1062 Chapter 28
shunted flux raises the efficiency of the head by
requiring less drive power.
The conductive shim is only effective at high
frequencies at which large eddy currents are generated
in the shim. As a result, focused gap recorders utilize
bias frequencies that are approximately ten times higher
than conventional systems.
In practical use the silver shim proved to be a major
problem because the soft silver would smear onto the
trailing pole piece of the head and short the head’s lami-
nations together.
A second technique, which yields similar results, is
the crossed field or X-field, Fig. 28-24A. This method
typically places a second bias-only head on the back
side of the tape to create a shaped bias flux field
jumping from one head to the other.
28.3.6.1 Biased or Anhysteretic Recording
The magnetization of the tape particles is not easily
changed due to the memory force or hysteresis of the
particles. In fact, the particles have a form of inertia that
must be overcome if a linear transfer is to be achieved.
If a rapidly varying signal of sufficient amplitude to
just begin magnetizing the particles is added to the
audio flux signal, the magnetic particles will more
readily conform to changes in the audio waveform. The
high-frequency biasing signal produces a hysteresis-free
or anhysteretic recording.
Fig. 28-25 shows a typical waveform of the current
in a low impedance Ampex record head that is
recording 10 kHz at a level of 250 nW/m. The bias
component of 7 mAp-p is approximately ten times larger
than the 10 kHz component at 650μA. (The voltage
waveform across the record head would be totally domi-
nated by the bias component due to the 6 dB/octave rise
in head impedance with increasing frequency, in this
case 35 V of bias versus 500 mV of 10 kHz or 70:1.)
The audio and bias signals must be added together in
a linear manner without generating any of the sidebands
that are present in either amplitude or frequency modu-
lation techniques. The short- wavelength bias signal can
therefore be easily filtered out during playback by the
gap and thickness losses so that only the audio signal
remains. (The high level of bias signal transformer
crosstalk that is present during sync/overdub operation
requires a sharp notch filter in the playback preamplifier
to remove the bias signal.)
Typical bias frequencies range from 100 kHz for
slow-speed recorders to over 10 MHz for high-speed
tape duplicators. Although high bias frequencies are
desirable to permit easy filtering and thorough tape
excitation, a practical upper limit for mastering
recorders is reached at 500 kHz due to a combination of
increased eddy current and hysteresis losses in the core
and the increase in bias drive voltage required due to the
inductance of the head.
Head losses can be reduced by using a very small
core to reduce hysteresis losses and by choosing either
thin laminations or a ferrite material to reduce eddy
current losses. If, however, the record head will also be
used for the reproduce function during sync/overdub, a
small core will cause serious long-wavelength contour
effects. The compromise hammerhead design shown in
Fig. 28-26 improves the playback performance of the
small core by adding extensions to the face of the core.
The tips function only to play back low-frequency
signals for which core losses are insignificant.The bias voltage required to drive a record head
doubles each time the bias frequency is doubled due to
the inductance of the record head. To keep the required
bias voltage within the range of common integrated
circuits, the inductance can be lowered either by
reducing the number of turns in the winding or by
lengthening the gap. Reducing the number of turns once
again degrades the sync/overdub performance by
reducing the playback voltage generated by the head.Figure 28-25. Record head current.Figure 28-26. Hammerhead cores.7 mA0.65 mA
Record head current 15 cycles of bias @150 kHz
for 1 cycle of audio @10 kHz. Amplitude ratio is 10:1