THD calculation & THD vs frequency graph smoothing

miniDSP and AV Nirvana
_redfox

_redfox

Registered
Thread Starter
Joined
Aug 21, 2025
Posts
7
Hello!
I've recently downloaded a new REW beta (5.40 beta 98 to be specific) and now I'm puzzled. It seems that new beta not only uses different algorithm to plot THD vs freq. data (graph seems less smoothed), but the entire calculation process has changed!

These are 2 exact files of sweep recordings I imported. The THD value at higher frequencies is almost one magnitude higher on the older version and the new one doesn't have noticeable "steps" from reaching Nyquist.
1755814686993.png

1755814667642.png


Please enlighten me on what's happening because this change alone can render an entire community's database of measurements basically useless.
REW 5.40 beta 98 and 5.31.3 were used. I couldn't find any release notes regarding this or settings in REW itself...
 
V5.40 beta 45 27th July 2024
Changed: Use frequency-dependent windows to reduce the noise floor of sweep distortion measurements

The above change eliminated the rising noise floor with frequency that previously affected sweep measurements. It does not affect genuine distortion that exceeds the noise floor. The traces you attached are noisy because the contributions are mainly noise.
 
John Mulcahy said:
V5.40 beta 45 27th July 2024
Changed: Use frequency-dependent windows to reduce the noise floor of sweep distortion measurements

The above change eliminated the rising noise floor with frequency that previously affected sweep measurements. It does not affect genuine distortion that exceeds the noise floor. The traces you attached are noisy because the contributions are mainly noise.
Click to expand...
Oh, I've read that note, but was unsure, whether it gives that effect or not. Thanks!
Precision aside, how can one compare older and newer results? You were absolutely correct about those graphs: it was a compander simulation test with distortion buried under the noise floor...
 
_redfox said:
Precision aside, how can one compare older and newer results?
Click to expand...
Best way is to make sure "capture noise floor" is checked for all measurements so you can compare it, and differentiate between noise and distortion signal. Without noise floor captured, you will have to make an assumption of whether the distortion level is a result of the measurement noise floor or not.

The update to REW's measurement capabilities that John highlighted above is a huge improvement, thank you again @John Mulcahy for this, you gave everyone a free upgrade in measurement capability!
 
Last edited:
dcibel said:
Best way is to make sure "capture noise floor" is checked for all measurements so you can compare it, and differentiate between noise and distortion signal. Without noise floor captured, you will have to make an assumption of whether the distortion level is a result of the measurement noise floor or not.

The update to REW's measurement capabilities that John highlighted above is a huge improvement, thank you again @John Mulcahy for this, you gave everyone a free upgrade in measurement capability!
Click to expand...
The problem is: this is a file import, not a realtime measurement...
 
sm52 said:
Does it work automatically with every measurement? Or should it be included somewhere?
Click to expand...
At this moment it uses new window system by default and I think you can't change it
 
Well, the question remains unanswered: how do I compare my new measurements with existing database?
It's obvious that new window produce different results. I want my measurements to be consistent and comparable with others', which were done with previous versions...
 
Nothing stops you using older versions. If you are trying to measure THD with harmonics that are below the noise floor the results aren't meaningful, whichever version you use, but the new windowing keeps the noise floor level with frequency and below the old versions. If some harmonics are significant you could limit the THD calculation so only those are included, and/or look at the individual harmonics as well as THD to have a better feel for what is significant and what isn't.
 
John Mulcahy said:
Nothing stops you using older versions. If you are trying to measure THD with harmonics that are below the noise floor the results aren't meaningful, whichever version you use, but the new windowing keeps the noise floor level with frequency and below the old versions. If some harmonics are significant you could limit the THD calculation so only those are included, and/or look at the individual harmonics as well as THD to have a better feel for what is significant and what isn't.
Click to expand...
Okay, thanks. THD+N just looks so strange due to lower noise floor in the newer version)
Will there be a menu option to toggle between window modes?
 
You must log in or register to reply here.
miniDSP and AV Nirvana

Popular tags

20th century fox 4k blu-ray 4k uhd 4k ultrahd action adventure animated animation bass blu-ray calibration comedy comics denon dirac dirac live disney dolby atmos drama fantasy hdmi 2.1 home theater horror kaleidescape klipsch lionsgate marantz movies onkyo paramount pioneer rew romance sci-fi scream factory shout factory sony stormaudio subwoofer svs terror thriller uhd ultrahd ultrahd 4k universal value electronics warner warner brothers well go usa
JTR Speakers SVSound Essence Audio

OUR MISSION

At AV NIRVANA, our mission is to explore audio and video systems that elevate the entertainment experience, allowing you to move beyond the ordinary and become fully immersed in music and movies. Our site is a gathering place for AV enthusiasts to share insights, experiences, and ideas—free from ego-driven debates—with the shared goal of refining and optimizing systems to achieve a true state of audiovisual bliss.

We take pride in fostering an inclusive and welcoming environment where discussions benefit everyone, from newcomers to seasoned experts, and where all levels of gear, from budget-friendly to high-end, are embraced. Above all, we encourage open, friendly conversations that inspire and uplift.

We invite you to join us in building a vibrant community of passionate enthusiasts who engage with respect, curiosity, and a shared love for exceptional sound and vision.

Quick Navigation

Buy us a cup of coffee!

The management works very hard to make sure the community is running the best software, best designs, and all the other bells and whistles. Care to buy us a cup of coffee (or two)? We'd really appreciate it! Check out our extra benefits for supporting members!

Premium Memberships
Community platform by XenForo® © 2010-2025 XenForo Ltd.
ALL Rights Reserved; Copyright © 2017– AV NIRVANA, LLC
XenPorta 2 PRO © Jason Axelrod of 8WAYRUN
Back
Top