I'll bet this capacitor is simply used for "DC Blocking" in this application, because a excessive amount of DC bias voltage could cause any noise present on the input line to false-trigger the Toshiba TC74HC04AP CMOS inverter chip. I believe that this chip is more than likely being used as a "input buffer" for the S/PDIF input circuit, but I don't have the board schematics to look at so I am just stating the most obvious use of a chip like this one in a digital input circuit.
You should take a look at the web pages on the Scientific Conversion Inc. web site for more information on the various trade-offs of designing circuits with pulse transformers as inter-device coupling isolators. There are three papers in PDF format that are exceptionally good reading on this subject!
http://www.scientificonversion.com/
The links are on the lower part of the column on the left side of the main page.....
HTH-
You should take a look at the web pages on the Scientific Conversion Inc. web site for more information on the various trade-offs of designing circuits with pulse transformers as inter-device coupling isolators. There are three papers in PDF format that are exceptionally good reading on this subject!
http://www.scientificonversion.com/
The links are on the lower part of the column on the left side of the main page.....
HTH-
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