Delta-T Curves for Measuring Magnetic Tape Link Jitter

Abstract

Magnetic instrumentation tape recording links are usually required to be substantially transparent data channels. One of the most important deviations from this requirement, both in FM and direct recording modes, occurs in the area of time delay error. A method for measuring the root-mean-square time delay error (Δ) between points on a record vs their separation in playback time (T) has been described in the literature (Refs. 1 and 2), but the implications and interpretation of this technique have not been fully developed. This paper derives the theoretical relationships between the Δ-T curve and the autocorrelation function of a pure sine wave recorded on the tape, and uses this correspondence to establish a connection between jitter spectra and Δ-T curves. A simple instrument for measuring these curves with high accuracy is described, and results of measurements made on a number of intermediate-band tape units in use at the Goddard Space Flight Center are presented. These measurements show that a low-mass tape unit may be superior to high mass units by more than an order of magnitude. The utility of the Δ-T measurement technique is discussed on the basis of its direct relationship to PCM and coherent recording applications, and its value as a standard means for evaluating time delay error in magnetic tape links is described. It is also shown that the technique can be used to determine FM modulation index.