Re: Recording digital data to analog tape... revisited

From: H. Peter Anvin (hpa_at_terminus.zytor.com)
Date: 10/03/04

  • Next message: Lewin A.R.W. Edwards: "Re: Recording digital data to analog tape... revisited" 
    Date: Sun, 3 Oct 2004 01:12:20 +0000 (UTC)

    Followup to: <415F4778.4080203@nospam.com>
    By author: Fred Bloggs <nospam@nospam.com>
    In newsgroup: comp.arch.embedded
    >
    > Do you think this is necessary for a kHz data stream? It's analog right-
    > so a simple lowpass schmitt should recover it nicely.
    >

    Well, it's sort of.

    A very good format for recording to magnetic tape in the low kbps
    range is MFM; it's simple, quite robust w.r.t. speed differentials,
    and polarity-independent (unlike Manchester coding.)

    MFM works like this:

    For each bit time, at t = 0 there is an edge (toggle whatever level
    the output is at the time.) If the bit value at 1, at t = 0.5 there
    is another edge; otherwise the level stays the same.

    During decode, use the first edge for synchronization; if there is
    another edge between t = 0.25 and t = 0.75 or so per your own clock,
    then record a 1 otherwise a 0. For higher bit rates you may want to
    try to produce a clock from the synchronization pulses instead of
    using your own "nominal" clock, but the error window is huge so it
    works well.

    This was used by the ABC series computers used in Sweden 1978-1986; a
    full schematic of the ABC80 machine is at:

    http://www.abc80.net/Datorerna/Luxor/Luxor_HW_Info/abc80schema.zip

    The ABC80 used 700 bps; the later 800 machine used 2400 bps but the
    same modulation technique.

    If you want higher bit rates you should start to consider some of the
    techniques used by modems, like quadrature decode and multilevel. The
    biggest issue about using those on a cassette tape is that you have
    nonlinear distortion due to speed differences; those are hard to
    analyze in the context of especially quadrature decode. An
    alternative is to use digital techniques which put in synchronization
    pulses in fewer places (if you remember RLL hard drives, that was one
    such technique.)

            -hpa

  • Next message: Lewin A.R.W. Edwards: "Re: Recording digital data to analog tape... revisited"

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