Audio Engineering

528 Chapter 17

wavelength further reduces, the familiar gap loss occurs, where the head gap is too big to
resolve detail on the track.

As can be seen, the frequency response is far from ideal, and steps must be taken to ensure
that recorded data waveforms do not contain frequencies which suffer excessive losses.

A more recent development is the magneto-resistive (MR) head. This is a head
that measures the fl ux on the tape rather than using it to generate a signal directly.
Flux measurement works down to DC and so offers advantages at low tape speeds.
Unfortunately, fl ux measuring heads are not polarity conscious and if used directly they
sense positive and negative fl ux equally, as shown in Figure 17.4. This is overcome by
using a small extra winding carrying a constant current. This creates a steady bias fi eld,
which adds to the fl ux from the tape. The fl ux seen by the head now changes between two
levels and a better output waveform results.

Recorders that have low head-to-medium speed, such as digital compact cassette (DCC) use
MR heads, whereas recorders with high speeds, such as digital audio stationary head (DASH),
rotary head digital audio tape (RDAT), and magnetic disc drives, use inductive heads.

Heads designed for use with tape work in actual contact with the magnetic coating. The
tape is tensioned to pull it against the head. There will be a wear mechanism and need for
periodic cleaning.

In the hard disc, the rotational speed is high in order to reduce access time, and the drive
must be capable of staying on line for extended periods. In this case the heads do not

6 dB per

octave

Comb filter effect

of finite head gap

‘Thickness’

loss

Level

Frequency

Figure 17.3 : The major mechanisms defi ning magnetic channel bandwidth.