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m920 USB / Computer Audio Configuration
ASYNCHRONOUS AUDIO
The asynchronous mode USB converter in the m920
represents a 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 USB DAC be-
comes 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. In addition, the USB
port ground is completely isolated from the m920
audio ground. This eliminates the possibility of noisy
computer grounds inducing any noise or impurity in
the m920 audio circuits.
COMPUTER AUDIO REQUIREMENTS
Regardless of the type of computer you will use to
playback audio from, it must have at least one avail-
able USB port. The m920 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 con-
nector to the USB m920 input.
The m920’s asynchronous mode USB DAC supports
standard driverless operation on MAC to 384kHz 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/sup-
port.htm
Driverless operation basically means ‘plug and
play’. The m920 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 and audio players will
pose their own set of complications in setting up
the m920 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 player and 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.
http://www.ayre.com/usb.htm
This is an invaluable resource for computer/USB
audio setup information for most current operat-
ing systems, and we strongly urge you to familiarize
yourself with the information pertaining to your spe-
cific OS. In the event that you have any setup issues
which aren’t addressed by these resources, feel from
to contact us directly at 303.823.8100 M - Friday 9 – 5
MST, or you can email our service department -
Operating the Headphone and Line Outputs
Both the headphone and the line outputs feature
precision level control from the front panel rotary en-
coder. 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 con-
trol. To toggle the output control selection, simply
press and release the rotary level encoder. You will
see the selection indicator change. Two different
modes can be used for toggling through each output
level. The first (L1) has the 2 line outputs ‘ganged’ to-
gether and they are controlled in unison, the second
(L2) is a ‘round robin’ type, where the line outputs are
controlled independently of each other. More infor-
mation 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 inde-
pendently. 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 m920 features an extremely high precision
analog volume control. Step size is 0.5dB and chan-
nel matching is within 0.05dB for all settings. Also,
embedded within the volume control architecture is
