M-AUDIO Delta TDIF - Manual (Page 41)

5. In order to monitor the digital signal coming into the TDIF, switch to
the Patchbay/Router page of the control panel software. In the "H/W
Out SP/ANA" column, select the radio button named "SP/ANA In."
This will copy all S/PDIF input audio directly over to the analog
outputs – therefore in this example, the monitor mixer will be
completely bypassed. Once this assignment is made, you may play
the DAT material at any time and verify that the signal is making it
into the Delta TDIF successfully. This can be done regardless of
whether or not your recording software is open.
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