While this article is written for the JSD-60 and JSD-100, the same principles apply to the USL JSD-80 and the DTS XD-10P (which is a rebranded JSD-80).
In order for digital audio to work one of the devices, generally, the sending device must establish a time base. This is referred to as the "clock". Without the "clock" no digital audio can pass.
The USL (DTS) processors indicate the presence of the clock signal by the status of the light under the DIGITAL or OPTION button (EXT2 on the JSD-80 or XD-10P). If the blue LED (on the JSD-80/XD-10P the LED is green) under the button is solid, the unit is receiving the clock signal from the server. If the blue (green) LED under the button is flashing, the unit is not receiving clock from the server. Troubleshooting should be directed to the d-cinema server or the cabling connecting the server to the processor.
This same principle applies if the COAX or TOSLINK inputs are being used. As a digital signal they also require receiving a clock signal from the sending device (generally a bluray player or cable/satellite receiver).