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14 Computer Audio Setup
ASYNCHRONOUS USB AUDIO
The asynchronous mode USB controller in the m905 represents significant improvement over any previ-
ous type of USB DAC. Previously, a USB DAC worked under standard adaptive mode USB audio, which
means the DAC’s clock would have to sync to the non-audio related computer USB buss master clock.
As you can imagine, the computer has a lot else to do, so the incoming clock signal to which the DAC’s
clock would have to sync was not ideal and would result in unwanted jitter.
With asynchronous mode USB, the m905’s USB DAC becomes the master to which the computer’s USB
buss gets synced. So the computer is now synced to a crystal-based audio clock signal and the system
works with dramatically lower jitter. No phase-lock loop or sample rate conversion is necessary, which
means bit-perfect playback from a computer with zero interface-induced jitter.
USB UP CHANNEL SETUP
In addition to playback, the m905’s USB interface allows for up to 10 channels to be transmitted back to
the computer. These ‘up channels’ allow the user to use the m905 as a simple I/O computer interface.
Up channel operation is configured in the PC’s USB Audio Driver Control Panel or Apple OSX Audio
MIDI Setup. Two modes are available. The first option configures the device for 10 input channels and
2 output playback channels. Input channels 1-2 are derived from the last selected digital input and
input channels 3-10 from the ADAT input. For stereo digital sources, PCM sample rates up to 192kHz are
supported as is DSD64. For the ADAT inputs, the m905 will send 8 channels of audio at 44.1/48kHz or 4
channels SMUX.
The second option limits the device to 2 input channels 2 playback channels. This configuration sup-
ports playback of PCM sample rates up to 192kHz, DSD64 and DSD128 (not currently available on Mac).
Input channels 1-2 are derived from the last selected digital input.
For Mac OSX, the input stream format is selected in the Audio Midi setup window as shown: