Bulk erase mini DV tapes?

[ricard]

When erasing with an AC bulk eraser, there's a certain time factor involved, not for the actual erasure, but in order to achieve a reasonably slowly decaying magnetic field when removing the tape from the actual erase magnet, to avoid leaving the tape magnetized. It may not make a difference in a MiniDV application, but for ordinary analog tape, if the tape is magnetized, then unused sections of the tape will exhibit noise, and possibly leak in from surrounding tracks when playing back new recordings, and might also make erasure difficult, if the tape is magnetized all the way to saturation.

When using a powerful enough permanent magnet, I would think that a second or two would be enough to erase the tape, it's not like it takes any time for the field to build up.

While a bulk eraser is in principle a secondary-less transformer there is one huge difference: The laminations are opened on one side to force the magnetic field out into the air (and through the tape to be demagnetized). It's really the same design as for a magnetic head, but on a larger scale. A properly designed transformer on the other hand has as small a leakage field as possible; it is usually still detectable and can affect nearby magnetically sensitive components, but it is way to weak to erase tape.

Similarly with speakers. Although I would certainly avoid leaving my tapes near speaker cabinets, remember that the magnetic field decays rapidly with distance, so in practice I've never noted any ill effects from tapes lying around speakers. If placed on the actual permanent magnet of a speaker element I would be worried. Also, think of how many tape recorders have built-in speakers, with the tape in close proximity to the speaker elements, and no special magnetic shielding.

[stevehertz]

So a permanent magnet will erase tapes? I thought it needed an AC induced magnet field to 'scatter' magnetic particles.

[TonyDuell]

A permanent magnet will erase tape, in fact 'The Philips Tape Recording Book' suggests using a permanent bar magnet to erase clicks, etc, on the tape, but I wouldn't worry about stray fields from speakers, etc.

As a (relevant) aside, the disk pack hubs in my DEC disk drives (RK05, RL01, RL02, RK07) and indeed the hubs of the 3.5" floppy disks used on micros, are held to the drive spindle by a permanent magent in the flange at the end of the spindle. I am sure the designers of said drives knew what they were doing.

[Simondm]

Originally Posted by ricard

When erasing with an AC bulk eraser, there's a certain time factor involved, not for the actual erasure, but in order to achieve a reasonably slowly decaying magnetic field when removing the tape from the actual erase magnet, to avoid leaving the tape magnetized. It may not make a difference in a MiniDV application, but for ordinary analog tape, if the tape is magnetized, then unused sections of the tape will exhibit noise, and possibly leak in from surrounding tracks when playing back new recordings, and might also make erasure difficult, if the tape is magnetized all the way to saturation.

When using a powerful enough permanent magnet, I would think that a second or two would be enough to erase the tape, it's not like it takes any time for the field to build up.

While a bulk eraser is in principle a secondary-less transformer there is one huge difference: The laminations are opened on one side to force the magnetic field out into the air (and through the tape to be demagnetized). It's really the same design as for a magnetic head, but on a larger scale. A properly designed transformer on the other hand has as small a leakage field as possible; it is usually still detectable and can affect nearby magnetically sensitive components, but it is way to weak to erase tape.

Similarly with speakers. Although I would certainly avoid leaving my tapes near speaker cabinets, remember that the magnetic field decays rapidly with distance, so in practice I've never noted any ill effects from tapes lying around speakers. If placed on the actual permanent magnet of a speaker element I would be worried. Also, think of how many tape recorders have built-in speakers, with the tape in close proximity to the speaker elements, and no special magnetic shielding.

The magnet of a speaker is designed to produce the strongest field through the voice coil. There can be a lot of leakage though, depending on the drive unit design. I have known of instances where recorded BBC type 200 tapes were damaged by a careless producer putting a box on top of a speaker, also when they came into proximity to an STC 4038 ribbon mic.
Many drive units from the mid 1980s onwards were indeed designed with magnetic shielding around the magnet, but the main reason was their effect on colour CRTs, where they upset the colour as the stray field caused the electron beam to miss the correct phosphor. It's also a nuisance for monochrome tubes, but not so obvious.

As discussed above, the coercivity of metal tape is very high, AND the tape is driven to saturation (in a digital context). These things together cause erasure difficulties with metal tape.

In contrast, analogue ferric-composition tapes are quite easily damaged. The BBC in London used to pay for taxis for studio managers and producers carrying programme tapes, to avoid them being taken on the tube (which was forbidden), although I never heard of a tape actually being damaged by the stray fields from underground train motors.

[ricard]

Originally Posted by stevehertz

So a permanent magnet will erase tapes?

Yes. In fact, there are tape recorders with permanent-magnet erase heads. They are mechanically kept swung away from the tape in all but record mode.

Permanent magnet erase heads seem to have been fairly unusual in European reel-to-reel recorders, but they were more common in the US, especially on low end recorders imported from Japan.

Some simpler cassette recorders (especially microcassettes used for dictation) also used permanent magnet erasure (and sometimes DC bias as well).

I think that in most cases, a more than one magnet is used to try and result in a zero magnetic field, but they rarely cancel out perfectly and therefore often leave "DC erasure noise" on the tape.

I thought it needed an AC induced magnet field to 'scatter' magnetic particles.

No, the AC is really there just to result in a zero field, assuming it decays at a reasonable rate. An erasure where the AC current is switched off suddenly is just as bad as DC erase, depending on exactly where in the AC cycle the current was turned off.

Think of magnetic tape as lots of small magnets. (Very small magnets). They can be magnetized, i.e. change their orientation, but they never move.

[Lucien Nunes]

I can confirm that a traditional bulk-eraser for audio tape spools will not touch DV cassettes for the reasons mentioned above. Twice, at the end of a film shoot, I have been presented with such a unit hired-in to erase surplus and unwanted video media, by the production office not checking what type of eraser was needed. I experimented with passing various cassette formats including Digibeta and DVCAM through numerous cycles, with no effect on the error rate as seen from the VTR channel monitor.

Whether PM / DC erasure is suitable depends on what you want from the erasure process. If it's being done data security, and you don't want to re-use the media, then it doesn't matter what condition the tape is left in so long as the recording is gone. But, if you are erasing to prepare the media for re-use then a proper AC erase that ramps down leaving it quiet is better than DC.

[woodchips]

Got an old transformer based arc welder? Lots of current there!

[stevehertz]

I think we're now going around and around the circle of "it can't be done it's metal tape", and "Use this type of magnet or transformer", with believers on both sides.