In general, magnetic disk drives utilize read and write channels to write and retrieve information from magnetic medium. In order to improve the recording density of the data storage system enhanced equalization and data detection systems are sometimes employed. One such scheme is the use of partial response equalization and Viterbi decoding or detecting in the read channel.
The read channels for magnetic disk drives sometimes employ a strategy of peak position detection with simultaneous amplitude qualification. Such read channels for magnetic disk drives are suitable if the inter symbol interference (ISI) is largely removed by an equalization system and if the signal to noise ratio (SNR) is adequate following such equalization. The adequacy is a function of the tolerable error rate within the overall read system. For example, if error correction capability increases, then the SNR may be decreased while still providing adequate overall system performance. In such reading systems, noise is boosted by equalization thereby decreasing SNR. As recording densities increase, the inter symbol interference becomes larger and it contains a larger portion of non-linear ISI which cannot be effectively removed by equalization. In any case, SNR decreases causing an overall degradation in error rate performance.
Some magnetic disk drives employ a technique known as partial response maximum likelihood (PRML) detection. In most cases, the PRML detection systems utilize some form of Viterbi detector. In general, density gain is achieved in PRML in two ways: Viterbi detectors tolerate larger amounts of ISI thereby minimizing equalization noise boost, and Viterbi detectors are inherently more robust than peak detectors.
One problem associated with utilizing PRML systems is achieving well controlled partial response pulse shapes at the output of the equalizer. The ability to achieve well controlled pulse shapes at the output of the equalizer is typically dependent upon the existence of mostly linear ISI (i.e. created from so called linear superposition of pulse shapes), and it requires sufficiently complex equalizers that generate the partial response pulse shapes. The classical Viterbi detector depends upon discrimination between amplitude levels, which are characteristic of linear ISI, from superimposed partial response pulse shapes.
The use of high linear densities in magnetic recording systems results in the ISI having a large non-linear component. Consequently, classical PRML error rate performance is thereby significantly degraded below the level of performance obtained if the ISI was entirely linear. Furthermore, the cost in logic complexity and power consumption for constructing long finite impulse response (FIR) equalizers to create the best possible pulse shapes is an undesirable burden. Therefore, compromises are typically made in equalizer complexity, including both length and precision, to limit the amount of logic and power consumption. It is highly desirable to develop an equalizer and detector system that is less demanding in terms of perfection in equalizer output.
The PRML systems have been used in magnetic disk drives as well as other applications. One such system for disk drives uses a PR IV (partial response, class IV) pulse shape and a 0,4 recording code. In such PRML systems, the Viterbi decoder was matched to the particular pulse shape and code selected. The PR IV and 0,4 recording code permitted a simple Viterbi decoder with excellent bandwidth. However, no compensation for non-linear ISI was incorporated.
Other disk drive systems have employed more elaborate PRML systems based on a 1,7 recording code and selectably either an EPR IV (extended partial response, class IV) or E.sup.2 PR IV pulse shapes. These disk drive systems included a Viterbi decoder to match the equalizer. The equalizer contains taps added to partially correct for pole tip undershoots, which are characteristic of widely used thin film recording heads. However, the system does not compensate for non-linear ISI except by use of a modest degree of target shape compensation.