Recording with the USBPre
Input Source Selection
USBPre has two input channels. Depending on the recording software used,
the inputs can be defined as a single, stereo input or as two separate mono
inputs. Each analog input on the USBPre can be set to different input types.
Since S/PDIF is a two-channel digital protocol, both channels are S/PDIF when
selected.
Input Pad
With some combinations of microphone sensitivity and sound pressure levels,
the microphone input of the USBPre can become overloaded. The bottom-side
panel input pad inserts a 15 dB pad to scale the input gain range for high
microphone input levels. This pad is selected per channel. The pad affects
only the microphone inputs.
Sample Rates
In Windows OS, software applications control the sample rate and bit depth
of the analog inputs of the USBPre. Valid sample rates for recording with the
USBPre are 32 kHz, 44.1 kHz, and 48 kHz. Valid bit depths for recording with
the USBPre are 8, 16, and 24 and 32-bits (32-bit is really the same as 24-bit
to the operating system). Recording software applications may show other
"valid" sample rates for the USBPre. These should not be used, as
the operating system performs a "sample rate conversion" while
recording. Sample rate conversion is very CPU-intensive and can cause choppy
audio and/or crashes. In Mac OS, the sample rate is selected by loading a
configuration file onto the USBPre (see Selecting Sample Rates for Mac OS).
When using S/PDIF inputs sources in both Windows and Mac OS, the sample rate
selected in software must match the sample rate that is at the S/PDIF input or
digital distortion will be heard.
Any sample rate from 5 to 55 kHz can be played back on the USBPre at 8 or
16-bit.
S/PDIF Inputs
The coaxial S/PDIF input on the USBPre accepts digital input sources with
bit depths of 16, 20, and 24-bit, and sample rates of 32, 44.1, and 48 kHz.
When S/PDIF input is selected on the hardware, the front panel selection LED
will flash until it receives a valid digital signal. Note that in Windows Me
and Mac OS any source with bit depths greater than 16-bit will be truncated to
16-bit.
In Windows the sample rate of the source material into the S/PDIF input
must match the sample rate selected in recording software, otherwise a rate
mismatch will occur and distortion will cause an unusable signal. In Mac OS
the sample rate of source material must match the sample rate selected in the
configuration file, otherwise a rate mismatch will occur. See S/PDIF in the
individual operating systems sections for more information.
It is not possible to directly listen to S/PDIF input signals via the
USBPre since the USBPre has its digital-to-analog converter connected to the
PC audio output. S/PDIF signals can be monitored after recording or streaming
via the PC Audio outputs. However, depending on the recording software used,
you may be able to route an S/PDIF input to an output for
"real-time" monitoring.
Metering
The input meter on the USBPre is calibrated in peak responding dB FS. If
recording software does not have its own level metering, metering from the
USBPre is necessary. When the red peak LED illuminates, the signal is very
near clipping. Reduce the input gain control to avoid overloading the inputs.
The front panel meter reads signals from the analog inputs only. If S/PDIF is
selected as an input sources the meters will not activate.
Phantom Power
USBPre provides phantom power for condenser microphones. When selected,
48-volt phantom power is applied to both inputs. Condenser microphones that
can operate on phantom voltages from 11-52 volts will function properly with
48-volt phantom.
Dynamic microphones typically do not require phantom power. A properly
connected balanced, dynamic microphone is not affected by the presence of
phantom power nor will it draw any current. However, it is good practice to
turn phantom power off when not needed. Poor or incorrectly wired microphone
cable can cause audible artifacts in microphone signals. Some wireless
receivers outputs are adversely affected by the presence of phantom power,
therefore, consult the wireless receiver documentation.
Headphone Monitoring
USBPre has a high current headphone amplifier that can monitor analog input
audio, computer audio, or a mix of both. When monitoring analog input audio,
the audio playing in the headphones is the direct analog signal before any
digital conversion has occurred and before it is sent to the computer. When
monitoring PC audio, the computer controls the digital audio. Note: The
Windows Volume Control (controlled with the "speaker"- icon in the
system tray) and the Mac volume control adjust the level of the computer audio
in the headphones and at the PC Audio output.
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