From the beginning digital recording was used extensively in peripheral devices of computers. However recent innovation in both professional and commercial technology have expanded the use of digital recording into such areas as audio and visual technology. Without digital recording it is doubtful we would have seen the development of multimedia which is such a hot topic recently. In digital recording, digital magnetic recording methods such as magnetic disk systems have played an especially important role.
When recording density was not as high as it is at present time, most research was concentrated on the development of magnetic heads and magnetic media, therefore there was not enough investigation into signal processing methods. However, the required recording density has increased year by year until it is now 750,000 times greater than it was just 40 years ago. With such a high recording density the performance of the total system is dependent not only upon individual components but also on signal processing methods. Thus, the development of superior signal processing system is now considered important.
Signal processing methods in digital magnetic recording are classifiable into 3 methods, channel coding method, equalizing method and signal detection method. Various type of recording codes have been developed in longitudinal magnetic recording. Among them, run-length limited (RLL) code is well known as a code with large minimum time interval between consecutive transitions and is now used extensively as the recording code in mass magnetic or optical disk systems. Recently (1,7)RLL code has been actively investigated. So far peak detection with cos^4 equalization has been widely employed as the signal detection and equalization method. These days partial response (PR) system as the equalization method and Viterbi detection as the signal detection method have been actively studied. Furthermore partial response maximum-likelihood (PRML) system combining these methods has attracted a lot of attention recently as a signal processing method which is well suited for high density recording. Generally, while the PRML system performs well, it requires a complex detector. Simplification of the system is also one of the important themes of research.
In this thesis named ``Studies on the PRML system and its simplification in digital magnetic recording,'' the signal processing method is mainly described. First the PRML system and the simplified system for (1,7)RLL code in present longitudinal magnetic recording is studied and compared with conventional EPRML and EEPRML systems. Secondarily to perpendicular magnetic recording expected to be the next generation of recording technology, application of PRML system using bi-layered main-pole head is studied. Then these PRML systems are systematicly studied and suitability of these systems with the bi-layered main-pole head is also studied.
In chapter 3, which examines PR(1,1)ML system in longitudinal magnetic recording, two kinds of simplified Viterbi detectors which expected a constraint of run-length of (1,7)RLL code were obtained. One of them was able to be composed using a very simple algorithm. Then at a bit error rate (BER) of 10^{-4} and at normalized linear density equal to 4, the improvements of signal-to-noise ratio (SNR) over EPRML and EEPRML were about 4.4 dB and 2.1 dB, respectively.
In chapter 4 through the computer simulations based on experimental reproducing waveform by the head in perpendicular magnetic recording using bi-layered main-pole head, the PRML systems are studied. As a result it was clear that PR(1,0, 0,0, -1)ML system could be comparatively simpler than the conventional PRML system, and furthermore that the improvement of required SNR over the conventional system was about 4.1 dB at BER of 10^{-4}.
Finally in chapter 5, PRML systems in perpendicular magnetic recording using bi-layered main-pole head, and suitability of these PRML systems with the head were systematicly studied and showed that the PR(1,1, 0,0, 0,0, -1, -1)ML system gave excellent performance when a nonmagnetic layer was twice as thick as each main-pole. Then simplifying this system, the system exhibited better performance than the PR4ML system using an ordinary single layered head such as when the normalized linear density was greater than 2.5.