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Discussion in 'Electronic Design' started by Dirk Bruere at NeoPax, Jun 7, 2010.

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  1. Dirk Bruere at NeoPax wrote...
    That's a simple AC signal to DC logic conversion,

    .. | \
    .. AC ---||---+---| >O--+----
    .. | | / |
    .. | |
    .. '---/\/\---'

    I'd say, yes, in the absence of any signal, an HCU04 gate
    would do a better job of settling at 1/2 Vcc, and perhaps
    do so with less class-A current, but in the absence of a
    signal, who cares what happens? In practice, S/PDIF lines
    have a continuous AC signal, so this simplified circuit's
    feedback RC node should find the average of the incoming
    data stream. Also, the continuous huge signal means the
    gates won't have a class-A rail-rail current problem.

    s/pdif data doesn't have a 50% duty cycle, which gives me
    pause, but Andrew Kilpatrick, Randy McAnally and ESP =
    Rod Elliott swear by it, so it must be adequate. :)

    OTOH, you can get official s/pdif receiver connectors and
    ICs for a few bucks, and that seems a bit more professional.
  2. Winfield Hill wrote...
    Check that, s/pdif's biphase mark code is more than
    close enough to a 50% average for good RC coupling.
  3. Grant

    Grant Guest

    Averages 50% in the longer term, that's why it's an AC coupled signal.
    Yes, but there's more to life than doing everything efficiently ;)

  4. In the following schematics page 17 upper left corner, you see a working
    solution (I have the eval board, its working nicely in coax SPDIF mode).


    Michael Randelzhofer
    FPGA und CPLD Mini Module
    Klein aber oho !
    Tel: 08131 339230
    Usst.ID: DE130097310
  5. M.Randelzhofer wrote...
    It's interesting that TI picks an 'U04. An 74LVC2GU04
    to be exact. They do use this simple ac-coupled-circuit
    in a PurePath Digital reference design for their rather
    sophisticated custom silicon for high-performance audio.
  6. Jim Thompson wrote...
    Right, I'm well aware of that. Old stuff, dating back to
    the 60s when I started using cmos, or COS/MOS as RCA called
    it back then. But the question was about the viability of
    non-U types for this, do you have an opinion about that?
    Not what's naturally best, but whether it'll work OK.
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