FFT of digital loopback

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#61
SergeG said:
@KSTR

Finally I’ve made the experiment for understating harmonics in in.wav. That’s curious. REW/WSAPI/48k on the loopback shown nearly the same absolute harmonics level varying input level from 0db, -0.1db and lower up to -40db (the lowers was not tried).
But if I switch “Lock generator to RTA FFT” on, all the harmonics disappeared completely, pure noise at the low end. And no harmonics appeared with the input level variation.
That IMHO means there is probably something foggy in FFT windowing.
So FFT specifics is rather than SRC involved into in.wav spikes

Moreover while I was trying to reproduce FFT from in.wav with precisely your settings I observed a bit different result (the same harmonics of course, but different noise curve width below -180db) To get exactly the same spectrum I had to decrease FFT size to 8 – 16k from yours 256k.
That also confirms there is something strange in FFTs of different REW versions.
Click to expand...
REW is mature software for years now and there are for sure no major bugs in the FFT and Generator section. You are quite quick in blaming the software while it is much more likely there is user error at work ;-)

I suggest always staying up to date, current version is 5.40 Beta 51.
I also suggest to make yourself familiar with all the different options, like for example, dithering which is really important to set correctly (enabled and set to 24 bit) for correct results for our test case.

Also I highly recommend to verify the expected result (which is output=input, true bitidentical bit stream) before you actually insert your loopback. REW offers saving of the generator signals and later re-import them in the FFT so you can try out everything without any hardware, driver or OS involved.

Once you feel comfortable with that, you can insert the digital hardware loopback you want to examine and check if it behaves any different... which it shouldn't. When it is different we can start digging in deeper whether it is a driver/OS issue or really a problem with the hardware itself and what exactly is going on. I will gladly assist in the process the moment we can be sure that there actually is a hardware or driver/OS issue.
 
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