Digital Audio Recording Basics 529contact the disc surface, but are supported on a boundary layer of air. The presence of the
air fi lm causes spacing loss, which restricts the wavelengths at which the head can replay.
This is the penalty of rapid access.
Digital audio recorders must operate at high density in order to offer a reasonable playing
time. This implies that the shortest possible wavelengths will be used. Figure 17.5 shows
that when two fl ux changes, or transitions, are recorded close together, they affect each
other on replay. The amplitude of the composite signal is reduced, and the position of
the peaks is pushed outward. This is known as intersymbol interference, or peak-shift
distortion, and occurs in all magnetic media.
The effect is primarily due to high frequency loss and it can be reduced by equalization on
replay, as is done in most tapes, or by precompensation on record, as is done in hard discs.
17.1.2 Optical Discs
Optical recorders have the advantage that light can be focused at a distance whereas
magnetism cannot. This means that there need be no physical contact between the pickup
and the medium and no wear mechanism.
N00(a)(b)(c)(d)SS NNN SSFigure 17.4 : The sensing element in a magneto-resistive head. Transitions are not sensitive
to the polarity of the fl ux, only the magnitude. At (a) the track magnetization is shown,
which causes a bidirectional fl ux variation in the head as at (b) resulting in the magnitude
output at (c). However, if the fl ux in the head due to the track is biased by an additional
fi eld, it can be made unipolar as at (d) and the correct output waveform is obtained.