Erase head
An erase head , or LK for short, is an electromagnetic transducer for erasing data stored in the magnetic layer of an analog audio tape . It is a specially designed magnetic head .
The deletion takes place through a demagnetization process . Since the magnetic layer can be magnetized to saturation by the existing signal storage, correspondingly strong fields are required for demagnetization.
The erase head is a ring magnet , consisting of a magnetic core, non-magnetic working gap and coil. The magnetic core consists of high-quality, soft magnetic metal ( MU metal , ferrite ) with special requirements with regard to high initial permeability , low coercive field strength and high specific resistance . To reduce eddy current losses , the magnetic core is laminated or sintered ferrite is used. During the erasing process, the magnetic layer is then moved through the stray field of the working gap and thus demagnetized. The course of the stray field is largely determined by the width of the gap.
The deletion process is, in the broadest sense, a recording process, but with the aim of keeping the remanent induction zero. This is achieved by making the working gap relatively wide ( s is about 0.2 mm) and using a high-frequency alternating field ( f HF between 80 kHz and 160 kHz). When passing the gap zone, the magnetic layer is first magnetized to saturation, in order then to let its induction gradually decay to zero through change of direction and field drop when leaving the gap.
The extinguishing attenuation is a measure of the extinguishing effect achieved. In the case of special erase head designs, the magnetic core is interrupted in the direction of tape travel by two non-magnetic working gaps (double gaps), the space between the two gaps being filled with a soft magnetic pole layer. The magnetic layer is thus subject to double demagnetization.
In the case of very cheap cassette recorders , the erase head can also be designed as a permanent magnet that can be folded away .
Digital recording
A separate erase head is not required for digital recording, because the old recording is always automatically deleted at the same time as it is spoken down to magnetic saturation.