magnetism and "safe zones"

[Rodrev]

More questions. Thanks to everybody who has answered me before

Im in the process of building a control room. In it, I plan to place my Fostex r8, as well as a PC (In a "soundproof" closet).

My question regards magnetism:

How far away from my speakers should the R8 be placed for it to be in the "safe zone" as for magnetism? My speakers are a pair of shielded Maudio BX5 monitors and I am planning to include a pair of HiFi speakers as well. Not to big ones tho'.

And is there any other elements that my recorder should be placed away from? I think of such equipment as CRT screens, the computer, amplifiers etc..

In other words: How far away from which pieces of equipment should my R8 be placed?

Please excuse and ignore me if this is covered in another thread. (Altho', I would utterly appreciate some hints on words to search for)

Baard

 

[Wil816]

Remember some small tape recorders have an unshielded speaker with its magnet directly under the tape path, in close proximity. There are also motors in the tape machine which, if they're shielded at all, it's to keep electrical noise out of the circuitry, not to protect the tape. The tape machine's power transformer might not be shielded either. Yet where the tape is, the strength of the magnetic field resulting from these things may or may not be any stronger than the earth's magnetic field. I have a microcassette recorder with a permanent-magnet erase head which is less than 1/8" away from the tape when in "play" mode. Don't worry about it.

 

[Rodrev]

Thanks! Everything became SO much easier now

Baard

 

[Beck]

It's good practice to keep tapes as far away from speakers as possible... at least 3 ft. However, since your monitors are shielded it's not as big a deal… maybe. It depends on how well shielded they really are.

Magnetism is a strange and fascinating beast. My Hand-D-Mag head degausser will cause my computer monitor to jitter and go psychedelic from 3 ft away. A small ceramic refrigerator magnet will cause a purple hue and jittering from 1 ft.

The thing to remember with hi-fidelity multi-track and master tapes is that your high frequencies are most vulnerable to stray magnetic fields. You can gradually lose important high frequency content with regular exposure. So when in doubt use best industry practices and keep tapes, disks, etc, away from magnets.

 

[Wil816]

Magnetism is a strange and fascinating beast. My Hand-D-Mag head degausser will cause my computer monitor to jitter and go psychedelic from 3 ft away. A small ceramic refrigerator magnet will cause a purple hue and jittering from 1 ft.

I would point out however that the deflection of the electron beam in the CRT has a linear response to magnetic fields, so any minimal field will have an effect. Tape on the other hand has a big hysteresis, and it takes a rather strong field to do anything at all to it. That's why bias is needed in the recording process, and the bias signal huge compared to the audio signal whose recording it makes possible. Again, motors, magnets, solenoids, relays, analog meters (yes, meters have strong permanent magnets in them, and they're usually not shielded) and occasionally even speakers reside inside tape recorders, sometimes a fraction of an inch from the tape path. If it makes you more comfortable to keep other magnetics farther away though, go for it.

 

[Beck]

Yes, I’m quite familiar with the components in a tape deck that create magnetic fields and the properties of magnetic tape. A properly built deck will shield the tape path from any potentially destructive magnetic fields. The example of the degausser was only to illustrate the far-reaching effect of this mysterious force.

Keep in mind my example involved a tape path degausser with about 350 gauss of magnetic field strength, whereas an good 8” speaker magnet can have upwards of 13,000 gauss.

As for the bias issue, tape needs to be excited by an AC bias signal with sufficient energy to create a quality recording with minimal noise and distortion. However, tape doesn’t need any bias at all if you don’t mind a poor quality recording that sounds like it was made in the 1930’s.

The point is the high frequencies can be erased by a relatively small magnetic field without supersonic excitation. Thus, the industry recommends degaussing across the board to prevent this gradual deterioration.

By the way, a magnetometer held near the front panel of a good tape machine will register nothing when engaging motors and solenoids. That’s the case even for my modest “semi-pro” TSR-8.

Bottom line… the practice of keeping tape away from speakers is based on long-established industry best practices, as is degaussing a tape path as part of a proper maintenance schedule. Same goes for getting your tapes well away from a machine during a degaussing session.

Tape handling is a science... elementary physics. If one isn’t careful one can lose the highest frequencies rather easily.

 

[cjacek]

tape doesn’t need any bias at all if you don’t mind a poor quality recording that sounds like it was made in the 1930’s.

Aha! So that's how that effect can be re-created!

 

[Wil816]

However, tape doesn’t need any bias at all if you don’t mind a poor quality recording that sounds like it was made in the 1930’s.

No, without bias, you won't be able to record any intelligible voice at all. The recording process will be virtually ineffective until the record signal is strong enough to distort severely.

That's not to say you need AC bias though. For inexpensive (non-music) applications, DC bias is used even today. (When I was in school, that was a silly trick question on a test: "When DC bias was first used, what frequency was chosen?") DC bias works fine for dictation and other applications where you don't have multiple voices or instruments that get intermodulated and produce objectionable distortion, and where there's usually an automatic-record-level circuit fitting the available signal into the smaller dynamic range. Nevertheless, you do need bias. Take it from someone who has designed this kind of thing 20 years ago for a private-aircraft application where one channel carried data and the other carried voice. We used a common inexpensive cassette transport that could be controlled electrically by the computer instead of by pushing mechanical-linkage buttons. To record, I found I had to put DC through the erase head in one direction, enough to saturate the tape, and through the record head in the oposite direction. To set the bias for cleanest output, I just adjusted the amount of DC current superimposed on the record signal at the record head.

One of my microcassettes has an erase head that is just a permanent magnet that is only pushed toward the tape when you push the "Record" button, and otherwise is about 1/8" away. I used to have another one that put it even closer. If you put a little screwdriver up to it with the machine turned off, the magnet still pulls it right in. There are no wires on this erase head.

So why use AC bias? For less distortion especially at high record levels, and lower background noise.

 

[Dr ZEE]

When I was in school, that was a silly trick question on a test: "When DC bias was first used, what frequency was chosen?".

Did the students who would give a definitive answer to the question (instead of feeling being insulted and thus getting pissed) still pass the test and later become designers of something for a private aircraft company or what not?
What did happen to those students who whould 'answer': "Professor, that was cute, but I would prefer Jack Lemon when need someone to make me feel like laughing. Now can we, please, get back to business so I don't feel like crying due to realisation that my dad wasted load of cash on my education. Thanks."
Those ones probably were kicked out of school for misbehavior.

Take it from someone who has designed this kind of thing 20 years ago

Taken.
Still keep the tapes away from speakers and/or anything with magnets in it though

 

[Beck]

No, without bias, you won't be able to record any intelligible voice at all. The recording process will be virtually ineffective until the record signal is strong enough to distort severely.

That's not to say you need AC bias though. For inexpensive (non-music) applications, DC bias is used even today. (When I was in school, that was a silly trick question on a test: "When DC bias was first used, what frequency was chosen?") DC bias works fine for dictation and other applications where you don't have multiple voices or instruments that get intermodulated and produce objectionable distortion, and where there's usually an automatic-record-level circuit fitting the available signal into the smaller dynamic range. Nevertheless, you do need bias. Take it from someone who has designed this kind of thing 20 years ago for a private-aircraft application where one channel carried data and the other carried voice. We used a common inexpensive cassette transport that could be controlled electrically by the computer instead of by pushing mechanical-linkage buttons. To record, I found I had to put DC through the erase head in one direction, enough to saturate the tape, and through the record head in the oposite direction. To set the bias for cleanest output, I just adjusted the amount of DC current superimposed on the record signal at the record head.

One of my microcassettes has an erase head that is just a permanent magnet that is only pushed toward the tape when you push the "Record" button, and otherwise is about 1/8" away. I used to have another one that put it even closer. If you put a little screwdriver up to it with the machine turned off, the magnet still pulls it right in. There are no wires on this erase head.

So why use AC bias? For less distortion especially at high record levels, and lower background noise.

We’re getting well off topic, but ok… again no you need no separate bias at all to arrange the domains sufficiently to imprint an intelligible signal… albeit a very poor, distorted one. In fact, high frequency content within audible range will contribute bias all by itself. For this very reason some bright individual at Dolby Labs came up with HX Pro to compensate. Yes, you are correct that the earliest biasing method was DC, but before that there was no dedicated bias circuit. The point being you need no bias at all to alter the magnetic particles on a tape, or in this case to damage prerecorded material.

You are correct that magnetic tape resists rearranging of the domains, but it is a far leap to say a stray magnetic field will have no effect at all, which is what you seem to be saying.

Are you saying that tape needs such a high level of biasing to record and therefore residual magnetism would not be strong enough to have an impact? That's what I'm getting.

I don’t really know what the prevailing thought was at TASCAM in the 80’s, but in every language; in every country, in small and large studios alike, whether TV, radio or music, tape handling and magnetic fields have always been an issue. Whether that is tape path maintenance or speaker/tape proximity it’s certainly one of the first things I learned as a teenage Audio/Video apprentice working in TV. We’re talking broad industry best practices here. Anecdotal evidence to the contrary is statistically insignificant.

For example, you won’t hear me saying something like “I had a TASCAM 32 for 20 years and it never needed degaussing, and I stored my tapes on top of my speakers.” Even if that were true it would be a fluke and completely contrary to the experience of thousands of recording professionals around the world over the last 50 years.

So, I tend to speak statistically when considering what recommendations I should make to members of this forum.

I think we certainly have different ideas of what we’re trying to accomplish. You have given the example of the micro-cassette with the permanent magnet erase head. Well, that is perfectly fine considering those devices are optimized for speech recording… maybe 5kHz or less depending on the model.

We’re primarily concerned with hi-fidelity music recording when discussing something like the R8. The same residual magnetism that will erase frequencies in the 12 kHz to 20 kHz range will have no audible impact on a micro-cassette voice recorder, and you won’t see a permanent magnet erase head on a half-track mastering deck.

The bottom line for the recording musician is that your high frequencies are vulnerable to stray magnetic fields from various sources, and it’s something you should be mindful of.

Does that make sense? It does to the preservationists and archivists of the world. If you’re just screwing around and don’t care what the tape you made today sounds like this time next year then by all means have fun and don’t give it another thought. But if you’re in it for the long term consider these statements form the Texas Commission on the Arts (I could have picked a thousand other organizations to quote basically the same thing (in several languages ), but this was handy)

“A tape's signal (the information carrier) is represented on a tape by the arrangement of the magnetic particles into a particular pattern. Strong magnetic fields can affect the signal on a tape, causing it to become unreadable or adding to errors in playback. Common mistakes include leaving tapes on top of, or next to, a television, computer monitor, speaker, or microphone. Motors, transformers, generators, and industrial cleaning equipment are other devices that can cause demagnetization if a tape is within close proximity.”

See McKnight’s papers on the subject… the first includes a simple test to determine if your tape path is slowly erasing your tape, starting with the high frequencies.

http://home.flash.net/~mrltapes/mcknight_demag.pdf

http://home.flash.net/~mrltapes/field-strength-for-partial-erasure.pdf

 

[Wil816]

It takes a certain amount of field strength to have an effect. Since I couldn't tell you how much, you had me interested in doing more tests. I don't have a magnetometer, but I took a small compass near my big speakers, and found the magnetic field around them becomes about as strong as the earth's magnetic field at 12-18" away around the front and sides. I have a smaller 50W speaker here and I had to get the compass within 3" to find a field about as strong as the earth's. Then I brought the compass near my Marantz CD-320 pro portable cassette deck (kind of a poor man's Nagra) and found the magnetic field to be much, much stronger, even inside the cassette compartment. (It was off, BTW.) IOW, it might be ok to get a tape within 12-18" of the big speaker, but it's not ok to put it in the cassette machine? My Philips open-reel machine is kind of inaccessible at the moment.

Next I put a cassette in the CD-320 and recorded about a minute of 10kHz at -10VU and observed the exact playback level both on the oscilloscope and the AC millivoltmeter. (IOW, I'm not relying on the VU meters.) I took the cassette out and rubbed it directly over the speaker for several seconds, where the compass's needle would stick hard to the bottom of the compass and couldn't move because the field was so strong. I put the cassette back in and played the 10kHz again. There was absolutely no change in the playback level. So I took the tape to the big speaker and put it up against the grill cloth right in front of the middle of woofer's big magnet, and then put it right over the tweeter, which although smaller, is closer, practically touching the cloth, and brought it back to play it again. Still no change in the 10kHz playback level.

As you can see, my thinking is that people are worrying about fields that are weaker than the earth's magnetic field, or at least weaker than what's encountered near the tape recorder itself.

You have given the example of the micro-cassette with the permanent magnet erase head. Well, that is perfectly fine considering those devices are optimized for speech recording… maybe 5kHz or less depending on the model.

We’re primarily concerned with hi-fidelity music recording when discussing something like the R8. The same residual magnetism that will erase frequencies in the 12 kHz to 20 kHz range will have no audible impact on a micro-cassette voice recorder

Remember that the tape doesn't care what the frequency is. Frequency is undefined until we give a tape speed. It is the tape speed divided by the wavelength on the tape. What matters for what you're saying is wavelength; and 20kHz @ 15ips will have the same wavelength as 10kHz at 7.5ips, 5kHz at 3.75ips, etc.. The microcassette specs say (I haven't measured it myself) 6kHz @ 2.4cm/second (less than 1 ips), which has the same wavelength on the tape as 95kHz (yes, ninety-five kHz) @ 15ips. (Cassette and microcassette frequency response however is never measured at 0VU, but at much lower levels.) Speech quality is still good at 1.2cm/second (less than half inch per second) where they claim 4kHz top end. That's like 125kHz @ 15ips. Now how important was it to keep that little magnet away?

I have another open-reel transport though, without electronics, which I want to bring in here to the workbench to try the recording on without bias again when I get some time. I haven't done that in years, and I doubt that I still have all my data from back then. I also want to try different amounts of erase current, relative to what it takes to get full erasure, and see the effects on pre-recorded solid high-frequency tones (10-20kHz @ 7.5ips).

 

[Dr ZEE]

Wil816, this sounds like bunch of cool experimenting. I've gotta try this sort of stuff myself sometime. May cure some major paranoia ..or may? - what ever, I would not know for sure untill I actually try it, I suppose.
/respects

 

[Rodrev]

Respects indeed. And thanks, to everyone who has replied.

Ive learned a lot here. Guess Ill try the thing with the compass, and see how strong the magnetical field around my speakers are. Ill also check what magnetism will do with a recorded signal. Guess that is the only real way to learn, by experience.

One other thing I thought of was the metal parts of the machine perhaps being magnetised by speakers nearby. But that is something I can prevent by keeping the machine out of the zone where the speakers magnetism is stronger than mother earth itselfs? I.e check it with a compass? (Great idea btw!)

Cheers
Baard

 

[Beck]

Kudos for the nice experiments, by the way Wil816… fun isn’t it?

However, there are a few problems.

I’m not really sure what your premise is (you still haven’t answered that). If it’s that tape is not vulnerable to magnetic fields, then you’re doing what would be equivalent to dropping Newton’s apple all over again. That is, you’re going back a bit too far and trying to prove or disprove something that’s already established… in this case that ferric-oxide is a magnetically soft material. We already know it is.

As you will note in McKnight’s experiments he is not trying to determine whether tape is vulnerable to residual magnetism; he is rather trying to determine whether it’s time to degauss the tape path. He already knows tape is vulnerable to relatively small magnetic fields as would be found on a tape path. A reading of as little as 2 gauss with an accurate magnetometer means its time to plug in the degausser and get to work.

Also, a compass isn’t an accurate magnetometer. Any nearby metal will skew deflection (Take my word for it… I’ve been orienteering, involved in wilderness search and rescue since the early 90’s) A compass will certainly react to magnets, motors, metal, degausers and such, but how does one interpret the deflection? Using a calibrated magnetometer we have established scales of measurement (though frankly, magnetometers aren’t perfect either at these levels).

Another thing to keep in mind is that high frequency signal loss due to residual magnetism is gradual… sort of like smoking. One cigarette won’t kill you (well, usually not) but over time it will catch up to you. If you take a look at McKnight’s experiment in the previously posted pdf you will note that high-frequency loss is cumulative, getting progressively worse with each pass. It’s going to be the same type of thing if you make a habit of storing tape in close proximity to a magnetic field. It will take some time. So again, how do we translate your experiments into studio life?

Another problem… the speed at which a frequency is recorded will determine how deeply it involves the magnetic particles on a tape. When you’re recording at one speed and playing back at another speed you’re opening new and exciting opportunities for bad science.

To use a radio term there is a “Skin affect” with high frequencies in relation to tape. Meaning higher frequencies don’t live as deeply in the oxide coating. For example, a 16 kHz frequency recorded at 15 ips will indeed reproduce an 8 kHz frequency at 7.5 ips, but the depth of the signal as recorded will not be the same as if originally recorded at 8 kHz at 7.5 ips. Thus the frequency will be more vulnerable to residual magnetism simply because it is on the outer most surface of the oxide.

If we could look at a cross-section of the oxide layer with the naked eye we would see the 16 kHz information living in the top layer, and something like 60 Hz involving more particles more deeply into the pigment. So, the speed at which a frequency is recorded is a crucial factor in making assumptions before moving on.

It’s important to understand that not all the material on a prerecorded tape is equal when it comes to how it’s affected by stray magnetism. With regular exposure the highs go first. And a magnetic field that might be close enough/strong enough to significantly bring down all information at 12 kHz and above may never affect frequencies below that no matter how long the tape is stored in that same proximity to the same magnetic field.

The issue has already been settled. You’ll find it in about every good recording book ever written, every good tape deck product manual, and if you want to go even deeper, more physics texts than you could possible read in a lifetime, not to mention countless articles in professional journals over the years. It’s a bit like pointing out the sun in a clear sky, so I won’t dwell on it.

But what the hell… have fun with it, but don’t forget to get back to recording before too long. A worthwhile experiment would be to verify that your monitors are shielded as advertised. Another might be to measure the aforementioned unshielded stereo speakers.

We already know stray magnetism can damage tape (well it’s known everywhere else but this forum at least ). So, we would all do well to spend our precious time verifying our tapes are in a safe environment.

 

[Dr ZEE]

heh heh
Yes, experiments are fun and they are what they are - no more, no less. Experiments are great to clear the "fog" in the mind however they should not introduce and fill the experimenter's mind with "smoke".
An experiment shall not lead to an unrelated conclusion/(conclusions).
Another words. The result of an experiment shall not give an experimenter an excuse for an introduction of any premisses other than what the result of the experiment makes clearly evident without necessity for an interpretation.
So, as example. If the experiment shows, that rubbing a specific cassette tape against a specific speaker in a specific location for a few seconds does not introduce any measurable difference to a recorded signal of specific frequency and at specific level means one and only one thing, which is: one single application of rubbing such specific cassette tape against that specific speaker in the that specific location for a few seconds will not introduce any mesurable difference to a recorded signal of that frequency at that level.

And Of course, just for convenience, we can put measuring equipment capability and measuring techniques that are being used in the experiment a side as an unrelated issue

 

[RICK FITZPATRICK]

We’re getting well off topic, but ok… again no you need no separate bias at all to arrange the domains sufficiently to imprint an intelligible signal… albeit a very poor, distorted one. In fact, high frequency content within audible range will contribute bias all by itself. For this very reason some bright individual at Dolby Labs came up with HX Pro to compensate. Yes, you are correct that the earliest biasing method was DC, but before that there was no dedicated bias circuit. The point being you need no bias at all to alter the magnetic particles on a tape, or in this case to damage prerecorded material.You are correct that magnetic tape resists rearranging of the domains, but it is a far leap to say a stray magnetic field will have no effect at all, which is what you seem to be saying.

Are you saying that tape needs such a high level of biasing to record and therefore residual magnetism would not be strong enough to have an impact? That's what I'm getting.

I don’t really know what the prevailing thought was at TASCAM in the 80’s, but in every language; in every country, in small and large studios alike, whether TV, radio or music, tape handling and magnetic fields have always been an issue. Whether that is tape path maintenance or speaker/tape proximity it’s certainly one of the first things I learned as a teenage Audio/Video apprentice working in TV. We’re talking broad industry best practices here. Anecdotal evidence to the contrary is statistically insignificant.

For example, you won’t hear me saying something like “I had a TASCAM 32 for 20 years and it never needed degaussing, and I stored my tapes on top of my speakers.” Even if that were true it would be a fluke and completely contrary to the experience of thousands of recording professionals around the world over the last 50 years.

So, I tend to speak statistically when considering what recommendations I should make to members of this forum.

I think we certainly have different ideas of what we’re trying to accomplish. You have given the example of the micro-cassette with the permanent magnet erase head. Well, that is perfectly fine considering those devices are optimized for speech recording… maybe 5kHz or less depending on the model.

We’re primarily concerned with hi-fidelity music recording when discussing something like the R8. The same residual magnetism that will erase frequencies in the 12 kHz to 20 kHz range will have no audible impact on a micro-cassette voice recorder, and you won’t see a permanent magnet erase head on a half-track mastering deck.

The bottom line for the recording musician is that your high frequencies are vulnerable to stray magnetic fields from various sources, and it’s something you should be mindful of.

Does that make sense? It does to the preservationists and archivists of the world. If you’re just screwing around and don’t care what the tape you made today sounds like this time next year then by all means have fun and don’t give it another thought. But if you’re in it for the long term consider these statements form the Texas Commission on the Arts (I could have picked a thousand other organizations to quote basically the same thing (in several languages ), but this was handy)

“A tape's signal (the information carrier) is represented on a tape by the arrangement of the magnetic particles into a particular pattern. Strong magnetic fields can affect the signal on a tape, causing it to become unreadable or adding to errors in playback. Common mistakes include leaving tapes on top of, or next to, a television, computer monitor, speaker, or microphone. Motors, transformers, generators, and industrial cleaning equipment are other devices that can cause demagnetization if a tape is within close proximity.”

See McKnight’s papers on the subject… the first includes a simple test to determine if your tape path is slowly erasing your tape, starting with the high frequencies.

http://home.flash.net/~mrltapes/mcknight_demag.pdf

http://home.flash.net/~mrltapes/f...ial-erasure.pdf

holy moly.....good grief Beck.

 

[cjacek]

I think this would summarize nicely our feelings for our own Tim Beck

Read the following while humming the theme from Shaft, only substitute where neccessary, like this:

Who is the man
that would risk his neck for his brother man?
(Beck!)
Can ya dig it?
Who's the cat that won't cop out
when there's danger all about
(Beck!)
Right on
They say this cat Beck is a bad mother..
(Shut your mouth!)
I'm talkin' about Beck!
​

 

[MCI2424]

I see big warning signs at all gas pumps everywhere. These signs say "No Smoking!" and "put gas cans on the ground to avoid static charges from igniting gas".

So, for over 30 years, I have seen (almost daily) people smoking around pumps, people filling gas cans with the can in the vehicle.

I have never seen any trouble from either of these activities in over 30 years of gas pump visiting. No stations blowing up.

Could gas ignite from these sources? Yes.

Now, magnatism and tape. You just finished a recording for a paying customer (say, The Beatles) and you just put that tape down for a moment near a magnetic source. That friggin ONE TIME it COULD happen, it does.

THAT is why people should be careful. Same with mic shock mounts, same with anything that COULD happen, even if it does'nt 99% of the time.

And, yes, just plug that good old vintage $10,000 ribbon mic in with phantom power ON. 99.9% of the cheap ones will never be harmed, but that 1 loved one will burn right up.

 

[Beck]

Damn it, now I have that song stuck in my head.

But for some reason I'm hearing TAFT!

 

[Rodrev]

Thanks Beck, for moving the thread back into topic, and rounding it off in such a convincing way.

Ill have no troubles keeping my tapes a long way from speakers, but the my CRT monitor worries me. Do they have a magnetic field when turned off?

I thought that the magnet in CRTs was the degausser, which is activated when the screen is turned on (and when you press the degaussbutton). Bet theese things could be a real tapekiller.

Baard