1. Field of the Invention
This invention relates to improvements in methods for operating a mass data storage device having an FIR filter in a read channel thereof that uses PRML equalization and dynamic coefficients, and more particularly to improvements in methods for modifying the dynamic coefficients thereof, and to improvements in mass data storage devices, filters used therein in PRML equalization environments, and more particularly to circuits for modifying the dynamic coefficients thereof.
2. Relevant Background
The PRML equalizaton of a magnetic recording read channel, is usually accomplished by a continuous-time filter, CTF, that is implemented using analog technique, followed by a finite-impulse response filter, FIR. The FIR filter can be implemented in one of two ways: either in a sampoed analog fashion or in a pure digital fashion. In the former case, a sample-and-hold circuit separates the CTF and FIR, whereas in the latter case, an analog-to-digital converter, ADC, separates the two blocks.
The chief purpose of the CTF filter is to provide an anti-aliasing filter to limit the spectral contents of the signal and noise beyond the Nyquist frequency. However, CTF filter are inherently difficult to tune and adapt. It is the FIR filter, nowadays that plays the najor role in achieving a fine signal equalization to the desired PRML targer. With the advent of deep sumicron CMMOS processes, the digital FIR filters with sophisticated Least Mean Square, LMS, coefficient adaptation algorithms are becoming less and lest costly, whereas the analog CTF circuits do not scale down very well. The LMS adaptation fo FIR coefficients is customarilyu done using a sign-sign LMS algorithm. Here, only a sign bit of the unequalized input data sample and a sign bit of the corresponding equalized output data sample error are used, instead of their full value representations.
Magneto-resistive heads have been recently more popular than their thin-film inductive head counterparts. This is due mainly to their lower cost. However, their response transfer function is nonsymmetric or positive and negative pulses, as well as the leading and trailing edges, and it exhibits a larger amount of nonlinearities.
There are two kinds of FIR cnnfigurations that are used to ensure its generally linear phase characteristic: odd- and even-symmetricity of the unit pulse response sequence. The linear phase property is required to minimize the frequency dispersion effects by nonlinear phase response in any data transmission application. However, a slight deviation from the perfect coefficient symmetricity is beneficial to compensate for the non-linear characteristics of MR heads and CTF filters.
Due to the problems of loop contention between the timing recovery and FIR coefficients adaptation loops, a symmetric-manner of coefficients adaptation is usually enforced. Not doing so is likely to cause a “runaway” situation for both loops. Oven though the initial FIR coefficient settings do not have to be symmetric, the LMS adaptation will be incomplete and not able to automatically compensate for the signal nonlinearities.