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I was making some FR measurements with a loopback timing reference and heard something strange: a second test tone started as the full 15Hz to 25kHzsweep reached about 1-2kHz. This second sweep began at around 500Hz but was time-delayed relative to the first sweep. Both sweeps played from there on up to the specified end frequency - sort of like a "Row, Row, Row Your Boat" duet. I know an audio recording would be better to help describe the issue, but I don't have one to share at the moment. It was a distinct, delayed copy of the original test tone but started at a higher frequency. It did not sound like traditional microphone feedback.

I've not heard this during normal sweeps without a timing reference. So is this normal? Did I miss something in REW's settings or maybe windows audio setup? AFAIK, My graphs look normal but I'm afraid the "feedback" somehow invalidates the measurements.
 

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Certainly a problem for measurements. There are a couple of possibilities: monitoring active on a soundcard or in a software mixer/PC audio settings (e.g. 'Listen to this device') or something in the chain is operating at a lower sample rate than the test signal and aliasing it down to lower frequencies. Neither is good for measurements so the cause needs to be tracked down and eliminated.
 

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Thanks for your patience in repeating this probably the umpteenth time! I've heard it before but wasn't sure if it applied this time. I did make sure monitoring was disabled on the sound card, but will need to check Windows settings. Thinking cap and troubleshooting hat ON!
 

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Certainly a problem for measurements. There are a couple of possibilities: monitoring active on a soundcard or in a software mixer/PC audio settings (e.g. 'Listen to this device') or something in the chain is operating at a lower sample rate than the test signal and aliasing it down to lower frequencies. Neither is good for measurements so the cause needs to be tracked down and eliminated.
We have a winner - you were spot-on, John! Thank you for saving me countless hours of debug time. I found the smoking gun in Windows 8 under:
"Control Panel > Hardware and Sound > Manage Audio Devices > Playback > LINE OUT - Scarlett 2i2 > Properties > Advanced > Default Format" and also under
"Control Panel > Hardware and Sound > Manage Audio Devices > Recording > LINE IN - Scarlett 2i2 > Properties > Advanced > Default Format"

Both Windows sample rates were set to 44.1kHz rather than to REW's assigned sample rate of 48kHz.
 
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