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ARSCLIST  January 2011

ARSCLIST January 2011

Subject:

Self erasing of magnetic tape?

From:

Goran Finnberg <[log in to unmask]>

Reply-To:

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

Date:

Fri, 14 Jan 2011 21:36:05 +0100

Content-Type:

text/plain

Parts/Attachments:

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text/plain (270 lines)

Tom Fine:

> Well, Goran, bully for your magic tapes! ;)

At least there is some substance in my claims.......:-)

> All of mine (Maxell, TDK and others) lost level just
> sitting around in typical northeastern US home environment
> conditions, and then on playback (on the same deck on
> which they were recorded) Dolby C mistracks on the
> high-frequency band.

But this is your highly subjective reaction many years after the cassette(s)
was recorded.

As for Dolby C mistracking how can you be so sure that it wasnt that way
from the beginning.....getting older means getting wiser hearing things that
went unnoticed years ago......and nowadays using much better equipment so
one can truly hear what is there.

My own memory would be totally unreliable at least and my ears are noway as
good hearing top today compared to way back so I have compared my own high
speed duplicated cassettes with the original 1/4" master tape in most cases
and both 1/3 octave analysis and the ear tells me that theres very little
difference.

But it should be as my target was +- 1 dB from 30 Hz to 18 kHz through the
whole chain from the rebuilt 3M79/Studer A80 who made the loop bin masters
on 1" and 1/2" tape. Replay in the Ampex BLM3200/Lyrec/Gauss 1200/2400 at
64X loop bins then onto the slaves at 64X speed.

The Ampex 3200 ran originally at 32X but I converted this to run at 64X
speed no problemo at all.

Output per hour is of monetary importance to the bean counters as always.
;-)

Absolutely no problems achieving this frequency response tolerance in
practice using the very stable AGFA cassette tape and using the Studer A80
QC for cassette tape monitoring.

> I would say that it's more high frequency dropoff
> than overall loss of level.


> I've read on this list and elsewhere that this problem
> also occurs on the professional Betacam tapes made
> by news organizations, which apparently also used Dolby C.

And was the Azimuth guaranteed to be correctly adjusted for each betacam
tape replay?

And can we be absolutely sure that the machines was correctly adjusted in
the very beginning of these tapes?

?

AND using a high frequency limiter in front of the Dolby C encoder?

See below near the end.

> As I understand comments on the Ampex list and elsewhere,
> tapes do self-erase over time.

Self erase?

Jay Mcknight and Bertram, world expert in magnetics says this should not
happen.

So far every time I hear about this there is not a shred of real substance
in these comments.

Most people just thinks that it sounds duller than CDs thats all....so we
are back to subjective arguments which are correct as far as is felt by
individual persons.

At least my own measurements on cassette test tapes done since 1973 from
RCA, BASF, Nakamichi and others using fluxloop and gap loss corrected
Nakamichi decks starting with the Nakamichi 700 lets you have an absolute
reference irrespective of time so I can remeasure a 35+ old cassette test
tape and compare it to measurements done when the test tape was new and so
far there is NO loss at either short or long wavelenghts IF the tapes have
been well cared for.

> I don't think anyone has figured out exactly why, have they?

Jay McKnight et al strictly denies that there is any such mechanism at all.

> With a Dolby B cassette, apparently self-erasure doesn't
> result in noticeable Dolby "pumping" or other mistracking
> problems as quickly as this happens with Dolby C tapes.
> At least not in my experience. What happens with Dolby B
> tapes is that they get to sounding "dull" and need a
> treble boost to sound "lively" again.

And how can you be sure that what it sounded X years ago is not the same
today?

CDs sound very bright on cheap equipment while on my reference system
sounds a bit dull which is exactly as it should be.

Feeding my DAC with a signal coming from my ADC straight from Ampex ATR102
or Studer A820 at 15/30" Master tapes and then switching between tape direct
and digitized tape shows near zero difference.

> By the way,  I've run into the same thing with Dolby A reels,
> with azimuth aligned with a scope, the level-set tones are
> sometimes lower than expected

As the tape machines once used to make these tapes cannot be guaranteed to
be set up correctly then I am not surprised at all that the tones can be all
over the map.

In an interview in RE/P Beatles Producer George Martin stated that in his
studio in Montserrat he always used Dolby A because he had an excellent in
house tech who made sure that things worked as they should by adjusting
things the way it was meant to be by the maker.

He stated that when working in other studios his preference was to not use
Dolby A NR because of the fact that many studios had tape machines and NR
which was not calibrated correctly.

Which makes good sense indeed.

> and in those cases it's great to have Dolby tones to get the
> CAT22 to track well on decode.

Hopefully azimuth and replay EQ matches exactly the machine used for
recording too....

> I've always ascribed this to self-erasure over time.

I sincerely doubt this self erasure subjective thing.

There is absolutely no evidence that this really has any basis in fact so
far.


> It's not drastic on the reels I've encountered,
> maybe 2-3dB at most. It seems to be more drastic
> on cassettes, I figure it's because cassettes have
> a much denser data-per-square-inch pack, so
> any sort of physical phenomena will have more
> drastic results on the audio.

So far nothing but subjective empirism.

If I may make an educated guess whats going on:

There IS indeed "High Frequency Self Erasure" going on in all magnetic
media.

Cassette tape is ONLY flat, Nakamichi using 0.8 M replay gaps, to 20 kHz at
around -20 dB relative 200 nWb/m reference fluxivity.

At 0 dB 200 nWb/m reference fluxivity the frequency reponse is maybe down
around -10 dB at 10 kHz due to "High Frequency Self Erasure".

This is due to the fact that magnetic tape compress at LF but self erases at
HF.

When you reach the self erasure point at short wavelenghts which is at
around -10 dB @ 10 kHz rel 200 nWb/m on good quality ferric tape @ 1 7/8"
speed then if you increase the input level at 10 kHz you will find that the
replay level GOES DOWN SHARPLY!!

And with a -4 dB 10 Khz signal the output has fallen say -15 dB in other
words it "Self Erases".

This is the reason why we got Dolby HX pro.

CRO2, Crome, tape ups the self erasure point to around -6 dB @ 10 kHz rel
200 nWb/m this using 70 S playback EQ so it is in fact nearly 9 dB better
than the best ferric tapes taking into account the use of 70 S eq vs 120 S
eq for ferric tape.

If you listen to the replay when fed large high frequency tones then you get
all sorts of nasty noises when things self erases in the treble.

Now when working professionally in the high speed tape duplicating buisness
the distorted spittiness on drums, voice, tambourine etc is unbearable so we
use the Ortofon STL 732 accelleration limiter as designed for disk cutting
to be equally usable for cassette duplication.

When I compared my high speed duplicated cassettes to the 15" master tapes
available to me I used my own STL 732 to have the master tape sound the way
they did when sent to the bin loop master.

Because, in fact, the transient attack in the high frequency area would be
some 10-15 dB higher at 10 to 15 kHz direct from the 15" master tapes.

If one is not using a treble limiter then the treble self erasure phenomena
will make the DOLBY B, C circuits mistrack horrendously since the decoder
will see a 10 -15 dB less level at the onset of the high frequency transient
compared to what the encoder got.

So it will sound very dull indeed using Dolby B or C.

Using a high frequency limiter in front of the Dolby B encoder will of
course dull the transient attack of the 15" master but it will make sure
that the dolby decoder sees the same signal as the encoder so it will sound
the same as the input.

We have exactly the same problem when cutting LP records where the frequency
response can be quite flat at the outside of the disk but near the center we
see losses of some 10 to 15 dB at 10 kHz too because of the slow speed of
vinyl past the facet of the cutting stylus which will self erase short
wavelenghts.

Some tried diameter eq to offset the loss but this only makes things sounds
screechy and distorted just as compact cassettes when fed to much high
frequency level and the very high frequency distortion created as it
literally self erases.

THIS high frequency self erasure, in my opinon, is what makes the compact
cassette sound dull no matter what.

Very few got it right in large scale high speed duplication in my
experience.

A very nice trick we did for some VERY picky customers was to use CRO2,
Crome, tape in the slaves using 120 S ferric EQ on the Gauss 2400 which
made the high frequency self erasure point at 10 kHz be at +1 rel 200 nWb/m
so one could have almost the full high frequency transient impact fom the
15" master onto the cassette even with dolby B.

Those sounded faboulus indeed!

Really bright detailed and punchy.....

But we had to use 32X duplicating speed since the Studer A80 MR at 3 3/4" on
the bin loop tape, which is used for 64X duplicating speed, self erased at -
6 dB at 10 kHz even with Dolby HX on the Studer A80 HX.

No one in their own homes have an inkling what is truly going on and dont
have access to a high frequency limiter so this type of compact cassette
have, most of the time various degrees of high frequency loss caused by high
frequency self erasure and such cassettes seems to age less well compared to
those using a professional high frequency limiter in _my_ experience
_subjectively_.


So.....Everyone is entitled to his opinon but none of the arguments  makes
any sense to _me_ so far.


Who during 15 years worked professionally in a high speed cassette
duplicating plant outputting some 20.000+ cassettes a day 5 day week and
during peak demand we were working 24 hour shifts 7 day week for months on
end.

Those were the days to remember having met so many of those experts, of
those days, in all areas who shared their expert knowledge with me.

Thanks guys for all your shared knowledge!


-- 
Best regards,

Goran Finnberg
The Mastering Room AB
Goteborg
Sweden

E-mail: [log in to unmask]

Learn from the mistakes of others, you can never live long enough to
make them all yourself.    -   John Luther

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