Cambridge Audio AZUR 851C User Manual page 10

Operating instructions cont.
Advanced USB audio
The 851C is both USB 2.0 (Hi-Speed) and USB 1.1 (Full-speed) USB port
compatible.
It should also work with the new USB 3.0 ports where the PC will simply
treat the 851C as if it were a USB 2.0 or 1.1 device.
The 851C also supports two USB Audio protocols (not the same as the
port types themselves) USB Audio 1.0 (which works over USB 1.1 ports
and supports up to 24-bit/96kHz) or USB Audio 2.0 (which requires a
USB 2.0 port and can support up to 24-bit/192kHz).
The default configuration is USB 1.1 and USB Audio 1.0 which works
with nearly all common operating systems and computer types without
drivers and supports up to 24-bit/96kHz audio, simply plug in and play.
In this configuration the 851C is able to work at up to 24-bit/96kHz
by declaring to your computer that it can handle any sample rate from
32kHz to 96kHz.
However in some Windows/Mac operating system variants the operating
system itself may restrict or fix the output sample rate or re-sample the
audio.
See our online guide at www.cambridge-audio.com/851CSupport on
USB Audio for more details on this. With careful choice of playback
software and settings many pitfalls can be avoided.
In particular our free Windows USB Audio 2.0 driver (available from our
website) supports up to 24-bit/192kHz audio and WASAPI Exclusive or
ASIO modes that can give enhanced performance.
See the Setup section on how to switch between USB Audio 1.0 and
2.0 modes.
A brief explanation of your choices are below:
Use with PCs
With the 851C switched to USB Audio 1.0 (this is the default setting) the
851C will work with the native Windows XP, Vista or 7 Audio 1.0 driver
(no need to load any new driver) and accept audio up to 24-bit/96kHz.
With the 851C switched to USB Audio 2.0 the 851C needs the Cambridge
Audio USB Audio 2.0 Driver to be loaded and can then accept up to
24-bit/192kHz (and support ASIO and WASAPI Exclusive if required).
The driver is available from www.cambridge-audio.com/851CSupport.
Use with Macs
No extra drivers are required. With the 851C switched to USB Audio 1.0
the 851C will work with the native Mac OS-X 10.5 (Leopard) or above
Audio 1.0 driver and accept audio up to 24-bit/96kHz.
With the 851C switched to USB Audio 2.0 the 851C works with the native
Mac OS-X 10.5 (Leopard) or above Audio 2.0 driver and can accept audio
up to 24-bit/192kHz.
Use with Linux
For most builds of Linux with the 851C switched to USB Audio 1.0 the
851C will work with the native Audio 1.0 driver and accept audio up to
24-bit/96kHz.
Some very new builds of Linux are now supporting USB Audio 2.0 for
which the 851C should be switched to Audio 2.0 to accept audio up to
24-bit/192kHz.
For both cases because Linux builds vary according to their creators
choice of software components including drivers it is not possible to
guarantee operation and Audio drivers may need to be loaded.
'Class drivers' as they are called for generic support of Audio Class 1.0
or Audio Class 2.0 devices may be available from the Linux community,
we do not supply these.
10
Bit perfect transfer – ASIO and WASAPI Exclusive
mode
The 851C USB interface hardware and software support bit perfect
transfer.
Whether or not the audio sent to the 851C is bitperfect (i.e. not resampled
or mixed etc. by the PC) is actually a function of the playback application
and the operating systems audio engine.
By default the standard windows audio drivers (often called WDM
drivers) included in Windows XP support MME or DirectSound transfer,
both of which include a kernel mixer and re-sampler stage.
One way round this is to use ASIO to bypass this. Another is to use a
playback application that supports its own form of kernel streaming, i.e.
if has its own way of outputting the audio without invoking the kernel
mixer.
For Windows Vista and Windows 7 a new method of audio transfer is
supported called WASAPI. This has two modes; Shared and Exclusive.
Shared mode is similar to MME or DirectSound but in Exclusive mode the
kernel mixer/re-sampler is bypassed and bit perfect transfer is possible
with only one audio playback program directed to the soundcard at a
time (for instance system sounds are not mixed in).
To ensure bit perfect transfer use a playback application that supports
ASIO, WASAPI in Exclusive mode (if you have Windows Vista or Windows
7) or some form of kernel streaming.
This is a fast moving area so we would encourage you to check the web
for the latest media players to support these more audiophile playback
options.
To use ASIO you will need the Cambridge Audio Sound Driver installed and
the 851C switched to USB Audio 2.0.
No further settings are required as the driver automatically supports
ASIO transfer, all that is required is to load and setup suitable ASIO
enabled playback software.
ASIO (Audio Stream Input/Output) is a non Microsoft audio driver
protocol for Windows originally invented by Steinberg ltd ASIO bypasses
some of the normal audio path from the playback application through
the Windows audio system (including the Kernel Mixer) for a more direct
path to the 851C.
You will need to install and use a suitable playback application
that supports ASIO output either natively or via a plugin. One
example of such an application with a suitable plugin is Foobar, see
http://www.foobar2000.org for more details.