back to article Reelin' in the years: Tracking the history of magnetic tape

Today marks the 80th anniversary of the first patent filing for a magnetic tape recording medium, though the tech I worked with was a bit more recent than that. Still, it has been quite some time since I last went shopping for tape. I recall the last time as being a deal on a load of JVC miniDV cassettes that I still haven't …

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Meh

Fascinating article, I'm now inspired to dig out my wire recorder.

Fascinating article, it's inspired me to dig out my old early-post war wire recorder and have a play. Mind you, the quality is marginal so it's only a talking point these days.

More importantly, what concerns me is the long term archiving of magnetic data. Even from my own expedience, the loss of magnetic remanence on tapes and other magnetic media is a problem. One has to assume the loss of a few percent each year and this doesn't bode well for the mountains of tape that lie unprocessed in archives.

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@RobHib - It's not only remanence, remember the Ampex 456 scandal.

Tape formulations varied from manufacturer to manufacturer, so maybe an Ampex tape would wear better in Hong Kong and BASF might run smoother in Belgrade.

I should have added it's not only magnetic remanence loss that's an issue affecting tape storage life but also the tape formulation itself is often a major problem. Remember the Ampex 456 scandal where the tape binders broke down and the tapes became sticky (and it wasn't the only one)?

This is very significant, as Ampex 456 was considered a mastering tape and used by many studios throughout the world for master recordings. I recall much effort went into fixing and transferring the recordings to other media.

Unfortunately, I'll bet there's still a great deal of 456 and other formulations that have already rotted lying in the archives still waiting to be discovered. Sticky-tape syndrome is really a significant problem.

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Re: @RobHib - It's not only remanence, remember the Ampex 456 scandal.

There's a good article on Wendy Carlos' site about this problem. Luckily she was able to rescue the master tapes for her Tron soundtrack. :D

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Re: @RobHib - It's not only remanence, remember the Ampex 456 scandal.

Thanks, I'd not seen that before. Anyway, that's the problem.

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Anonymous Coward

Could someone clarify bias "mixing" please?

I've never been sure, and would appreciate an answer (wiki et al don't make it plain enough for me).

When they say the bias is MIXED with the audio, is it :

a) Additive Mixing - like the bias signal is an inaudible (too high) signal on another fader on a mixing desk, combining the bias 'tone' and audio in such a way that you could still plug in a speaker and hear the audio.

or

b) Multiplicative Mixing - as in superhet radio : the audio modulates the bias like a carrier? In that case you'd hear nothing on a speaker as the whole signal would be up around the bias carrier in terms of frequency.

As a radio ham to whom mixing can mean either thing, I suspect 99% that it's a) but wouldn't want to bet my life on it :)

And a follow-up question is about the bias adjust - it's the level of the bias, not the frequency, that the typical control changes?

Thanks!

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Re: Could someone clarify bias "mixing" please?

This topic could span pages, hopefully I'm not misinterpreted by simplifying.

Essentially, bias is a way of overcoming the non-linearity at the crossover point in the magnetic media (magnetic materials). If you look at the transfer (B/H) curves for magnetic materials you'll find that near the crossover point (i.e.: swinging from say positive to negative), the 'curves' are not straight but curves (i.e. they're not linear as is the line y=x, nor is the gradient/differential of this curve particularly 'linear' either).

This, non linearity, of course, adds harmonic distortion (which theoretically can be calculated mathematically if you know the equation for the curve). Well, there's a way of cheating. If you superimpose (mix) a RF (HF) bias (a high frequency signal that's well above the top audio frequency that's being recorded) you effectively 'lift' the main audio signal off the non-linear points. As the bias swings +/-, is has the effect of cancelling the non-linearity in the baseband (main audio) signal.

It's a bit more complicated than that and it's hard to fully explain without some graphs and a bit of maths. Also, the bias frequency, it's level, and how it's applied (mixed) etc. is the key to good audio tape recording. In high quality tape recorders, it's a big deal--probably the biggest design issue outside tape transport (wow and flutter etc.).

If you're an amateur radio operator, you'll know the purpose of grid bias on radio and transmitting valves (i.e. for linearity, modulation, class of operation for TX tubes, 'C' etc.). In vales the bias is usually a DC offset which can determine both current through the valve as well as its transfer linearity. Well, tape bias serves a similar function. In tape recorders, the bias is essentially a dynamic AC signal whose magnitude is sufficiently large to overcome the inherent non-linearity at the extremes (it's extremely important at low levels, as that's where the non-linearity is worst (or until to hit clipping, but clipping isn't as bad as you can avoid overload whereas you can't avoid non-linearity at the 0-point, that's where HF bias turns out to be a godsend.

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Anonymous Coward

Re: Could someone clarify bias "mixing" please?

I appreciate your efforts and willingness to help, but an A or B would have done - and I have to say I'm still only at 99% because you didn't clarify the type of 'superimposing' or 'mixing' :)

OK, maybe 99.5% because it's sounding more like additive mixing the more I read, but I'm still chasing that final 0.5% certainty!

Thanks anyway.....

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Re: Could someone clarify bias "mixing" please?

In modulation mixing, you have products: f1, f2 ==> f1, f2, f1+f2 , f1-f2 etc. This is non-linear as the product frequencies weren't thereto begin with. Product frequencies mean distortion. Simple products, then simple harmonic distortion.

Superimposition ('mixing') means no product mixing, it where f1 + f2 ==> f1 + f2 only. Sometimes referred to a longitudinal mixing (as in longitudinal hum). Assuming no non-linearity (which is always problem to some extent in electronics), no products are created.

In a tape recorder, linearity is key, products are not wanted (as certainly 'f1-f2' etc. would end up in the baseband, thus audible). Thus, logically, bias is linear superimposition. Try this:

http://artsites.ucsc.edu/ems/Music/tech_background/TE-19/teces_19.html

The Wiki article on hysteresis explains the problem in a more detailed way. Also, there's a good AES article (PDF) if to type in 'tape bias aes' into Google, it explains all.

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Re: Could someone clarify bias "mixing" please?

I like to think of it this way. The tape is magnetized positively and negatively (or plus and minus) many times by the bias during the time a point on the tape passes over the gap in the head. On the average, the magnetization is zero because the bias signal is an AC sine wave, with no DC component. As the audio is superimposed onto the bias, which looks to the head gap like a slowly varying offset in the bias magnetization, the audio causes that average to go positive and negative, and so the average magnetization varies with the audio signal. This is what is left on the tape. And the bias changes polarity so many times during the time the tape is in the gap that there is no bias frequency on the tape. What you hear is just the audio.

Some very cheap tape recorders used a magnet to give the tape head the 'DC' bias, instead of the AC bias signal. Magnets were also used to erase the tape. But the AC bias gave a much better erasure.

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Re: Could someone clarify bias "mixing" please?

When I said superimposed, I should have said that it was the same as mixing the audio with the AC bias. This was done at the head. But you could have thought of the AC bias as being just another audio signal mixed with the audio.

Of course the AC bias was more powerful than the audio because it had to overcome the hysteresis in the magnetization. Also the same AC bias was used to erase the tape.

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Acme Fixer - Re: Could someone clarify bias "mixing" please?

Yeah, that's a simpler way of putting it. (A problem on this forum is that you can't assume IT types have electronics knowledge--yet there's stacks who do. Seems yuh damned whatever level an explanation is pitched at.)

The other way I've explained tape bias in the past is basically the same as yours. Essentially, the bias is slew-rate limited by the bandwidth of the head/tape system on record, thus the HF on the tape is negligible. But as the bias originally swung +ve an -ve the integrated output on the tape (mostly) cancels. Once you get past this point it gets a bit tricky as the remanence is still 'settling down' and the tape is still moving--so we've the dynamically collapsing field problem, the head's field is collapsing but the tape's magnetic domain has moved on somewhat (forget Maxwell, just pick up the tweakers and CRO).

It's fascinating to have CROs and spectrum analysers on both the record and playback heads simultaneously, adjusting bias levels etc. all that B/H stuff becomes immediately obvious and falls right into place just as the textbooks say it does.

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@Acem Fixer - Re: Could someone clarify bias "mixing" please?

"...When I said superimposed,.."

Right. As mentioned in the other post, technical discussions on these forums can get a bit messy. I've accidentally made the mistake previously (and more than once) of assuming that everyone uses the same nomenclature on El Reg. One gets used to the terminology of one's training institution, workplace or electronics magazine then assumes that everyone across the world/internet uses the same. Not so.

I was guilty of that earlier in first post where I used 'mix' (mixing) too loosely without definition. For me, 'mixing' usually refers to the non-linear kind (diode mixer etc.) where there's new products (f1-f2 etc.), there I used it in the linear sense. It was inconsistent with my normal usage (and it somewhat contradicted what I said later on, although I corrected it on-the-fly with qualifiers 'linear/longitudinal' etc. (Shame El Reg has no after-post editor as many other forums do).

I've been taken apart and hammered for oversimplifying when I explained something to a neophyte, and also for explanations in quick/short replies. It was drummed into me years ago by a lecturer to let equations do the work--to explain and argue the case, that way one doesn't get into difficulty. Trouble is, either the audience gets glassy-eyed or it vanishes, either way discussion stops.

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Great article

Great article. Nuff said. Look forward to the second instalment.

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@El Reg / Bob Dormon

"It should come as no surprise that GCHQ would want to record telecommunications broadcasts.... ...They could run at up to 240 inches per second - that's 20 feet spooling in front of you every second, 400 yards a minute, or close to 14mph."

Approximately when was that? And what was the upper frequency limit (-3dB point or whatever cutoff was)? I assume that's no longer classified as that very-high speed tape recording went out of practice quite some decades ago.

Back then (during the Cold War), to overcome the problems of super-speed tape recording, the US and I presume others, adapted the Ampex Quadruplex 2-inch video recorders to record RF from a low baseband to somewhere in the HF band so that the signals could be analysed at leisure (i.e.: not analysed in real time). This method of recording was much more convenient than using machines that would probably kill you if a tape flew off the spool.

Exactly how this adaption was done was a military secret at the time but I recall seeing declassified NORAD photographs in tech journals of the interior of B52s or whatever showing the Quadruplex recorders. They flew as close to the USSR etc. as possible and recorded the radio traffic. It's also my understanding, that as the Quadruplexes only had a HF limit of broadcast video ~~5/10MHz [perhaps the modded machines were different], that higher frequencies were 'superheterodyned'/down-converted to fit within the Quadruplex's bandwidth.

I'd imagine CGHQ did something similar with its super-H/S recorders.

--

BTW: presumably, you're El Reg's backdoor on the NSA!

...Duh sorry, 'twas too good to miss. ;-)

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Open sore

A sound engineer at the Open University once told me he always ordered cases of blank TDK cassettes, not just because of quality, but because they contained a consistent amount of tape. Cheaper cassettes tended to come up short, a problem when you have to distribute audio lessons to hundreds of students.

Needless to say he had a constant battle with our good friend Purchasing who would want to substitute whatever brand fell of the back of a lorry lately....

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Facepalm

Important safety tips ..

Back when they had Cathode Ray Tube televisions, with an electron beam being deflected by coils at the back of the set, it wasn't a good idea to store your music tapes on top of the telly.

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Re: Important safety tips ..

It wasn't the deflection coils that caused the problem, as much as the degaussing coil around the front of a colour CRT that was specifically designed to demagnetize anything in the vicinity when switched on! Our cleaning lady had a bad habit of neatly stacking any left-around video tapes across the top of the TV...

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And I'm reminded of my days at Bradford, when we were all ushered into a little room in the back of the university of photography, film and television and given a highly entertaining lecture on the evolution of magnetic recording media fro the very first to the very latest.

I mean it when I say highly entertaining. I've forgotten the chap's name now, but he could weave a story. It helped that he had a lot of props to hand around, including what he claimed was a length of the tape from those old BBC recorders, which was accompanied by a story of the day he took part in or observed an interview that used them. According to him, the operator had at least one finger missing on each hand.

I know it sounds silly and that digital recording has essentially no downsides, but there was always something so very tactile about recording to tape or playing back from it. On the other hand I'll not miss the amount of time it took to transfer rushes from DV to the computer.

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Back to analog

I bought an Otari MX 70 last year.

Although it takes time to align it, clean it, demagnetise the heads, etc, it definitely adds something to the recording.

I've tried plugins for tape saturation effects and some are pretty decent but definitely not as good as the real thing.

Also, when you work all day on computers its really nice to record music without having to stare at a bloody screen!

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FAIL

Unfortunately.......

......magnetic tape never was, is or will be a GOOD medium.

It always was, is and will be CRAP because of the inherent flaws in the fundamental design of magnetic tape.

Shame really, 'cos you got a lot of data/sound/video in a relatively small, cheap space. But there again, you get what you pay for.

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Bah and Double Bah!

For all the talk of degradation, tapes I made on a budget JVC deck in the 1980s still play just fine with no audible artifacts, even though some of them have been stored in a damp basement and some of them have been cooked in the 100+ degree F heat my car can achieve of a summer's day.

Reel to Reel tapes I cut in 1963 still played on the old Stellaphone, albeit with loss of top end, until my dad in a fit of lunacy erased them "to test if the recorder was working properly", thereby committing audio recordings of the Xmas '63 Top of the Pops and a number of rare audio gems (the soundtrack to Steptoe and Son - the attempt to holiday in France episode for one) to the great bit bucket in the sky.

What was really annoying was that he could have bought tape from Radio Shack for a few dollars that would have been in better shape for recording than the fifty year old Phillips Standard Play substrate. Then he wanted me to actually listen to the rubbish he'd recorded, which was just noise from the local radio broadcast that day.

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Posting out of love for the article's title reference to Steely Dan

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