Overwrite as a function of record gap length

In disk recorders, a common head is used for both recording and reproducing. Old data are not erased but are simply recorded over. The partially erased old data signal appears in the new data as error-producing interference. The amount of erasure of the old data signal is called "overwrite", and 30 dB is typically required. To avoid the reproduce gap null, the head gap is usually chosen to be less than half of the 2F (or bandedge) flux change length. To achieve adequate overwrite using a gap this small, it is necessary to use a very thin recording layer. A simple explanation for this, involving greater depth of penetration for low density signals, is at best incomplete. The data below show that the overwrite phenomenon is complex. Overwrite spectra measured using a thick medium show large resonance peaks whose amplitude and position depend on the record gap, record current, and the densities of the overwriting signal and the signal being overwritten.