Areal-Density Capability of a Magnetic Recording System Using a "747" Test Based Only on Data-Block Failure-Rate

The "747" test provides a well-established basis for determining the areal-density capability of a magnetic recording system. When new record areal densities are announced, they are usually accompanied by a 747 plot to support the claim. The 747 curve plots how far the read-head can be moved off-track versus the distance to an adjacent "squeezing" track. The criterion for off-track failure is defined as a threshold on raw error-rate, e.g., 1-bit error per 1000 customer bits. There are two concerns with this approach. First, the raw error rate from the readback channel is not the final measure of performance and may vary from channel to channel or, for next generation channels, may not even be defined. Second, the areal densities achieved are usually quoted exclusive of the necessary error correction code (ECC) overhead. So there is no credit given for channels that require little or no ECC. This paper describes the traditional 747 test and then a modification of that test that relies only on measuring the final data-block failure rate. Also, the proposed new definition requires that the resulting areal-densities be quoted inclusive of any added ECC overhead.