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spdif and toslink switch

Discussion in 'Electronic Design' started by JESSD, Jan 2, 2004.

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  1. JESSD

    JESSD Guest

    I'm designing a spdif/toslink (digital audio) switch and would like
    some advice. The switch can convert from optical to coax, coax to
    coax, optical to optical, or coax to optical automatically. I've got
    a circuit that works for the output portion that can drive both the
    optical and coax. The input portion is a little trickier.

    This is what I have so far:

    10 nF | |
    || | |\ | |\
    COAX/SPDIF|-----------+---||---100R--+--| O--+---+---| O---- (output
    | || |/ | |/
    75R 10K
    | 74HCU04 |
    --- ---

    I want to add in the Optical audio receiver to the front end of this
    circuit without affected the 75 ohm termination resistance needed for
    the coax connection. The user would only connect either the coax or
    optical to the input, never both.

    The output of the optical TOSLINK receiver is TTL.

    I thought of just connecting the optical output through a diode to the
    coax input. My thinking is that the termination resistance would
    remain the same since the diode should look like an open circuit one
    way. See below:

    Optical Output (TTL)
    \/ Diode +--10K--+
    -- 10 nF | |
    | || | |\ | |\
    COAX/SPDIF|-------+---+---||---100R--+--| O--+---+---| O---- (output
    | || |/ | |/
    75R 10K
    | 74HCU04 |
    --- ---

    When the user plugs in the optical cable the diode would let the
    signal through. When not connected the coax shouldn't notice the
    extra circuitry hanging on since the diode would always be reverse

    Any suggestions/comments or other options to consider are appreciated.
    Thanks in advance.
  2. What is the idle output level of the receiver, ie. with no signal at the
    input? Low or High?
    Hm - how about connecting the TTL optical input signal to the lower end of
    the 75R terminating resistor?

    10 nF | |
    || | |\ | |\
    COAX/SPDIF|-----------+---||---100R--+--| O--+---+---| O---- (output
    | || |/ | |/
    75R 10K
    | 74HCU04 |
    | ---
    Optical Input(TTL) ---+

    Depending on output impedance of the Toslink receiver and idle polarity, you
    might have to insert an additional inverter or buffer (better CMOS than
    TTL), and maybe lower the 75R value so that the sum of output impedance and
    resistor is again 75R.
    Remember to put isolation transformers on the coax outputs. These are easily
    made using a small core for a few parts, or readily available for higher
    volumes. For my home wiring (about 20m of S/PDIF running through CAT5
    cabling), I got good results using 100BaseT ethernet transformers (ST6118T
    or similar) which I had around.

  3. If you use and AND gate or OR gate you can control the impedance of both
    inputs independently. Choice of gate type is dependent on which logic
    level you get from an idle input.

  4. JESSD

    JESSD Guest

    The output of the toslink receiver in idle mode is high, but I planned
    on using an inverter to change that to low.

    That's something I didn't think about. I wonder what the output
    impedance is of an inverter? I also worry that it won't be consistant
    from inverter to inverter. I'll try this out though.
    Yes, that is a good idea. Why is CMOS better than TTL?

    I would but the cost of the transformers I have seen is a little too
    high. I'll check out the ST6118T you mentioned. What is the
    disadvantage of not using the transformer?
  5. I would guess some 10 Ohms - but you should better measure the result.
    However, with the 74HCU04 used as shown, you will need to control the exact
    type of inverter anyway.
    Without looking it up, I would expect a better (lower) "L" output level.
    The possibility for ground loops.

    I know Pico has ready-made S/PDIF transformers, but nearly any ring-shaped
    core with a few windings of insulated wire (with the ratio matched to your
    output stage) will usually work fine.

  6. I read in that Michael Schwingen <[email protected]> wrote (in <bt4fsl$kn6$-
    Aachen.DE>) about 'spdif and toslink switch', on Fri, 2 Jan 2004:
    Indeed. If you put 10 ohms on the input, you must measure -10 ohms at
    the output. It's an inverter!
  7. JESSD

    JESSD Guest

    I originally tried or'ing the output of the optical receiver and the
    output of the inverter feedback network. The problem was the output
    of the inverter feedback network started oscillating when there was no
    coax input present. When I managed to get rid of the noise and the
    oscillation, the level would sit at 2.5V. I wanted the output to be
    low when no input was present so that I could properly or the signals

  8. JESSD

    JESSD Guest

    I tried it out and it worked. I measured 60 ohms using the 74HC04
    using only 1 inverter. If I used all 6 inverters on the 74HC04 in
    parallel I could get that number down quite a bit I suspect.

    I'll have to get a cmos chip and try it out to see.

  9. Making sure that an idle input stays on one side is crucial, of course.
    This is quite easy when you use a differential receiver. Are you
    committed to a 74HCU04 or are you free to use whatever chips you like?

  10. I can use whatever chips I would like. Is there something that might work

  11. With a differential receiver you can bias the positive and negative
    input separately, thereby defining the idle polarity. It is also easy to
    control the hysteresis by using a small amount of positive feedback.
    This should kill oscillations.

    You may use an RS422/RS485 receiver. These have some hysteresis built in
    already. They also often have a defined output state when the inputs are
    open, so you may save the biasing resistors. Such things are available
    in small packages, if you want. See the MAX3280E for an example. Note
    that RS422 receivers are also quite resilient against damage by ESD or

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