Sound Devices USBPre 2 - Manual (Page 20)

USBPre 2 User Guide and Technical Information
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not attached to a computer). The USBPre 2 can be forced into Stand-alone mode when connected to a
computer supplying USB power by holding down the Meter Source button when attaching the USB
cable.
In Stand-Alone mode all front panel controls perform the same functions as when the USBPre 2 is
in Interface mode, except that the Meter Source button is disabled and the meter mode is locked to
PRE. Since there is no reference clock from the computer, the operating sample rate is set using the
back panel DIP Switches.
See DIP Switch Options .
Sample Rate
In Stand-alone mode, the sample clock is generated by the USBPre 2’s internal sample clock genera-
tor, and the Sample rate is set by adjusting the DIP switches on the back panel of the USBPre 2.
See
DIP Switch Options . This will affect the internal sample rate of the USBPre 2 as well as the sample
rate output from the digital outputs.
Sample rates below 32 kHz are not supported on the coaxial or optical (TOSLINK) digital interconnec-
tions. 192 kHz sampling rate is not supported on the TOSLINK interconnections.
In Interface mode, the USBPre 2 operates at the sample rate set from the host operating system and
the DIP switches related to sample rate are ignored. It is further possible to change the sampling
rate of the USBPre 2 within some applications’ audio preferences. In Windows XP there is no global
sample setting, and the sample rate must be set by the application in use.
Locking Sample Rate to External Sources
The sample rate of the USBPre 2 can be locked to the sample rate of an external S/PDIF signal. When
a digital signal is connected to either the coaxial or optical input, the S/PDIF Input Source LED will
illuminate to indicate that the USBPre 2 is locked to the sample rate of the incoming digital signal.
The S/PDIF Input Source LED’s will fl ash for 30 seconds when the signal is removed or turned off.
To make a digital recording with the computer at a locked sample rate, be certain that the incoming
digital signal is valid (solid S/PDIF Input Source LED’s) and that the software driver is set to the
same sample rate as that of the incoming digital signal (
See Installation and Connection section for your
operating system ).
If the USBPre 2 senses a discrepency in the incoming digital signal such as a clocking error, the Meter
Source LED’s will fl ash until the Meter Source button is pushed.
Setup Examples
Test and Measurement
The high quality preamplifi ers and portability of the USBPre 2 make it a perfect interface for test and
measurement applications. A requirement for measuring a transfer function is the ability to moni-
tor the output reference signal from the computer. The LOOP source of input 2 enables a copy of the
computer audio left channel to be internally routed to input 2. This allows reference signal to be sent
back into the measurement application without additional cabling.
The USBPre 2 supports native sample rates below 44.1 kHz to provide better FFT resolution for low-
frequency audio measurements.