I would check to make sure the outputs on the cable box are putting out 2 channel pcm and not surround sound 5.1. If it's putting out 5.1 you won't likely get signal out of the DAC.
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I had an issue trying to figure out how to force the output to LPCM. I initiated a chat with Comcast who provided a link to the cable box manual. I reviewed it and found the procedure which is as follows:
1. Power off the box from the remote, 2. Select the Menu button on the remote to invoke the cable box menu, 3. Scroll down to HDMI Settings and select by depressing the OK button, 4. Change the Audio Ouput Mode from Auto to LPCM by depressing the OK button to toggle between the different options, 5. Once selected, use the left button to navigate to the main menu then hit the power button to exit.
This worked for me. Good luck!
PS Below is a link to the manual for comcast model rng110 in case you have the same box.