[Ted Kendall]

The main difference between record and replay heads, when these are purpose-designed, is the gap length. The gap on a record head has to be long enough for the flux generated across the gap to penetrate the full depth of the tape oxide - the precise gap length is of less significance, as recording actually takes place slightly beyond the trailing edge of the gap, as the tape leaves the bias field. The gap on a playback head is critical to quality of reproduction, as it has to be long enough to capture sufficient flux to give a decent signal-to-noise ratio, but short enough for the extinction frequency (when the recorded wavelength equals the gap length) to be above the highest frequency of interest.

Record heads have a higher inductance than play heads - more turns, in other words, to ensure the tape can be saturated, whilst the number of turns on a play head has to be balanced against other losses.

In domestic machines, where the coating thickness of the tape is thinner than professional tapes, it is possible to make one head perform both functions adequately.