[EliasGwinn]

The AP does not output USB. We play a "bit-test" audio file using a media player (Foobar, for example) through the stock USB port on the computer. Then we convert the I2S output from the USB chip (TAS1020B) to SPDIF and send that back to the AP. The AP compares the SPDIF signal to the bit-sequence it uses to create the "bit-test" audio file. It reports any errors - that is, descrepancies in digital data - to assure that it is the same data, bit-for-bit.