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Old 10-27-2017, 06:05 PM   #1
Tom41
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Default Audio tapes and magnets

Not sure whether I should post it here or in the general electronics, but it is related to audio equipment!

As you know, signals on audio tapes will print through to adjacent layers if the tape is left wound up for a long period of time. In the past I have had the misfortune of leaving a tape near a weak magnet, and hearing print-through and partial erasure on just the part of the reel that was exposed to the magnet. I have therefore experimented using an old tape with recordings I donít care about.

My first experiment was to recreate the print-through effect I heard on the tape that was accidentally damaged. After recording some speech, I rewound the tape and put a weak magnet up to the edge of the cassette for a couple of seconds. Upon playback, I heard the print-through effect just like on the accidental one - mystery solved.

Where it gets more interesting is when I put the magnet on top of the tape reel and moved it in a circular motion 5 times. This time, upon playback, I got a very noticeable pre-echo AND a post-echo of the speech throughout the recording!
Later on I took a neodymium magnet out of an old hard disk and rubbed that on the outside of the cassette shell, left to right and up to down several times. Upon playing the tape, the recording was gone - but the tape wasnít silent. Instead there was a rhythmic Ďthumpí noise. I canít explain why this would happen, can you?
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Old 10-28-2017, 11:20 AM   #2
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Default Re: Audio tapes and magnets

Since the beginning of time we were always warned to never place audio magnetic tapes near a magnet or in a magnetic field, as the audio information may be erased or damaged. In an empty audio magnetic tape the magnetic dipoles are haphazardly arranged, so no audio. During recording the dipoles are arranged according to the input signal by the tape recorder head and so on playback you hear intelligible audio. To bulk-erase a magnetic tape an electro-magnet powered by ac is used to haphazardly rearrange the dipoles so the audio is lost or erased. The 'thump' you hear is due to a couple of the dipoles retaining residue magnetism and so the playback head on the player amplifies that as a low frequency. Hope this helps you.
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Old 10-30-2017, 09:53 PM   #3
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Default Re: Audio tapes and magnets

Interesting. I've never had cassettes print-through their audio on the adjacent layers (or if they did, it was probably below the noise floor, so it cannot be heard), and some of my cassettes have been in storage for over 10 years and not touched.

That said, I did read that it is possible to get print-through with cheaper cassettes that use thinner plastic tape. Also read it is more likely to happen on longer tapes (like 90 minutes plus), as the cassette has to have thinner tape layers in order for the reel to fit inside. A 45-74 min cassette doesn't have that problem, as the tape reel will usually fit fine, even if it's of normal thinkness.
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Old 10-31-2017, 12:14 AM   #4
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Default Re: Audio tapes and magnets

I've always been wondering what it'd take to do that, and I have a C120 to try it on
Actually microcassette tapes are approximately the same thickness as a C120 tape so it should be more susceptible to this phenomenon.

Yeah that thumping problem is due to the "sharp" cutoff as you leave the tape area with your erasing magnet, and you have this cutoff every layer of the reel (but the shape will sort of change as you get farther from the magnet). There exists dedicated electromagnet erasers that are indeed AC powered, and some even use "diminishing ring" that will carefully "automatically" lift the eraser from your media so these sharp cutoffs never show up. Ideally you can simply cover the whole tape with the erase magnetic field so all of the domains are at least magnetized in the same direction and then you won't hear any strange artifacts. The slow removal AC ideally will try to magnetize everything near the center mark, so it won't be as hard to equalize to the signal you subsequently actually want to record.
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Old 11-01-2017, 08:30 PM   #5
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Default Re: Audio tapes and magnets

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Originally Posted by eccerr0r View Post
Ideally you can simply cover the whole tape with the erase magnetic field so all of the domains are at least magnetized in the same direction and then you won't hear any strange artifacts. The slow removal AC ideally will try to magnetize everything near the center mark, so it won't be as hard to equalize to the signal you subsequently actually want to record.
Well, the thumping sound is actually due to a step-shaped sound waveform appearing on the tape from the strong magnet, as the strong magnet orients all of the magnet domains on the tape in one direction (either + or - ). You want the magnetic domains on the tape to be all in random directions so that their net magnetic field is neutral, as that is indeed the center mark. Hence the reason for fading AC magnetic field erasers.

It's the same with a CRT monitor degauss circuit - you need a fading alternating magnetic field to demagnetize the monitor. Because otherwise you know what happens when you put a strong magnet to a CRT screen.

On that note, I had the degauss relay loose contact midway through degauss on one of my CRT monitors, and the colors were absolutely out-of-whack after that. Took a few normal degauss cycles to get it back to normal and it's still not perfect IMO. Kind of a shame, as it was my nicest-looking 21" CRTs.
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Old 11-01-2017, 11:41 PM   #6
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Default Re: Audio tapes and magnets

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Well, the thumping sound is actually due to a step-shaped sound waveform appearing on the tape from the strong magnet, as the strong magnet orients all of the magnet domains on the tape in one direction (either + or - ).
Well if they're all facing the same direction, then there will be no net movement of current when it passes over an electromagnet(head)! If the whole tape was dumped into a coherent field, all of the particles would be pointing in the same direction. However when recording new material, the ones facing the "wrong" direction will require more time to equalize with the record head producing sound that may not be accurate. But if a physically small magnet was used - now in this case as you draw the magnet away, some of the domains will lose the field first and stop changing (and possibly influenced by the other pole of the small magnet since it's close by), while the others will still have the magnetic field and maintain their orientation for longer. This would generate the thumping that's repetitive because the tape is more or less the same length from layer to layer wound on the reel.
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Old 11-06-2017, 12:25 AM   #7
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Default Re: Audio tapes and magnets

Magnetic tape particles have coercivity, which is the threshold flux required to alter its magnetic polarization. When exposed to a field significantly below the coercivity value, not much will happen to the state of the magnetic particles - there is a field potential that has to be overcome before anything can change.

Print through has to be caused by some sort of a field influencing the magnetic particles that store a tape’s signal. This field could be simply the field from an adjacent tape layer’s particles, but it can also be ‘helped along’ by an external, additive field that allows the adjacent layer’s field to rise above the tape particle’s coercivity value.

If the field is increased, then it may get closer to the coercivity value, and permanent changes could result. This can happen when an external field is placed closer to the magnetic particles - these external magnetic fields are not uni-polar, but instead, they are dipoles, so even a strong dipole at a distance will largely 'cancel out'. However, placing a tape closer to a dipole will likely expose it to higher magnitude fields, since the field strength of the dipole will increase greatly as the distance to the dipole decreases.

Motion is also an important factor. Placing a tape close to a strong static dipole where there is significant relative motion can induce a high magnitude as well as high 'delta' field gradient, which can influence a tape. Bulk eraser machines expose a tape to a high, changing magnetic field. A high static field magnitude alone won't do that much, but if the field alternates polarity or magnitude, then the particles that are located in a position where the changing field is above the coercivity value will get 'erased'.

Finally, print through with a typical recording tape is largely a function of the quality of the magnetic particle size grading used to construct the tape. Small magnetic particles, unlike the desired long particles, are most susceptible to picking up print-through, since they are able to be altered by a smaller strength field. A smaller particle has less magnetic moment, and can thus be altered by a smaller field. So, some tape formulations will exhibit less print through using the same recording flux levels, and the same backing thickness - they simply have fewer small ‘junk’ particles, which will dominate the layer-to-layer print through mechanism when there are no external fields exposed to the recorded tape.

An oddity is that tape machines have been built with ’skimmers’ which are essentially erase circuits designed to erase only the small particle stored field, while leaving the large particle fields alone. So, a tape that has print through not caused by large external fields can have that print through ’skimmed’ away, without altering the quality of the primary recorded signal, stored by the long magnetic particles. This is a godsend for archivists, since a tape can be scrubbed and played back while eliminating layer-to-layer print through, which will start again as the tape it wound to the takeup reel as it is being transferred.

Sorry for the huge core dump, but I hope that it will help you to understand why and how print through and erasure happens with magnetic tape recordings. Yes, it’s hugely complex, but it’s part of how tape somehow works pretty well to store a signal, despite a number of potentially dangerous external influences.
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Old 11-06-2017, 12:48 AM   #8
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Default Re: Audio tapes and magnets

Quote:
Originally Posted by montemcguire View Post
Motion is also an important factor. Placing a tape close to a strong static dipole where there is significant relative motion can induce a high magnitude as well as high 'delta' field gradient, which can influence a tape. Bulk eraser machines expose a tape to a high, changing magnetic field. A high static field magnitude alone won't do that much, but if the field alternates polarity or magnitude, then the particles that are located in a position where the changing field is above the coercivity value will get 'erased'.
I misspoke here - a high static field can alter a magnetic tape's stored signal, as long as the field's magnitude is above the coercivity of the tape's magnetic particles. However, a physically moving or changing magnitude field will be more effective in changing a tape's stored signal. So, a changing magnetic field, due to mechanical motion or a changing field, is able to alter a tape more easily than a static magnetic field. Thanks for your patience!
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Old 11-07-2017, 07:58 PM   #9
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Default Re: Audio tapes and magnets

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Thanks for your patience!
No, thank YOU for posting all of the above info. Was a very interesting read, to say the least.

By the way, does anyone here still keep cassettes and VHS tapes? I know I've held onto my my collections as they still work to this day. Maybe not the best quality, but some of them are over 25 years old now and still work.

Also, I bought a cool cassette deck a few years ago (Marantz SD-4000) for cheap and I have to say I am impressed by its recording audio quality. Using my computer to transfer music onto cassettes, I get pretty awesome sound quality.

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Old 11-09-2017, 05:02 PM   #10
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Default Re: Audio tapes and magnets

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By the way, does anyone here still keep cassettes and VHS tapes? I know I've held onto my collections as they still work to this day. Maybe not the best quality, but some of them are over 25 years old now and still work.
I do! I haven't tested any of them for years though.
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Old 11-09-2017, 05:54 PM   #11
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Default Re: Audio tapes and magnets

I still have a whole bunch of old mag tape equipment, nothing fancy.
Haven't gotten rid of my 8mm tape backups yet either... not sure how well those work.
Video leaves a bit to be desired, but NTSC wasn't ever that hot. However compact cassette with *good* media and a *good* recorder/player still sounds good, even compared to some digital formats...
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Old 11-11-2017, 04:23 PM   #12
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Default Re: Audio tapes and magnets

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Video leaves a bit to be desired, but NTSC wasn't ever that hot.
TBH, I don't think it was that bad either. In fact, VHS did have *some* advantages over DVD - mainly to do with color compression, or lack thereof. On that note, cable TV seems to be the worst when it comes to that - even for HD channels. This is probably easiest to see in dark scenes, where the picture will start to look like an 8-bit BMP image.

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However compact cassette with *good* media and a *good* recorder/player still sounds good, even compared to some digital formats...
Indeed.
Most 128 kbps MP3s just sound ugly (very few are properly compressed to yield reasonable quality). I found several computers full of music - about 50-60 GB worth of content. Yet most of it was 128 kbps, so I ended up saving very little out of that. What's even more amusing is that the people with those collections appear to have purchased the music through iTunes, lol. Can you actually believe iTunes charges people for that shit?!

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I do! I haven't tested any of them for years though.
Cool. BTW, nice to see you back goodspusearch!
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Old 11-19-2017, 06:38 PM   #13
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Default Re: Audio tapes and magnets

I really missed this forum
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