Fiio M9 - Coaxial Digital Output; Gain

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Coaxial digital output
You can choose between DOP and D2p modes. In D2P mode, DSD will Converted to PCM. In DOP
mode, it will output whatever you input (The DAC device should support DSD or there will be noise when
you play DSD in DOP mode)
Gain
Sets the Gain level of the headphone output to be either High (high volume) or Low (low
volume).