M-AUDIO Delta TDIF - Manual (Page 55)

Problem: The sound from the monitor mixer is muffled. It sounds as
if it were running through a mixer with the treble control turned all the
way down!
Possible Cause: The current Delta TDIF sample rate is too low. The
monitor mixer is a digital device that runs at the current sample rate of
the Delta TDIF board. The frequency response of the mixer is roughly
one-half of the sample rate. There is no way to prevent this lost frequency
response while playing back or recording at a low sample rate. However,
it may be prevented when the system is idle. See the "CODEC SAMPLE
RATE" paragraph of the section ‘Hardware Settings Page’ for more
information.
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