Test of audio & video tapes

Forensic examination of magnetically stored information on data tapes



Function and application

Magnetic tape is a storage medium for magnetic recording, made of thin magnetizable coating on a long narrow strip of plastic. Regarding to their form of usage, magnetic tapes are comercially available as video, audio and data tapes. Especially forensical investigations of magnetic audio-, video- and data-tapes gain a huge advantage when using a magneto-optical system.

It provides the possibility to clearly identify traces of manipulation on magnetic tapes, for example audio tapes, which have clearly noticeable lift-off and drop-down marks when beeing rewritten or manipulated (see image). mo-tapeSometimes these marks are audibly recognizable, but mostly there is still the need for verification, for example for trials. With magneto-optical inspection devices, as the cmos-magview, these marks can be made visible and can be proven from a technical viewpoint. Another, non the less important application, is the reconstruction of damaged magnetic tapes. When using magnetic tapes it should be clear that its re-readability and re-writeability is mostly definied by the quality of single magnetizations. Especially when using a tape very frequently, the lifetime will decrease extremely, due to a intense wear of the tape.

Also when just storing magnetic tapes for long times the „sticky-shed syndrome“ is a well known problem. Additionaly to these ordinary forms of wear, magnetic tapes are often used in very rough conditions, where other forms of damage can not be excluded at all. When thinking of magnetic tapes used in flight recorders (also for Flight Dara Recorders / Cockpit Voice Recorders) or also in space (due to their higher resistance to cosmic radiation) it is obvious that magnetic information structures are affected by several forces of destruction as physical force, heat and fire and also radiation.

Tapes exposed to these forms of destructive forces have a special need for extremely sensitive handling in order to keep the remaining information in best possible quality. In addition mostly they are no more reconstructable by magnetic measurement probes due to important parts of the data are damaged. By optical analysis of the remaining information, the tapes can be handled as carefully as possible. Furthermore there are often traces of magnetic information which automated probes are not capable to identify. Here magneto-optical systems are able to provide new techniques for investigation and reconstruction.



of magnets


of the magnetic field on linear encoders


of electrical steel sheets

Structural investigation

and quality control for magnets

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