Upper bounds on achievable storage density: a two-dimensional approach

This paper compares two upper bounds on achievable storage density in rigid-disk magnetic recording systems. Each upper bound equals the capacity of a communication channel that models a physical recording system. The upper bounds arise by relaxing physical constraints to make the models tractable. Medium capacity is an upper bound on the density of information that a magnetic medium can accommodate. It is independent of the particular method of storage and retrieval. We model the magnetic medium as a two-dimensional waveform channel with additive white Gaussian noise, and we compute medium capacity as the Shannon capacity of this channel. Single-head capacity is an upper bound on the density of information that a single-head recording system can reliably store and retrieve from the medium. We model the single-head recording process as a one-dimensional waveform channel with linear impulse response, correlated Gaussian noise, and crosstalk. We show that for a typical thin-film system, medium capacity exceeds single-head capacity by more than a factor of 40. This result suggests that the use of novel recording techniques that account for the two-dimensional nature of the medium can achieve significant gains in storage density