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Subject: Digital Audio Tape

Digital Audio Tape

From: Gilles St-Laurent <gsl>
Date: Thursday, May 19, 1994
Richard Pearce-Moses <iacrpm [at] asuvm__inre__asu__edu> writes

>Aside from the general problems of magnetic media, is anyone aware of
>particular problems or benefits of digital audio over analog audio for
>tape recordings?

DAT was originally designed for the consumer market and for various
reasons made its way into professional use. There are several advantages
to DAT technology:

    *   It is a standardized digital medium.

    *   Any place on the tape can be numbered for quick reference (the
        code is imbedded on the tape).

    *   Some players have a shuttle wheel to allow playback at various
        speeds (forwards and backwards) while maintaining proper pitch;
        in other words one can play a tape at half-speed while
        maintaining the original recorded pitch. This can be very useful
        for the comprehension of difficult to understand material.

    *   The DATs can hold up to two hours of material in a small
        cassette (including case roughly 3 1/8" by 2 1/4" by 5/8").

    *   Time base errors (tape not running at steady speed) can be
        corrected by buffering (information loaded into memory chips and
        played-back at a regular rate).

    *   DATs can be copied in the digital domain, bypassing the Digital
        to Analog and Analog to Digital converters for better quality

    *   There are powerful error correction circuits built into the
        playback to compensate for some random errors due to dirty
        heads, tape dropouts, etc.

    *   Because the information is recorded with some redundancy,
        certain errors can be corrected fully.  The DAT player can
        rebuild the information.

Unfortunately for long term storage purpose, I believe that these
advantages are outweighed by the following disadvantages:

    *   Because the information is recorded so densely, a small
        imperfection on the tape due to dust, physical damage or aging,
        can affect a large portion of the recorded material.

    *   If an error is sufficiently large that the information cannot be
        rebuilt, the player will interpolate ("guess") what the
        information should be.  If the error burst is too large to
        interpolate, then the signal is muted until the machine can find
        useable information.

    *   Because DAT uses a rotary head to record and playback (like a
        VHS recorder) the tape is subjected to greater stress and wear
        than stationary head media (reel to reel tape, cassette, etc).

    *   With any machine readable technology, one is at the mercy of the
        manufacturer for any repair to the equipment.  Will the
        manufacturer still have parts in a few years or decades when,
        invariably, the technology becomes obsolete?  Can the technology
        be rebuilt easily?

    *   Because DATs cassettes are so small, security becomes an
        significant issue.

    *   Though DAT is standardized, not all DAT players are created
        equal.  For instance, there is a TV broadcast standard, yet
        there is considerable difference in picture quality between
        different television models.

The periodical "Studio Sound and Broadcast Engineering" undertook a
series of accelerated aging tests on various brands of DAT tapes.  The
results are fascinating.  That article and other articles relating to
DAT technology were republished by HHB (not surprisingly the
manufacturer of the first-placed tape) in a single 18  page pamphlet. If
you are considering DAT technology, it would be wise to read it.  The
pamphlet is available through:

    HHB Communications Ltd
    73-75 Scrubs Lane
    London NW10 6QU
    Fax: 081-960-1160

    Independent Audio
    295 Forest Avenue, Suite 121
    Portland, Maine 04101-2000
    Fax: 207-773-2422

    Studer Revox
    1947 Leslie
    Toronto, Ontario
    M3B 2M3
    Fax: 416-510-1294

Gilles St-Laurent
Music Division
National Library of Canada

                  Conservation DistList Instance 7:84
                   Distributed: Sunday, May 22, 1994
                        Message Id: cdl-7-84-001
Received on Thursday, 19 May, 1994

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