Degauss 'er? But I just...

[jbsegal]

Anyone in MetroBoston - ESPECIALLY either who can come to MIT tonight or who is coming to our openhouse tomorrow - have a degausser/bulk tape eraser I can borrow? I've got this stack of DATs that need to zero'd.

If you do, could I borrow it for a day or 2? (If you're not going to be at either place, let me know where you are and when I can come by to pick it up...)

Thanks!

Comments

[persis.livejournal.com]

I have one... Spencer says it is a radio shack one... and he's not sure how well it will work, but I'll bring it along anyway...

[jslove.livejournal.com]

I should explain that I am not sure the field is strong enough for digital tape, especially newer, high-density tale. It's an older unit.

It works great on audio tapes, and I think it's OK on regular VHS or 8mm video. I think it's kind of so-so on DAT1 and audio DAT. The last time I tried to erase Hi8 (higher density 8mm) video tape, which is used for 8-track digital audio recording and is essentially similar to the old Exabyte tape, I don't think it worked fully. I mean, it did render the tape unreadable, but it didn't erase the formatting information well enough to let me convert them from 48 KHz to 44.1 KHz audio formats. When I tried to erase Hi8 video, for some reason, only the bottom 25% or so of the image was erased. The top didn't look too good, but there was enough there to allow the VCR to track, and the middle of the image (middle of the tape) was not erased.

So if you have DDS2 or DD3 (is there as DDS4?) 4mm helical-scan tapes, I don't know if this eraser will even touch them. As the bits get smaller (densities higher), the required fields to erase them go up. Because the heads are very close to the media, and probably closer for each generation of the technology, the fields from the heads may not increase increase in intensity, but the strength of an external field may need to be stronger.

[jbsegal.livejournal.com]

Never actually thought of that...
http://www.athana.com/ddequip/degaussingfacts.html turned up with a quick google.
Any idea what the (Oersted?) output is of your degausser?

[jslove.livejournal.com]

In spite of what follows, I am sending the unit in with persis.

Permit me to call your attention to the manual for my "High Power Video/Audio Tape Eraser", Radio Shack catalog number 44-233A:

http://www.radioshack.com/graphics/uc/rsk/Support/ProductManuals/4400233_PM_EN.pdf

There is no mention of field strength in the manual. It does point out that this unit can be used for approximately one minute, and then has to cool for 30 minutes. This may not be a good enough duty-cycle for your purposes. This thing was not expensive at Radio Shack, so it may be worth putting up with. Professional level degaussers (many of which still have duty-cycle limitations) seem to range in price from $80 to $5,000.

My unit is rated at 8.5 amps, max. I have no idea how many turns are in the magnet. I've forgotten nearly all the relevant physics in the 30 years since I learned it, so I can't give advice on measuring the field, either. I speculate below.

The table you point to does not give values for audio tapes (especially "metal") or 8mm video tapes. Here is a reference for 1150 Oe coercivity for metal audio cassette tape:

http://www.adelcom.net/DenonTape.htm

Hi8 video tapes are to 8mm video as SVHS is to VHS, but due to higher data density require a stronger field to erase them. It also does not specify whether the disks it can erase are just DD or include HD as well. The manual for the eraser does say that it will erase 8mm video tapes, but does not mention either SHVS or Hi8. I can't find a coercivity spec for non-Hi8 8mm video tape, but Hi8 seems to be up to 1600, which is similar to DDS1 (DAT) tape in the table, and much more than 1150.

Perhaps Hi8 is out of reach (at up to 1600 Oe), or perhaps I did not adequately follow the instructions in the manual to ensure erasure. I forget. At a minimum, Hi8 would probably require more passes. That audio "metal" tape requires 20 passes suggests that such tapes are probably at about the limit of this eraser's capability.

It would appear that this unit could work if it generates as field that could erase 700 Oe. The recommendation I have seen is that the field to perform the erasure should be 2-3 times the coercivity of the tape. This suggests that perhaps it generates a field of at least 2300 gauss, but it might be as little as 1400 gauss. Since DDS1 has a coercivity of about 1600 Oe, it seems marginal at best. Here is a reference for a 2800 gauss unit similar to the Radio Shack unit (and cheap), which claims to work on DAT but not Hi8 tape (the width of the tape?):

http://www.datalinksales.com/degaussers/pf215.htm
http://www.datalinksales.com/degaussers/home.htm

The best way to test erasure would be with a DAT deck, its manual, and a scope, but you can test whether playback works somewhat more easily. Good luck, JB. This reply will self-destruct in... Oh, dear, that function is not implemented.