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The Magnetic Tape Head
Most professional analog recorders use three magnetic tape heads, each of which performs a specialized task:
The function of a record head (Figure 1) is to electromagnetically transform analog electrical signals into corresponding magnetic fields that can be permanently stored onto magnetic tape.
In short, the input current flows through coils of wire that are wrapped around the head’s magnetic pole pieces.
Since the theory of magnetic induction states that “whenever a current is injected into metal, a magnetic field is created within that metal” … a magnetic force is caused to flow through the coil, into the pole pieces and across the head gap.
Like electricity, magnetism flows more easily through some media than through others. The head gap between poles creates a break in the magnetic field, thereby creating a physical resistance to the magnetic “circuit.” Since the gap is in physical contact with the moving magnetic tape, the tape’s magnetic oxide offers a lower resistance path to the field than does the nonmagnetic gap.
Figure 1: The record head.
Thus, the flux path travels from one pole piece, into the tape and to the other pole. Since the magnetic domains retain their polarity and magnetic intensity as the tape passes across the gap, the tape now has an analogous magnetic “memory” of the recorded event.
The reproduce or playback head (Figure 2) operates in a way that’s opposite to the record head. When a recorded tape track passes across the reproduce head gap, a magnetic flux is induced into the pole pieces.
Since the theory of magnetic induction also states “whenever a magnetic field cuts across metal, a current will be set up within that metal” … an alternating current is caused to flow through the pickup coil windings, which can then be amplified and processed into a larger output signal.
Note that the reproduce head’s output is nonlinear because this signal is proportional to both the tape’s average flux magnitude and the rate of change of this magnetic field.
This means that the rate of change increases as a direct function of the recorded signal’s frequency. Thus, the output level of a playback head effectively doubles for each doubling in frequency, resulting in a 6-dB increase in output voltage for each increased octave.
Figure 2: The playback head.
The tape speed and head gap width work together to determine the reproduce head’s upper-frequency limit, which in turn determines the system’s overall bandwidth.
The wavelength of a signal that’s recorded onto tape is equal to the speed at which tape travels past the reproduce head, divided by the frequency of the signal; therefore, the faster the tape speed, the higher the upper-frequency limit. Similarly, the smaller the head gap, the higher the upper-frequency limit.
The function of the erase head is to effectively reduce the average magnetization level of a recorded tape track to zero, thereby allowing the tape track to be re-recorded.
After a track is placed into the record mode, a high-frequency and high-intensity sine-wave signal is fed into the erase head (resulting in a tape that’s being saturated in both the positive- and negative-polarity directions). This alternating saturation occurs at such a high speed that it serves to confuse any magnetic pattern that existed on the tape.
As the tape moves away from the erase head, the intensity of the magnetic field decreases, leaving the domains in a random orientation, with a resulting average magnetization or output level that’s as close to zero as tape noise will allow.