Audio Engineering

Digital Audio Recording Basics 527

of the magnetic coating is recorded, although this cannot be done if the same head is
intended to replay.

Figure 17.2 shows what happens when a conventional inductive head, that is, one having
a normal winding, is used to replay the track made by reversing the record current.
The head output is proportional to the rate of change of fl ux and so only occurs at fl ux
reversals. The polarity of the resultant pulses alternates as the fl ux changes and changes
back. A circuit is necessary which locates the peaks of the pulses and outputs a signal
corresponding to the original record current waveform.

The head shown in Figure 17.2 has the frequency response shown in Figure 17.3. At DC
there is no change of fl ux and no output. As a result, inductive heads are at a disadvantage
at very low speeds. The output rises with frequency until the rise is halted by the onset
of thickness loss. As the frequency rises, the recorded wavelength falls and fl ux from the
shorter magnetic patterns cannot be picked up so far away. At some point, the wavelength
becomes so short that fl ux from the back of the tape coating cannot reach the head and
a decreasing thickness of tape contributes to the replay signal. In digital recorders using
short wavelengths to obtain high density, there is no point in using thick coatings. As

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Transitions

S SSSNN NN

(a)

(b)

(c)

Figure 17.2 : Basic digital recording. At (a) the write current in the head is reversed from

time to time, leaving a binary magnetization pattern shown at (b). When replayed, the

waveform at (c) results because an output is only produced when fl ux in the head changes.

Changes are referred to as transitions.