Subaru WRX Owner's Manual page 298

Experience
The HD Channel is unavailable caution
message appears when digital reception is not
possible.
Cause
.
When the system determined that the SPS
station could not be received while receiving
a digital broadcast and SPS station.
.
When the SPS broadcast is ended
(stopped) by the radio station while receiv-
ing SPS.
Audio/Audio set
Action
.
When the status is changed to acquiring digital
data because the reception condition of SPS
station has improved.
.
When the received station is changed.
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