Audio Engineering

Tape Recording 753

on the audio signal, thereby biasing the audio signal portion of the overall magnetic effect
into the initial linear region of the BH loop. In practice, such a scheme has a number of
practical disadvantages. Instead a system of ultrasonic AC bias is employed, which mixes
the audio signal with a high-frequency signal current. This bias signal, as it is known,
does not get recorded because the wavelength of the signal is so small that the magnetic
domains resulting from it neutralize themselves naturally. It acts solely to ensure that the
audio modulation component of the overall magnetic force infl uences the tape in its linear
region. Figure 26.4 illustrates the mechanism.

It is hardly surprising that the amplitude of the superimposed high-frequency bias
signal is important in obtaining the best performance from an analogue tape machine
and a given tape. Too high an amplitude and high-frequency response suffers; too low a
value and distortion rises dramatically. Different tape formulations differ in their ideal
biasing requirements, although international standardization work [by the International
Electrotechnical Commission (IEC)] has provided recommendations for the formulation
of “ standard ” tape types.

26.5 Equalization ..........................................................................................................

For a number of reasons, both the signal that is imprinted upon the tape by the action
of the record current in the record head and the signal arising as a consequence of the

Input

Distorted

output

Input

Undistorted

output

Figure 26.4 : Linearizing effect of AC bias.