Choose the Output Device and Input Device you wish to use for measurements. After the devices have been selected the particular Output and Input can be chosen. The channels used for output and input are selected from the drop downs to the right of the output and input selections, most outputs and inputs are stereo so you have the choice of using the left or right channel, or (for output only) both channels. Some soundcards only provide mono inputs or outputs, if that is the case the channel selectors will be disabled. Note that for ASIO drivers (only available on Windows) all channels are mono.

Make sure that the input channel is the one connected to your SPL meter (or mic preamp output). If Use loopback as timing reference has been selected in the Analysis Preferences the other channel will be used a reference to remove time delays within the computer and soundcard, this requires a loopback connection on the reference channel.

If the soundcard (or something else in the input chain) inverts the signal check the Invert box to have REW automatically correct the inversion. If the input has a DC offset check the High Pass box to have REW automatically apply a 2 Hz high pass filter.

If you select Default Device REW will use the devices which have been set as the defaults for your Operating System - under Windows these are specified via the Sounds and Audio Devices entry in the Control Panel, under OS X via the Audio Midi Setup tool in Applications - Utilities. In that case the Output and Input used will be those which have been selected in your soundcard's mixer. Make sure that the input is not feeding into the output signal in your soundcard's mixer and that monitoring is not enabled (if available).

If you choose specific devices, outputs and inputs from the REW selection boxes you have the option to always adjust volume settings yourself via your OS controls or to allow REW to set them to the last used values on startup. This is determined by the check boxes above the output and input level controls. Note that REW may not be able to control levels under some operating systems or for some soundcards, in those cases uncheck the boxes or leave the Output Device and Input Device set to Default Device.

The Output Device Buffer and Input Device Buffer controls set the size of the buffers used when accessing the soundcard. The default settings are 32k (meaning the buffer sizes are 32,768 pairs of audio samples). If you experience occasional glitches or interruptions in the signal generator output try increasing the output device buffer size, but note that there are other possible causes of this, such as interference from wireless cards. Similarly if the captured audio signals (as shown in the Scope graph) have occasional dropouts try increasing the input device buffer. Using larger buffers will increase latency (delays when starting and stopping replay and recording) but should otherwise not be detrimental. If you are not experiencing any problems with audio input or output you may wish to reduce the buffer sizes to minimise latency.

It is a good idea to measure the soundcard using the Calibrate... button in the Calibration section before making measurements to allow the response of the soundcard itself to be compensated for. Before making measurements you can use Check Levels to make sure the replay and input levels are set correctly.