Figure 1-4. Playback process
Erasure is accomplished by applying a high-frequency, constant-amplitude AC signal to the tape
with an erase head. This head has a wide gap which allows the flux to reverse itself many times as the
tape passes the gap. The tape becomes demagnetized because the erase current is large enough to over-
ride the recorded signal and the negative and positive half cycles cancel each other across the gap.
Learning Event 2:
HIGH FREQUENCY BIAS IN AUDIO RECORDING
If a graphical plot of a signal current through the record head against residual flux density
produced in the tape is made, a nonlinear magnetic transfer curve is obtained. In audio recording on
magnetic tape, the distortion due to nonlinearity is reduced by adding a high-frequency, constant-
amplitude alternating current to the signal current in the coils of the record head (fig 1-5). Both the
upper and lower peaks of the envelope then trace out replicas of the signal current. The combined effect
is the same as if two signals were applied to the tape; one confined to a linear segment in the upper part
of the transfer curve, and the other to a linear segment in the lower part of the curve.
Each segment provides a separate output but the two signal currents are in phase and are added
to each other on the tape. The audio bias frequency is generally at least five times as high as the highest
signal frequency and the amplitude is about ten times greater than the signal to be recorded. The effect
of the bias on the reproduced signal is negligible because it has too high a frequency to be reproduced.