Abstract :
Digital magnetic recording on flexible media has grown from ¿ in tape in 1959 to include mass storage systems, digital cassettes, data cartridges and flexible disks in 1980. The properties of greatest interest are the capacity of the removable unit, the access time to the data and the rate at which data can be communicated to another device. To the customer the price he must pay per bit of data stored is very important but he must also consider the price of the drive unit and not just the price of the removable unit. The characteristic by which different storage devices are most commonly compared is their storage capacity. To increase this continuously in response to the ever-increasing amount of information that must be stored in machine-accessible form requires a steady improvement in the amount of data that can be packed into each unit of area. The problems encountered in increasing this areal storage density are discussed in detail. Three kinds of limit to bit and track density are considered: the fundamental limit, the technological limit and the economic limit. No estimate is attempted for the economic limit which is strongly machine-dependent but the fundamental limit to bit and track density is given as 20 000 bits or tracks per mm (500 000/in). The technological limit for bit density is given as 4000¿8000 b/mm (100 000¿200 000 bpi) and for track density 600 t/mm (15 000 tpi).
