Check the option reference input and use the setup shown in the image.
If you have the option, use a special cable that returns a mix of line out left and right line input. This will compensate for almost any properties of your sound device. The mix can be done with two 10 kΩ metal film resistors as shown at the right. This will only work if you use an external microphone amplifier, because the microphone input has a too high sensitivity and is often only mono.
Uncheck the option reference input and use the setup shown in the image.
If you do not have the option for the loopback above DRC123 can also use an internal loopback. In this case it is recommended to measure only in stereo mode because otherwise the delay between the left and the right channel is unknown and will cannot be compensated. Furthermore you should work with a calibration in this case.
It is not required that the response is approximately at 0 dB.
Repeat the procedure for different locations of the microphone and save
the result to different file names.
It is not required that all measurements are done with exactly the same
parameters. But if you measure the left and right channel separately you
must not change anything between this measurements.
If you want to verify the results from DRC123 you may check the Verification mode box. This will apply the currently selected deconvolution filter to the reference signal before output. This is how your music is played with DRC123. Ideally the response should be flat in this mode, but usually there are some residuals because the correction is not exact and the response depends highly on the microphone position.