Logic inverter feedback resistor, SPDIF Input/Ouput circuit
They are intended to bias the inverter into operation as a linear amplifier, but this is likely to result in high frequency oscillation unless you use an unbuffered inverter such as the 74HCU04. 

When it works, the input sits at about midway between the logic levels of Vih and Vil or about 1.4V in this case. Since the input is AC-coupled that doesn't matter much. 

You can learn more from this web page. The author echoes my experience that a buffered gate as shown will often oscillate (he says 240MHz and getting very hot as a result). The reason is that buffered gates (the usual type these days) have two additional inverting gain stages internally and have inaccessible nodes that would be required to stabilize the circuit as an analog amplifier. In fact, with feedback, the circuit resembles a ring oscillator.