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High DAT head rotational speeds coupled with significant head penetration
(not just "in contact" but actually deforming or dimpling the tape) results
in removal of some of the coating. With repeated passes, the debris can
"snowplow" or accumulate at the beginning and end of the area used, and also
can accumulate on the head, resulting in redeposits of the debris elsewhere
on the tape or the infamous "head clogs". This issue represents a serious
disadvantage of both 4 mm (DAT) and 8 mm helical scan tapes.

Although DAT and 8 mm have powerful Reed-Solomon Product Code error
detection and correction capabilities, dropouts from this debris can
accumulate to a level where error correction is no longer possible. The
referenced "cleaning" actions can burnish, or polish, the tape thereby
removing some but not all of this debris. Because the debris originates from
the coating, it bonds tightly to it when redeposited, and tape with problems
remains suspect. Furthermore, head cleaning does not remove all of the
material that has accumulated on the head, and may necessitate replacement
of the head or even the drive.

In years past, Media Sciences used helical scan tape testers from Media
Logic to evaluate 4 mm and 8 mm quality. We found big differences between
media and also between drives. Also, subsequent generations of drives often
provided higher capacities, and their designs were very different from their
predecessors even though they were backward compatible. For example, an
early generation drive might have a larger head gap than a later version,
and might be able to read some tapes that later drives could not.

Jerry Hartke
Media Sciences, Inc.

> -----Original Message-----
> From: Association for Recorded Sound Discussion List
> [mailto:[log in to unmask]] On Behalf Of Shai Drori
> Sent: Thursday, January 21, 2010 4:15 AM
> To: [log in to unmask]
> Subject: Re: [ARSCLIST] DAT ripping
> 
> Okay, that I understand, but I am thinking about correctable errors. We
> are then assuming that both systems will correct the errors the same way
> since both use the schemes implemented. How do we know which system has
> fewer errors over the other? My experience with rotary head systems is
> that sometimes second or third reading yielded better results, I think
> due to "cleaning" actions of the previous playing. Maybe we should
> compare five readings of the same cassette?
> Shai
> 
> Ted Kendall wrote:
> > In my view, yes.
> >
> > Consider - you have two data files. One is a text document (for the
> > sake of argument). The other is a digital audio file. Both have errors
> > in the storage medium. This is inevitable, so we devise error
> > correction strategies (redundancy, check codes, etc). These allow us
> > to correct errors completely and accurately.
> >
> > Suppose now that there is an error in the storage medium which is too
> > large to be corrected. This will cause an obvious error in the text
> > file, which is unaceptable, so the system does not allow for it and
> > declares the file corrupt. The audio file, however, can be rendered
> > inoffensive by interpolation, and this is implemented in the DAT audio
> > format. If we retrieve DAT audio in a system which does not admit of
> > interpolation, we therefore know that the data are accurate, as any
> > uncorrectable errors are recorded as such.
> >
> >
> >
> > ----- Original Message ----- From: "Shai Drori" <[log in to unmask]>
> > To: <[log in to unmask]>
> > Sent: Thursday, January 21, 2010 7:37 AM
> > Subject: Re: [ARSCLIST] DAT ripping
> >
> >
> >> My own experience with dat is that almost all tapes have some form of
> >> errors on them. I think the idea in dds is that errors are better
> >> fixed than dat machines. There were many machines that came off
> >> assembly lines not at spec, thus making the tape not a standard tape.
> >> Some machines are better at coping with these (my experience with
> >> Sony is better than tascam for example, but I suspect this is highly
> >> subjective). All in all, I think the DAT format was the word digital
> >> format I have ever come across.
> >> Also' checking two files one against the other will not necessarily
> >> prove one format better than the other. If you get some audio, how
> >> can you be sure one stream is correct and the other is corrupt?
> >> Either the dat or dds stream could be better, or am I missing
> >> something in the methodology?
> >> Shai
> >>
> >> Tom Fine wrote:
> >>> I'm happy to do a SPDIF to hard drive transfer and then exchange
> >>> tapes with someone using a PC-drive transfer system so we can do the
> >>> comparison Richard mentions.
> >>>
> >>> Please ping me off-list if you have a working PC-drive transfer
> >>> chain and want to exchange DATs and computer files.
> >>>
> >>> -- Tom Fine
> >>>
> >>> ----- Original Message ----- From: "Richard L. Hess"
> >>> <[log in to unmask]>
> >>> To: <[log in to unmask]>
> >>> Sent: Wednesday, January 20, 2010 5:14 PM
> >>> Subject: Re: [ARSCLIST] DAT ripping
> >>>
> >>>
> >>>> Hi, Tom,
> >>>>
> >>>> After I saw your post and re-read Jim's post, I think I understand
> >>>> where he is coming from.
> >>>>
> >>>> What we _should_ be able to do is take the DDS ripped file and an
> >>>> AES/SPDIF'd copy of the DAT from an audio DAT machine, align the
> >>>> starts, invert the phase of one, and get dither or silence.
> >>>>
> >>>> In both instances, we're pulling numbers off the tapes (although
> >>>> the basest representation of the numbers is analog on the tape, the
> >>>> processing in both instances interprets these analog signals as
> >>>> either ones or zeros).
> >>>>
> >>>> I would not, without doing the tests that Jim is talking about, be
> >>>> 100.0000% confident that the two files are identical.
> >>>>
> >>>> I think that the DDS reading could be "better" than the audio DAT
> >>>> reading as there is no error concealment stage in a data recorder,
> >>>> so if you grabbed all the bits via the DDS route, you could be sure
> >>>> that they were correct.
> >>>>
> >>>> These are all subtle differences and are probably not as large as
> >>>> the "Interstitial Errors" that Chris Lacinak is talking about here:
> >>>> http://www.avpreserve.com/wp-
> content/uploads/2010/01/Digital_Audio_Interstitial_Errors.pdf
> >>>>
> >>>>
> >>>> I would expect some burst differences between the two methods, and
> >>>> those bursts would be where the audio DAT's error concealment
> >>>> kicked in. Other than that, they should be identical, presuming you
> >>>> haven't introduced an interstitial error in one copy or the other.
> >>>>
> >>>> I'm glad to see Chris offering to help. I am interested in this. I
> >>>> would also like to know who is set up with the DDS Mass Ingest of
> >>>> DATs as I am sometimes asked who can do large DAT collections. At
> >>>> the moment, I am not interested in doing any because of anticipated
> >>>> remaining headlife on my machines, the growing lack of parts for
> >>>> DAT machines, the need to transfer my own DAT collection first, and
> >>>> the analog work that I have piling up.
> >>>>
> >>>> Cheers,
> >>>>
> >>>> Richard
> >>>>
> >>>> At 04:27 PM 2010-01-20, Tom Fine wrote:
> >>>>> Hi Jim:
> >>>>>
> >>>>> How could the data be "better" than a direct-digital out from a
> >>>>> properly-working player (ie no head problems or mechanical
> >>>>> issues)? I thought the main advantage of the computer-drive method
> >>>>> was to save time. Is there more to it?
> >>>>>
> >>>>> -- Tom Fine
> >>>>>
> >>>>> ----- Original Message ----- From: "Jim Sam" <[log in to unmask]>
> >>>>> To: <[log in to unmask]>
> >>>>> Sent: Wednesday, January 20, 2010 12:45 PM
> >>>>> Subject: Re: [ARSCLIST] DAT ripping
> >>>>>
> >>>>>
> >>>>>> All,
> >>>>>>
> >>>>>> First, Dave, that information is very helpful.
> >>>>>>
> >>>>>> That said, I didn't ask because I'm worried about the theory.  I
> >>>>>> was asking
> >>>>>> for a collaborator in testing.
> >>>>>>
> >>>>>> The theory's been discussed before on this list, and I'm aware
> >>>>>> that more
> >>>>>> than one person/organization has experimented with this to some
> >>>>>> success.  It
> >>>>>> was also *briefly *discussed at last year's conference in DC.
> >>>>>> However,
> >>>>>> every time I've seen a discussion about the topic, it has never
> >>>>>> come along
> >>>>>> with what matters to me: testing to make sure what's coming off
> >>>>>> the DDS
> >>>>>> drive is the same (or better) data than what would go down the
> >>>>>> AES/EBU
> >>>>>> pipeline.
> >>>>>>
> >>>>>> I'm still extremely interested in this situation, and after
> >>>>>> having had to
> >>>>>> deal with other similar formats, I've got ideas for testing that
> >>>>>> I'd like to
> >>>>>> do.  But I don't have a working DDS setup here.  I could build my
> >>>>>> own, which
> >>>>>> I might do, but that's a can of worms, and there's other things
> >>>>>> to be gained
> >>>>>> by having a collaborator in these tests.
> >>>>>>
> >>>>>> Thanks,
> >>>>>> Jim
> >>>>>
> >>>>> Richard L. Hess                   email: [log in to unmask]
> >>>>> Aurora, Ontario, Canada       (905) 713 6733     1-877-TAPE-FIX
> >>>>> Detailed contact information:
> >>>>> http://www.richardhess.com/tape/contact.htm
> >>>>> Quality tape transfers -- even from hard-to-play tapes.
> >>>>
> >>>
> >>
> >