A first write encoder encodes digital data in a first high density format. A second write encoder encodes digital data in a second low density, i.e. ANSI format. A first read decoder decodes digital data from the first format. A second read decoder decodes digital data from the second format. A disk drive receives a replaceable 90 millimeter optical disk. A read/write head reads encoded data from and writes encoded data to a 90 millimeter optical disk received by the drive. In a first mode, the first encoder is connected between a source of digital data and the read/write head, and the first decoder is connected between the read/write head and utilizing apparatus. In a second mode, the second encoder is connected between the source and the read/write head, and the second decoder is connected between the read/write head and the utilizing apparatus. Control electronics switches between the first and second modes, depending upon the format in which data is recorded on the disk received by the disk drive. An 8/9 GCR encoder and a laser pulse generator are used in the first write encoder to record data on the optical media to increase the storage capacity. Improvement in timing is accomplished with write compensation and signal equalization.