1082 Chapter 28
wider bandwidth, however, requires a method of actu-
ally sensing any errors when the head begins to slip off
the center of the slant track. One of several techniques
for this purpose is utilized in the 8 mm helical format.
As shown in Fig. 28-50, low-frequency tracking signals
are added to the high-frequency data. Four different
frequencies are recorded on four sequential passes, with
the frequencies chosen so that the difference between
adjacent frequencies is either 16.5 kHz or 46.2 kHz.
If the video head on track f 1 plays back the signal
accurately, only the signal f 1 (102.5 kHz) is reproduced.
If the video head deviates toward the f 4 track, it will
pick up both signals f 1 and f 4 (148.7 kHz). The differ-
ence between these two signals, f 4 – f 1 , will give an
error signal 'f of 46.2 kHz, causing the video head to be
moved back toward the f 1 track immediately. If the
video head shifts to the f 2 track, another error signal
f 2 – f 1 of 16.5 kHz will be produced, and the video head
will be moved back toward the proper f 1 track. Thus, the
video head can be made to accurately follow a previ-
ously recorded track.
The close packing of adjacent tracks would cause
serious fringing problems if the data signals on the
tracks contained any long-wavelength information. To
avoid any such problems, the audio signal is encoded
using either digital or FM techniques to shift the
frequency content upward and eliminate all low
frequencies.
Isolation of the short-wavelength encoded signals
between adjacent odd and even tracks is further
improved by offsetting the azimuth tilt of the heads
during recording and playback as shown in Fig. 28-51.
The resulting azimuth error for any signal leaking from
the adjacent tracks will partially attenuate any crosstalk.A helical scan recorder must have additional
circuitry to assemble the digital data from several tracks
into a serial stream that is recorded as blocks of data by
the scanning head. The data must usually be replaced as
an entire block, necessitating a complete rewrite of all
channels if any channel is changing.
All of the digital circuitry of a helical recorder can
be squeezed into just a few custom integrated circuits.
The newer generations of ADAT machines, for
example, adopted digital servos for controlling the
transport so that all of the motor servos could be consol-
idated into a single chip, eliminating the need for any
analog servo adjustment potentiometers. The digital
signal chain is also highly integrated, resulting in an
amazingly uncomplicated main circuit board with just a
few ICs for the entire machine.28.5.5 Heads for Digital Tape Recorders and Hard
Disk DrivesThe packing density of the data on hard disks in 1990
was around 100 mb/in^2. At the time of this publication,
it is over 200 Gbits/in^2.^ Fig. 28-52 shows a thin film
digital tape head.
As the areal density of the data on tapes and disks
increases, each bit must shrink in size. The smaller bits
contain less magnetic energy and generate smaller elec-
trical pulses in the coil of a read head. The resulting loss
in SNR eventually imposes a useful lower limit on the
size of the bits.
This limit has been pushed back by read head tech-
nology called giant magnetoresistive (GMR) or spin
valve heads. (The term giant differentiates these very
high-output heads with giant output signals from earlier
low-output magnetoresistive heads.) The GMR head is
fabricated by vacuum deposition, creating a sandwich of
metals that changes resistance when excited by aFigure 28-49. A dynamic head positioner using a bimorph.
Figure 28-50. Pilot signal tracking servo showing pilot fre-
quencies.
Video
headUpper head drumPiezo bimorphLower head drumTapepV Error signalPilot
frequency102.5 kHz
119.0 kHz
165.0 kHz
148.7 kHzf 1 f 4 f 1 f 1 f 2f 1f 1
f 2
f 3
f 4f 4Figure 28-51. Differential azimuth recording technique.Horizontal sync signal Playback head
Track B
Track A
Track B
Track ARecording head^2 FFF
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