Problem with high (jumping) input level when Calibrating sound card with Focusrite Scarlett 18i8

[Tandemrx]

I am trying to calibrate my soundcard with the loopback test and every time I attempt to do this and increase the input to match output the input level jumps dramatically from a low level (say -70 dBFS to highest possible level (red halo on my focusrite 18i8, 0 dBFS on REW meter). It seems impossible to match input and output because input always does this massive jump without intermediate increase using either the input volume control in REW or the gain knob on my Scarlett device.

Any thoughts or others that have had this problem?

 

[Tandemrx]

actually, getting anything other than minor input signal on the loop back test is challenging to me. I must have some incorrect setting. hmmmm.

 

[John Mulcahy]

Monitoring is probably active, so the input is getting mixed back to the output creating a feedback loop. Need to ensure there's no monitoring, the output signal should only come from the PC.