and a digital portion  308 . An analog-to-digital converter (ADC)  310  receives an output from the analog portion  306 , converts the output to a digital data sample signal, and provides the digital data sample signal to digital portion  308 . Analog portion  306  includes a preamplifier  312 . 
   Read head  302  outputs an analog read signal that is received by preamplifier  312  which amplifies the signal. Next, the amplified signal is input into ADC  310  which converts the analog signal to a digital signal of data samples. This digital signal is then input into digital portion  308 . 
   Digital portion  308  includes a digital filter  314 , preferably implemented as a finite impulse response (FIR) filter, for filtering noise from the received digital signal. FIR  314  outputs a signal that is input into a PRML digital pulse shaping FIR filter  318 . Pulse shaping FIR filter  318  generates a sampled signal levels signal  316 . Signal  316  is received within by a detector, preferably implemented as a Viterbi detector  320 . 
   A digital automatic gain control device  322  receives the sampled signal levels signal  316 , generates a gain control signal, and provides the gain control signal as an input into digital FIR  314 . Automatic gain control  322  generates its gain control values from the conditioned data samples included in sample signal levels signal  316  output by the digital pulse shaping FIR filter  318 . AGC  322  provides its output as an input into digital filter  314 . Alternatively, AGC  322  could instead provide its output as an input into pulse shaping FIR  318 . ADC  310  and digital portion  308  receive and are synchronized to variable rate clock  304 . 
     FIG. 4  is block diagram of a third embodiment of a read channel that includes a digital filter that adjusts automatically as a variable rate sample clock changes in accordance with the present invention. PRML read channel  400  receives an electronic signal from a read head  402 . Read head  402  is included in a storage device, such as a disk drive or tape drive, and is used to retrieve data stored using the device. The storage device is capable of operating at a variable speed. Thus, PRML read channel  400  must receive a variable rate clock  404  as an input. 
   PRML read channel  400  includes an analog portion  406  and a digital portion  408 . An analog-to-digital converter (ADC)  410  receives an output from the analog portion  406 , converts the output to a digital data sample signal, and provides the digital data sample signal to digital portion  408 . Analog portion  406  includes a preamplifier  412 . 
   Read head  402  outputs an analog read signal that is received by preamplifier  412  which amplifies the signal. Next, the amplified signal is input into ADC  410  which converts the analog signal to a digital signal of data samples. This digital signal is then input into digital portion  408 . 
   Digital portion  408  includes a digital filter  414 , preferably implemented as a finite impulse response (FIR) filter, for filtering noise from the received digital signal. FIR  414  outputs a signal that is received within a digital pulse shaping FIR filter  418  which shapes the received signal and generates a shaped signal output. The shaped signal is then received by a detector, preferably implemented as a Viterbi detector  420 . Viterbi detector  420  outputs a sampled signal levels signal  416 . 
   A digital automatic gain control device  420  receives the sampled signal levels signal  416 , generates a gain control signal, and provides the gain control signal as an input into digital FIR  414 . Automatic gain control  422  generates its gain control values from the conditioned data samples included in sample signal levels signal  416  output by the Viterbi detector  420 . AGC  422  provides its output as an input into digital filter  414 . Alternatively, AGC  422  could instead provide its output as an input into pulse shaping FIR  418 . ADC  410  and digital portion  408  receive and are synchronized to variable rate clock  404 . 
   The description of the present invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.