Abstract: In a magnetic recording playback device, since minimum redundant bytes required for satisfying a target error rate are added, the addition of more redundant bytes than are necessary is reduced, whereby overwrite of the drive due to the redundant bytes is reduced. The size of redundant bytes to be added in a formatter block is varied according to the defect condition or electric characteristic of a medium on which data are to be written, or the electric characteristics of a write head for writing the data and an MR head for reproducing the written data.

Abstract: In a magnetic recording playback device, since minimum redundant bytes required for satisfying a target error rate are added, redundant bytes to be added more than necessary are reduced, whereby overwrite of the drive due to the redundant bytes is reduced.

Abstract: A semiconductor laser device is capable of producing an increased optical output power with improved optical characteristics without being subjected to mode distortions in the output beam, while retaining advantageous features inherent to a semiconductor laser device of a buried heterostructure. The semiconductor laser device comprises an optical confinement region which is constituted by at least first, second, third and fourth semiconductor layers successively laminated on a predetemined semiconductor substrate. The second semiconductor layer has a relatively small refractive index and a relatively wide band gap as compared with those of the third semiconductor layer, while the first and the fourth semiconductor layers which are of the conductivity types opposite to each other have relatively small refractive indexes as compared with the second and the third semiconductor layers.

Abstract: A semiconductor laser device capable of emitting highly collimated beams, especially of a narrow beam divergence, is disclosed. A striped hetero-junction is formed on a predetermined semiconductor substrate by a first semiconductor layer (refractive index: n.sub.1, band gap: Eg.sub.1, thickness: d), a second semiconductor layer (n.sub.2) and a third semiconductor layer (n.sub.3), and the hetero-junction is sandwiched between portions of a fourth semiconductor layer (n.sub.4, Eg.sub.4) into a buried structure. At this time, the various materials are selected to follow the relations of d.ltoreq..lambda. (where .lambda. denotes the oscillation wavelength of the semiconductor laser), n.sub.2, n.sub.3 <n.sub.4 <n.sub.1, and Eg.sub.1 <Eg.sub.4, and the effective refractive index of the first semiconductor layer (n.sub.1eq) is made smaller than n.sub.4.