Modulation noise in thin-film metallic media: elements of pulse/transition modulation theory

This paper is a mathematical preliminary to a pulse/transition modulation theory in magnetic recording. Equations are given for single-tone pulse modulations of different types (pulse position, width, shape). The theory is applied to noise generation in thin-film metallic media. As longitudinal bit size becomes comparable to the grain size, medium noise manifests itself as significant errors in the traditional mapping of write current to media magnetization. Thus, noise has a character of random multidimensional modulation of pulse parameters of the magnetization derivative. Transition jitter in recording is followed by the transformation of double-sideband recording modulation into single-sideband readback modulation, because of the bandwidth limitation in the readback transducer. Readback output becomes modulated 50% in pulse amplitude and 50% in pulse position. The fluctuation of transition length in recording causes pulse shape modulation. This amounts to simultaneous pulse-width and pulse-amplitude modulations acting against each other, so that virtually no modulation sidebands are generated. The compensation process is hidden in the recording cycle and is not visible in the frequency domain of the readback signal. Therefore, the traditional technique of media noise measurement with a spectrum analyzer is flawed, because the analyzer is blind to pulse shape modulation, which is the dominant source of sampling errors in practical channels.