The Influence of an Ambient Magnetic Field on Magnetic Tape Recorders

Abstract

Magnetic recorders are susceptible to external magnetic fields and hence prone to data degradation. This is primarily observed in instrumentation (space and ground) and computer recorders, where little or no shielding of the heads is provided. The magnetic core material in the heads attracts flux lines and will with certain orientations concentrate these in the record, play back or erase gaps. During recordings a foreign field may therefore be superimposed on the intended recording field and may cause errors in the form of phase shift and dc offset; these affects are agravated when AC bias is used in the recording process. If an external field is present during playback only, partial or complete erasure may take place. One external field is always present: The earth s magnetic field which has a magnitude of roughly one Gauss (or Oersted). The writer is not aware of any errors per se caused by this field, but it has numerous times caused difficulties in achieving a perfect demagnetization of heads (An oscillating and simultaneously decreasing field from a degausser does then in essence record a permanent magnetization into the heads, which in turn will result in noisy and distorted recordings). Other fields are man made, such as originating from heavy currents in cable harnesses. The analysis presented in this paper was undertaken to establish susceptibility limits for a field generated by a magnetic attitude control system for spacecrafts. This is illustrated in Figure 1, where three orthogonal electromagnets on board a spacecraft generate a magnetic moment (M), variable in magnitude and orientation. The attitude correcting torque (T) on the spacecraft is expressed as the cross product between this moment and the earth's field. Recording equipment may be located within a few feet of the center of the attitude control field, which must be limited in magnitude or the recorder shielded to avoid data errors and/or erasure.