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m903 USB Input and Computer Audio Configuration
aSynCHronoUS aUDio
The asynchronous mode USB converter in the m903 represents a significant improvement over any previous 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 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 systems 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. In ad-
dition, the USB port is completely ground isolated from the m903 audio ground. This eliminates the possibility of noisy computer
grounds inducing any noise or impurity in the m903 audio circuits.
This asynchronous USB DAC code was developed by Gordon Rankin at Wavelength Audio. We, along with a select few other
audio companies, have chosen to license this technology because it is simply the best computer audio playback technology avail-
able.
CoMPUter aUDio reqUireMentS
Regardless of the type of computer you will use to playback audio from, it must have at least one available USB port. The m903
ships with a standard USB type A to type B mini cable. The type A connector plugs in to the computer and the type B mini connec-
tor to the USB m903 input.
The m903’s asynchronous mode USB DAC supports standard driverless operation on MAC to 192kHz and on PC to 96kHz. For
sampling rates above 96kHz, PC users will need to download and install a free driver on their computer. This driver can be found
on our website -
http://www.gracedesign.com/support/support.htm
Driverless operation basically means ‘plug and play’. The m903 will automatically show up in your computer’s list of supported
audio devices as ‘Grace Audio Device’. In most cases, simply choose that as your audio playback device and the system will work.
Different operating systems may pose their own set of complications in setting up the m903 as the audio playback device. In the
event that ‘plug and play’ operation does not occur, you will need to look at some specific setup variables for your OS. In this case,
we will direct you to a very well written and comprehensive document by our friend and colleague Charles Hanson from Ayre
Acoustics.
This is an invaluable resource for computer/USB audio setup information for most current operating systems, and we strongly
urge you to familiarize yourself with the information pertaining to your specific OS.
Another excellent resource is Wavelength Audio’s website:
In the event that you have any setup issues which aren’t addressed by these resources, feel from to contact us directly at
303.443.7454 Monday through Friday 9 – 5, MST.
oPeratinG tHe HeaDPHone anD line oUtPUtS
Both the headphone and the line outputs feature precision level control from the front panel rotary encoder. Each output can be
completely independent of the other in terms of level and balance control (see the setup menu section for further description of
this feature).
The output selection indicator LED’s show which output is currently under control by the volume control. To toggle the output
control selection, simply press and release the rotary level encoder. You will see the selection indicator change. Note: two different
modes can be used for toggling through each output level. The first (L1) has the 2 outputs ‘ganged’ together and they are con-
trolled in unison, the second (L2) is a ‘round robin’ type, where the line output are controlled independently of each other. More
information on these two modes is found in the set up menu section of this manual.
The volume encoder is used to adjust the level of both the headphone and line level outputs independently. The level of the
currently selected output is displayed in the 7-segment display. Turning the encoder clockwise increases the output level and
counter-clockwise decreases it.
The m903 features an extremely high precision analog volume control. Step size is 0.5dB and channel matching is within 0.05dB
for all settings. Also, embedded within the volume control architecture is a 3 stage acceleration curve. The encoder has 24 posi-
