Longitudinal and perpendicular recording media used in disk drives are associated with different signal and noise characteristics at the output of the read-head. For example, in the frequency domain a perpendicular channel has a pulse response with an abrupt amplitude change around the “DC notch.” The abrupt amplitude change in the perpendicular channel results in a pulse response with a long tail in the time domain. The pulse response and, in particular, the long tail, occur because of the physical properties of the head and the recording media. The long tailed response is further affected by the high pass filtering of the signal produced by the read head. The superposition of the pulse tails is commonly referred to as “baseline wander.” The baseline wander acts as a noise source, and it can become a major source of impairment. Furthermore, the baseline wander is data dependent, and for pathological data it significantly degrades drive performance.
An ideal approach to handling baseline wander is to incorporate the pulse tail into the pulse response that the read channel detector expects at its input, that is, into the “target impulse response,” or TIR. The complexity of the detector grows exponentially with pulse length, however, and it becomes impossible to realize such a detector.
To limit baseline wander, modulation codes known as “DC-free codes” can be used to encode the data bits. These codes impose constraints in the data patterns such that the total accumulated data charge and subsequently the baseline wander is close to zero. Unfortunately, the code-rate loss associated with such DC-free codes is significant.
Another approach to limiting baseline wander is to modify the amplitude change at the DC notch from an abrupt change to a gradual one. Since there is no abrupt amplitude change in the frequency domain, the corresponding time domain pulse tail is very short and can be included in the TIR. Although the baseline wander is essentially eliminated by the pulse shaping, the shaping removes a significant portion of the pulse energy at low frequencies, which results in higher error rates.
Still another approach is baseline wander compensation. Additional circuitry is included to estimate and then compensate for the baseline wander. Such circuitry is referred to as a “DC-loop.” The DC-loop estimates baseline wander from the associated error signal at the detector, i.e., from the difference between observed and expected samples. Under present loop delays and high-pass cutoff frequency requirements, known baseline wander compensation circuits suffer performance loss, especially with worst case data sequences.
It is also possible to combine multiple approaches. For example, a weak DC-free code, a DC-loop and a TIR with attenuated DC content may all be employed in the same architecture to minimize baseline wander at the detector input. However, known prior systems suffer some or all of the problems discussed above.
While perpendicular recording offers the potential for better error rate performance at the detector output than longitudinal recording due to the additional signal power at low frequencies, such gain remains partly illusive because of the presence of baseline wander. Thus, an effective approach to baseline wander compensation in perpendicular recording systems remains a major signal processing challenge.