audio-cassette copy-protection?

[J]

Archived from groups: rec.audio.tech (More info?)

The library at school has several instructional audio cassettes they
lend out. Several of these tapes have become somewhat garbled, fading
in and out and echo-ing. The library staff claims that this is
because someone has attempted to copy the cassettes. I say this is
hogwash, as I am unaware of any schemes for copy-protecting audio
cassettes -especially that would destroy the source recording! I'm an
electrical engineer and a recording hobbyist. Am I simply ignorant of
the methods? If I'm right, what is the most likely explanation for
this type of damage? Heat? Magnetic destruction? Wear?

 

[Guest]

Archived from groups: rec.audio.tech (More info?)

>The library at school has several instructional audio cassettes they
>lend out. Several of these tapes have become somewhat garbled, fading
>in and out and echo-ing. The library staff claims that this is
>because someone has attempted to copy the cassettes. I say this is
>hogwash, as I am unaware of any schemes for copy-protecting audio
>cassettes -especially that would destroy the source recording!

I believe you're correct. If you can play back a cassette, then you
can dub the resulting signal onto another medium. Neither the deck
used for playback, nor the tape itself, will have any idea that this
is occurring. There's no way to make the signal uncopyable, nor any
way to "feed back" something to the cassette which would damage the
recording.

The only kind of copying process which might affect the original tape,
is the high-speed thermal-contact method which is (or was) sometimes
used for bulk duplication. It's extremely unlikely that anyone would
have tried this on a cassette of that sort.

> I'm an
>electrical engineer and a recording hobbyist. Am I simply ignorant of
>the methods?

I don't think so. I think you've been told something which is almost
certainly false, by someone who does not understand the situation at
all well.

> If I'm right, what is the most likely explanation for
>this type of damage? Heat? Magnetic destruction? Wear?

Could be any of the above, plus a few more.

Sufficient heat (e.g. leaving the tape on the dashboard or in a glove
box, when the car's out in direct summer sunlight) could possibly
cause the tape base itself to warp or deform, so that it doesn't make
good contact with the heads.

The tape may be shedding its magnetic oxide, either due to wear
(numerous plays), heat damage (affects the binder which "glues" the
oxide to the tape), or a defective binder which either dries out and
sheds, or suffers from moisture-absorbtion "sticky shed".

The tape may have been played in a deck which had an extremely high
level of residual magnetism in one of the heads, partially erasing the
recording as it passed over the head.

My vote is for "cheap bulk-duplication tape stock, which is simply
wearing out after some heat exposure and numerous plays on
poorly-maintained equipment with dirty, abrasive heads."

--
Dave Platt <dplatt@radagast.org> AE6EO
Hosting the Jade Warrior home page: http/www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

 

[Guest]

Archived from groups: rec.audio.tech (More info?)

J wrote:

> The library at school has several instructional audio cassettes they
> lend out. Several of these tapes have become somewhat garbled, fading
> in and out and echo-ing.

The above is a good clue to what happened. Echoing equals print through.
That's when there is a transfer of information from one layer of tape to
another. There is no recovery from this. Prevention requires the tape be
left in a "loosely" wound condition. For most tape machines, never
rewind a tape until needed as fast winding gives a tighter wrap.

> The library staff claims that this is
> because someone has attempted to copy the cassettes. I say this is
> hogwash, as I am unaware of any schemes for copy-protecting audio
> cassettes -especially that would destroy the source recording! I'm an
> electrical engineer and a recording hobbyist. Am I simply ignorant of
> the methods? If I'm right, what is the most likely explanation for
> this type of damage? Heat? Magnetic destruction? Wear?

You are correct, hogwash.

 

[Guest]

Archived from groups: rec.audio.tech (More info?)

In <f9832f34.0409220853.1e7d6673@posting.google.com>, on 09/22/04
at 09:53 AM, baritone_4@hotmail.com (J) said:

>The library at school has several instructional audio cassettes they
>lend out. Several of these tapes have become somewhat garbled, fading
>in and out and echo-ing. The library staff claims that this is
>because someone has attempted to copy the cassettes. I say this is
>hogwash, as I am unaware of any schemes for copy-protecting audio
>cassettes -especially that would destroy the source recording!

There was a proposed (to the US Congress) cassette copy protection
scheme that involved an in band signal and a cooperating recording
deck. If a tape was "marked" as protected, the recording machine would
stop recording. As one would expect, the in band marking scheme was
found to be too noticeable and the whole scheme was scrapped and the
RIAA was unhappy.

> I'm an
>electrical engineer and a recording hobbyist. Am I simply ignorant of
>the methods? If I'm right, what is the most likely explanation for
>this type of damage? Heat? Magnetic destruction? Wear?

If the tape gets too hot, print through (magnetizing adjacent layers)
increases.

If the playback machine's heads are magnetized, some of the high
frequency information on the tape can be damaged.

Heat can physically distort the tape.

A poorly wound tape pack makes edge damage more likely.

Some old tapes will exhibit oxide shedding. This can be so bad that a
"skin" will form on the playback head, act as a spacer and cause high
frequency loss. Cleaning the playback head will correct the problem for
a short while, but the skin quickly reforms. (In my experience, this
problem is more typical of reel-to-reel decks)

Some old tapes are leaching adhesive, which can cause the tape to stick
to the playback machine's guides and heads. This can cause a variety of
problems, including severe scrape flutter or so much tape friction that
the tape may stop.

Dirty playback heads can cause temporary loss of high frequency
information.

----

Examine the tape. Is it in good condition? Dirty cassette machines can
cause tape "creasing" or stretching. The damaged tape can cause a
variety of playback problems. Some or all portions of the tape could be
damaged.

Is the cassette shell in good shape? Is the pressure pad damaged or
missing? Is the playback machine in good working order?

-----------------------------------------------------------
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wordgame:123(abc):<14 9 20 5 2 9 18 4 at 22 15 9 3 5 14 5 20 dot 3 15
13> (Barry Mann)
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[Guest]

Archived from groups: rec.audio.tech (More info?)

Barry Mann wrote:

> In <f9832f34.0409220853.1e7d6673@posting.google.com>, on 09/22/04
> at 09:53 AM, baritone_4@hotmail.com (J) said:
>
> >The library at school has several instructional audio cassettes they
> >lend out. Several of these tapes have become somewhat garbled, fading
> >in and out and echo-ing. The library staff claims that this is
> >because someone has attempted to copy the cassettes. I say this is
> >hogwash, as I am unaware of any schemes for copy-protecting audio
> >cassettes -especially that would destroy the source recording!
>
> There was a proposed (to the US Congress) cassette copy protection
> scheme that involved an in band signal and a cooperating recording
> deck. If a tape was "marked" as protected, the recording machine would
> stop recording. As one would expect, the in band marking scheme was
> found to be too noticeable and the whole scheme was scrapped and the
> RIAA was unhappy.

It involved a notch filter at around 1kHz IIRC. Also required all cassette
recorders to be fitted with a matching anti-copy 'chip' or circuit as would
be more likely back then..

You might be surprised how far it got before the powers that be were shown
that it made sound quality dreadful and therefore a *bad thing*. RIAA
didn't care about the quality issue for sure.

Graham