Scientists at the Georgia Tech Research Institute (USA) have been busy with a project for the military that may have future application to audio and video surveillance and forensics by law enforcement. It addresses the problem of not having a universal eraser for all magnetic media.
With the explosion of new types of digital recording devices and storage media, equipment and procedures for reliably and efficiently erasing the media will likely become a pressing issue. The community has been through this before though. When digital audio tape (DAT) came out, quite a few professionals made the mistake of thinking that their old bulk (analog) tape erasers would wipe them clean, but the magnetic fields used for analog tape eraser were not strong enough for digital tape.
ScienceDaily link is here.