Generator/RTA issue at 24 bits

[firage]

[REW V5.20.4]

The signal is from the REW generator, 997Hz 0dBFS sine at 24-bit PCM 48kHz, dithered at 24 bits. There's an even order harmonic distortion in this spectrum that is not ideal. I don't see the problem in 16-bit or fp32 signals.


These are good. 16-bit dithered at 16 bits and full-scale 32-bit float dithered at 25 bits.

 

[John Mulcahy]

Can you give some details of the setup, which audio driver (Java/ASIO) used, which OS, nature of loopback...?

If I internally loop the generator (float) output to the input capture all is clean.

 

[firage]

Saving to file is the same whether using ASIO + EMU 1616m or Java with various sound cards. It's a Windows 11 system. I can dig a little deeper into it if you're unable to reproduce this.

Generator / Save to file / Format 24-bit PCM. RTA / Open WAV. The other bit-depth options are clean.

ASIO out->in loopback through EMU PatchMix shows the exact same thing in real-time.

 

[John Mulcahy]

Thanks. The even harmonics in the 24-bit WAV file are due to the inherent bias in the default Java rounding method. It is easily fixed by switching to an unbiased rounding, I have done that for the next build. The ASIO loopback is a little different, even though the effect is the same. In that case if the ASIO driver is using a 32-bit integer sample format the conversion from float to Int32 is problematic since the integer format has higher resolution than the floats. I have also fixed that for the next build.

As an aside, you can drag and drop WAV files onto the RTA rather than using the file dialog.

 

[firage]

Wow, that’s quite interesting. Thank you for the explanation. And thank you for the software - it’s amazing!