Rick,
You don't say exactly what the source of your spdif signal is.
Most all of the RS422 receivers that I'm familiar with require
an absolute minimum of 400 mV peak-to-peak input voltage swing
in order to recover the signal.  If your source doesn't produce
at least this much signal when it is terminated, it is simply
out of spec.  If it does, then another possibility is unwanted
loss in your input transformer.  Which part are you using?

PS - I added the [EE] tag to your header.

Brian Aase


> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Rick Mann
> Sent: Wednesday, June 12, 2002 12:23 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Help with "proper" SPDIF interface
>
>
> Anyone out there know how to "properly" interface coax SPDIF to
> TTL? Here's
> what I tried:
>
> SPDIF coax input to Pulse Eng isolation txfmr. Txfmr terminated
> with 75 ohm
> resistor. Txfmr signal through 0.01 microfarad DC blocking cap,
> into + input
> of a National RS-422 receiver. - input of receiver thought 0.01 cap to
> ground.
>
> Now, as it happens, the TTL out from the receiver only gives a
> clean signal
> if I remove the terminating resistor. Furthermore, the open-loop signal
> coming from the SPDIF source is 1 V p-p, rather than the 2 V I would have
> expected. If I put in the 75-ohm terminator, too much power is consumed by
> it, and it won't trigger the receiver.
>
> Any ideas? Please let me know if you need more details! TIA!
>
> --
> Rick
>
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