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ARSCLIST  April 2015

ARSCLIST April 2015

Subject:

Re: More tales of woe from the tape vaults

From:

Marie O'Connell <[log in to unmask]>

Reply-To:

Association for Recorded Sound Discussion List <[log in to unmask]>

Date:

Thu, 9 Apr 2015 18:46:04 +1200

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (284 lines)

Thank you Richard Hess, you do rock!

On Thu, Apr 9, 2015 at 5:03 PM, Jeff Willens <[log in to unmask]> wrote:

> Richard,
>
> You rock!
>
> I'm not sure that either Partial Erasure or Magnetostriction are involved
> here simply because this issue ran across more than one reel coming from
> different batches, different studios and different years, yet all were
> Ampex, mid-1970s. But who knows, since all those tapes may have been pulled
> for the same type of project years before.
>
> Really the most likely causes I'd think would be:
>
> (4) loss of mag coat due to normal unbaked sticky shed deposits on heads
> and
> guides
>
> (5) increased spacing loss due to warping or embossing of the tape or other
> mechanical/chemical defects
>
> And possibly this, or some variation thereof:
>
> (7) Loss of magnetic information where the magnetic particles stop being
> permanent magnets.
>
> Has this ever been investigated as a side effect of too much cumulative
> bake
> time in an oven over a tape's lifespan?
>
> I had a 2" multi that I had to bake in order to transfer to make a safety
> copy. No matter how long I put it in the oven (hours, days, etc.), it still
> would not play without shedding. I was finally able to get exactly one
> pass,
> and that was it. There was just no way this tape was going to withstand
> another baking regimen.
>
> Jeff
>
>
>
>
> On Wed, 8 Apr 2015 08:58:36 -0400, Richard L. Hess
> <[log in to unmask]> wrote:
>
> >Jeff,
> >
> >That is scary.
> >
> >There are two magnetic-related failure modes I can think of that MIGHT
> >have contributed. I'm mentioning them to see if you think either (or
> >both) might be at play.
> >
> >(1) Partial erasure: this is the more common and better understood
> >failure mode. It involves relatively low-level magnetic fields. These
> >are also applied for "skimming" to reduce print through and can be
> >unintentionally applied by magnetized heads and guides, though uncommon,
> >can happen.
> >
> >(2) Magnetostriction: This is less-well understood and the only case I'm
> >aware of that it was consistently applicable was on cassettes and since
> >this is arguably a wavelength dependent failure mode, related to sharp
> >bends of short-wavelength recordings, is probably not applicable.
> >
> >Then, as you (or someone) mentioned, there are the physical damage modes:
> >
> >(3) loss of mag coat due to ripping out by an unbaked playback attempt
> >
> >(4) loss of mag coat due to normal unbaked sticky shed deposits on heads
> >and guides
> >
> >(5) increased spacing loss due to warping or embossing of the tape or
> >other mechanical/chemical defects
> >
> >(6) poor azimuth performance due to country lane-ing usually resulting
> >from poor slitting but can be warped in that direction as well due to
> >poor winding and storage.
> >
> >(7) Loss of magnetic information where the magnetic particles stop being
> >permanent magnets. This is called the "Curie temperature" or "Curie
> >point" (Tc) and is usually not considered a factor. For two common iron
> >oxide formulations, it is:
> >Iron(III) oxide (Fe2O3) 948 K 675 °C 1,247 °F
> >Iron(II,III) oxide (FeOFe2O3) 858 K 585 °C 1,085 °F
> >
> >BUT, Chromium dioxide's Tc is lower:
> >CrO2 386 K 113 °C 235 °F
> >While that is above the recommended baking points (by more than a factor
> >of 2), it is not out of the question that these temperatures could have
> >been reached without destruction of the tape base film. The glass
> >transition temperature (Tg) of PET is about 67-81 C and that of the mag
> >coat is lower, sometimes below room temperature. The Tg is the point
> >where the polymer becomes rubbery. The melting point of PET is at least
> >250 °C and the boiling/destruction point is at least 350 °C. While I
> >quickly grabbed these PET numbers from Wikipedia, I recall that they are
> >in keeping with what I've seen from other sources.
> >
> >Anyway, I'd appreciate your thoughts on how these magnetic records were
> >damaged (or rendered not playable) at that point in time.
> >
> >Cheers,
> >
> >Richard
> >
> >
> >On 2015-04-08 1:06 AM, Jeff Willens wrote:
> >> Worn out tapes are absolutely in the vaults right now, and have been for
> >> some time. I've come to find out (and perhaps others can correct me),
> that
> >> there is a finite number of hours a polyester-backed tape may be baked
> >> cumulatively before it stops working on the binder and/or it stops
> giving
> >> the fidelity it used to offer.
> >>
> >> As for tapes in record company vaults, having worked at more than one
> major
> >> label, I can say that you just don't know who's been on a tape before
> you
> >> get hold of it. Some engineer may have tried to play a shedding Ampex
> 456
> >> tape for a transfer without baking it, and lost a ton of oxide in the
> >> process years before you. If so, there went your high end, and all your
> detail.
> >>
> >> I had to do tape transfers of a very prominent 70s R&B band (who will
> remain
> >> nameless) for an outside mastering engineer doing a CD compilation.
> They HAD
> >> to be from the "original masters". All the masters were on Ampex 456
> and 406
> >> 1/4". All had been baked and used scores of times over the years. I
> checked
> >> and baked the tapes for our usual amount of time, set up my machines,
> did
> >> the azimuth, etc. All was set to go. I put up the tapes. And they
> sounded
> >> like crap. Every last one of them. I double checked my entire set up. I
> >> checked the tape path. I had other engineers checking everything.
> Nothing
> >> made sense. All the tapes sounded like I was playing them from the
> outside
> >> -- muffled, muddy, and smeared. Still, I transferred them flat and sent
> them
> >> on.
> >>
> >> 2 days later, an extremely irate mastering engineer called me up and
> >> demanded to know why I transferred the tapes inside out. When I
> explained
> >> what happened, he couldn't believe the tapes could just "wear out" like
> >> that. But that is what happened. They were utterly shot. No amount of
> >> baking, EQ, azimuth tweaking, or sonic wizardry at the time was going to
> >> bring these things back from the dead. The ME used other sources.
> >>
> >> So it's not necessarily "shelf life" that contributes to a reel of
> tape's
> >> demise, but the use and handling it may get over the years that does the
> >> ultimate damage. And that damage can be irreparable.
> >>
> >>
> >>
> >>
> >>
> >> On Tue, 7 Apr 2015 19:06:13 -0400, Robin Hendrickson
> >> <[log in to unmask]> wrote:
> >>
> >>> I had a conversation once with someone who worked in the
> >>> Capitol/EMI/Universal system, and this person told me that the "the
> >>> old tapes are wearing out" and that all tape has a maximum shelf life
> >>> of 30 – 40 years.
> >>>
> >>> That sounded like BS to me; I know there are older tapes out there
> >>> that still work fine. (No guarantees, of course.)
> >>>
> >>> This Tom Petty story made me wonder whether there is some fallacious
> >>> conventional wisdom out there that would lead one to rush to a verdict
> >>> of "Yep, no way these old tapes are gonna work, we have to use
> >>> something else."
> >>>
> >>> The mind reels. Excuse the pun, this is a serious matter indeed.
> >>>
> >>> Robin
> >>>
> >>>
> >>>
> >>> On Tue, Apr 7, 2015 at 5:26 PM, Richard L. Hess
> >>> <[log in to unmask]> wrote:
> >>>> On 2015-04-07 3:18 PM, Eric Jacobs wrote:
> >>>>>
> >>>>> 2-inch tapes with sticky shed may not respond as well to baking as
> >>>>> 1/4-inch.
> >>>>>
> >>>> That is true in the sense of in the same or similar time frame.
> >>>>
> >>>> I don't think there are precise formulae for predicting the time
> either to
> >>>> achieve thermal equilibrium or to achieve moisture equilibrium in a
> tape
> >>>> pack. Vos (1994) inspired me to develop a rule of thumb that moisture
> >>>> equilibrium appears to take 1500 times as long as thermal equilibrium
> in a
> >>>> one-inch tape, based on my extrapolations from his curves.
> >>>>
> >>>> We have long suspected that the width of the tape was a large
> modifier of
> >>>> this ratio. I based my estaimate on Vos's graphs which seemed to
> indicate
> >>>> that a 1-inch tape pack, might achieve thermal equilibrium might in
> 100-200
> >>>> minutes while it might take 100-200 DAYS to achieve moisture
> equilibrium. I
> >>>> felt that a factor of 1440 implied far too much precision in the
> >>>> calculation, so I rounded it to 1500.
> >>>>
> >>>> Further pointing to this is what Stuart Rohre has reported on the
> Ampex
> >>>> mailing list and elsewhere. He has been responsible for retrieving
> the most
> >>>> information possible from some 1-inch instrumentation tapes which are
> >>>> 15-inch diameter tape packs on glass precision Corning reels with no
> >>>> windows. The windowless reels further slow moisture diffusion. He had
> >>>> originally said they were baking for several days and could get
> through
> >>>> about half the tape and then had to rebake, but they also had to run
> the
> >>>> tape through their Bow tape cleaners. Partially at my suggestion and
> >>>> partially on his own initiative, Stuart found that if he baked the
> tapes for
> >>>> 30 days, they would play through without the need for any tape
> cleaning or
> >>>> re-baking and he was getting very clean signals off the tapes at that
> point.
> >>>>
> >>>> So, are the two-inch tapes not responding to baking or simply in need
> of
> >>>> more of it?
> >>>>
> >>>> One 7-inch reel of 1/4-inch tape that had been exposed to high
> humidity
> >>>> cycles overnight had a very easy-to-remove mag coat when first
> inspected.
> >>>> When it was stored in my air-conditioned home (minus the economizer
> cycle
> >>>> bringing in Los Angele's famed "Marine Layer" of "night and morning
> low
> >>>> clouds") for 3-4 months, the same test that initially showed mag coat
> >>>> removal could not be duplicated and the tape binder seemed very
> secure at
> >>>> that point.
> >>>>
> >>>> The "more baking" concept pertains to tapes like Ampex 456, 406, and
> 407 as
> >>>> well as the instrumentation tapes made by Ampex at about the same
> time. It
> >>>> may also apply to Scotch 226 and 227 and possibly Scotch 250. It
> probably
> >>>> does not apply to Agfa tapes which have some of their own nastiness.
> >>>>
> >>>> This web page attempts to categorize tapes by degradation modality,
> and
> >>>> degradation modalities are currently described more by what can
> ameliorate
> >>>> their effect than by the actual chemical/mechanical failure modes. My
> >>>> decade-long goal of a "pool-test kit" for tape degradation
> measurement is
> >>>> farther in the distance than it was when I started the quest.
> >>>>
> >>>>
> >>
>
> http://richardhess.com/notes/formats/magnetic-media/magnetic-tapes/analog-audio/degrading-tapes/
> >>>>
> >>>>
> >>>> Cheers,
> >>>>
> >>>> Richard
> >>>> --
> >>>> Richard L. Hess email: [log in to unmask]
> >>>> Aurora, Ontario, Canada 647 479 2800
> >>>> http://www.richardhess.com/tape/contact.htm
> >>>> Quality tape transfers -- even from hard-to-play tapes.
>

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