MadSci Network: Engineering

Re: Is it even possible to recover an erased tape?

Date: Wed Dec 27 10:06:15 2006
Posted By: Dr. Fred Jeffers, Staff, Magnetic Recording Research, Iomega corp.
Area of science: Engineering
ID: 1164414779.Eg

     I was not involved in trying to recover the 18 minutes that were 
erased from Nixon's tape but I watched it in the news with some interest.
     The problem is that the person who did the erasure apparently did it 
several times.  Some data indicated that there were as many as nine 
separate erasures.  
     There is always some wander of the tape over the head. If they had 
erased only once or twice there may have been some signal left at the 
track edges that could be read with special heads.  
     The erasure was apparently done on another recorder than the one 
used to record.  The track widths of the two heads are always somewhat 
different.  If the trackwidth of the erasing head were a little smaller 
than the recording head again there can be enough signal at the track 
edges to recover.
     Most audio heads of that vintage are made by laminating several thin 
layers of high permeability metal to avoid eddy current problems.  The 
layers are electrically isolated from one another and so there is a tiny 
gap between the layers that is not recorded or erased.  These gaps in the 
erasure can be read by special heads.          
     There are sizeable physical forces between the particles in a tape.  
The particles are held in a plastic binder.  If the tape is recorded and 
then aged or heated for some period the particles can move very slightly 
in response to these forces.  This can leave a tiny residue of the 
original signal even if the tape is completely erased magnetically in a 
very large magnetic field.  Exposing the erased tape to a constant field 
can give a signal that is the second harmonic of the original signal.
     The original tape is stored under carefully controlled conditions to 
wait for improvements in technology.  Perhaps it is now time to try 
again.  Magnetic Force Microscopes can now detect the field emanating 
from a single magnetic particle and hard disk heads are now being 
routinely made with trackwidths approaching the width of the "large" 
particles used in the Nixon tapes.  With enough work perhaps the tape 
could now be read.  However, would anyone really care enough now to try?  
Everyone now knows that Nixon was involved in the coverup, even without
the 18 minutes.

Fred Jeffers 

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