The Sound Devices USBPre overcomes many of the shortcomings of internal PC sound cards. Important benefits include balanced inputs, phantom power, analog level controls, meters, bus-powering to eliminate batteries and “wall warts,” and of course high-quality audio inputs. Prior to the USBPre, an external mixer was required for PC-based sound system measurements. The USBPre provides the important functions of the external mixer in an extremely compact, high performance package. The USBPre is the ideal partner for the SIA Smaart dual-channel FFT software application. Used together, the USBPre and Smaart allow the transfer function of a system or device to be measured. While several connection methods are possible, the one that I typically default to is as follows:
- Connect PC Audio R on the USBPre to Tape Input 2 on the USBPre using a short phono-to-phono cable. This provides the reference signal that Smaart will calculate the transfer function against.
- Connect PC Audio A on the USBPre to the input of the system under test. This can be accomplished with an unbalanced connection for short cable runs. A “half unbalanced” connection or balancing transformer is suggested for driving long lines.
- Connect the measurement mic to the Mic 1 input on the USBPre. Make sure that phantom power in enabled (if required by the mic).
- As an option, the output of the system equalizer can be connected to Line/DI 1 on the USBPre. A properly wired twisted-pair shielded cable should be used along with a TRS plug to maintain an electrically balanced connection. This allows the transfer function of the equalizer to be measured if desired.
- Make certain that the proper inputs are selected on the front of the USBPre. The Mix control should be set fully clockwise (PC position). The headphone output is not being used in this configuration.
Launch the Smaart software application and make certain that the USBPre is selected for both recording and playback under Devices. Select Pink Noise as the source under Generator. The output level control (Speaker icon) on the Windows task bar can be used to set the noise level to properly drive the system being tested. The front panel controls on the USBPre can be used to trim each input level to 12 dB on the USBPre’s input level meter, a value that should provide adequate headroom for the signal. If you wish to measure the equalizer response, use the front panel control of the USBPre to select the Line/DI 1 input (rather than the mic). Use the input level control to adjust the incoming signal to 12 dB and run the Smaart Delay Locator to adjust for the time offset between the channels. That’s it! You can now perform transfer function measurements in Smaart in the usual manner. The USBPre now lives in my computer bag, providing the ability to perform high resolution transfer function measurements on demand. Sound Devices thanks Pat Brown for providing this information to our test and measurement customers.