Benchmark DAC1 PRE Instruction Manual page 23

Microsoft Windows® Test Results
Windows® 2000 and XP operating systems
have a digital mixer known as 'Kmixer'. By
default, all audio streams go through the
Windows® Kmixer to reach native USB audio
devices. The performance of Kmixer is critical
to any native USB audio solution, so it was
tested extensively by the engineers at
Benchmark.
The results indicate that Kmixer can perform
with full transparency under the correct
conditions. However, under the wrong
conditions, Kmixer can do a great deal of
damage. Kmixer's sample-rate-conversion is
of very poor quality (under XP and 2000) and
must be avoided. Benchmark's
AdvancedUSB Audio™ solution allows
Kmixer to default to a transparent mode of
operation that avoids sample rate conversion.
In contrast, sample-rate-conversion is
outstanding in Vista. By default, Vista up-
samples to the highest sample rate supported
by the connected audio device. This up-
sampling is so well designed that it should not
be capable of generating audible artifacts.
In any Windows® operating system, true bit-
transparency is only achieved when the all
volume controls are set to maximum.
Nevertheless, we discovered that the
Windows® volume controls are very well
executed and are distortion-free when
streaming to a 24-bit output device.
Whenever audio is originated from a single
application, Benchmark's AdvancedUSB
Audio™ solution prevents Kmixer's sample-
rate-conversion so that bit-transparency can
be maintained. It also forces Kmixer into a
24-bit output mode so that the Windows®
volume control does not degrade the audio
quality if it is used.
Benchmark's AdvancedUSB Audio™ solution
offers users the convenience of simultaneous
high-quality playback from more than one
DAC1 PRE Instruction Manual Rev J
Windows® application. Kmixer's sample-
rate-conversion is disabled as long as all
applications are playing files at identical
sample rates. If the sample rates do not
match, sample-rate–conversion is only
applied to the lower sample rates, and the
high sample rate signals remain at high-
quality.
Mac OS X Test Results
The system sample rate must be set to
appropriately to optimize playback quality.
This setting is different depending on
software version and media player. The user
is strongly encouraged to check the latest
information on this by going to:
http://www.BenchmarkMedia.com/wiki
Like the Windows® XP and 2000 operating
systems, most versions of OS X have very
poor-quality sample-rate-conversion. The
system sample rate is manually set and must
be set to match the sample rate of the audio
being played.
OS X is capable of bit-transparent audio
playback when the system sample rate is set
to match the audio, and all volume controls
are set to 100%.
TIP – To access the most current
information regarding settings for high-
quality computer playback, visit our
computer audio application pages:
www.benchmarkmedia.com/wiki
This site provides the latest information
on media players, media servers,
operating systems, and audio-related
computer accessories. It also includes
general instructions for maximizing the
performance of media servers.
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