Crackling noises during tempo automations

[Master Mi]

I have a final problem with my newly finished Crisis Core remix, where I added some tempo automations to slow down the final chords gradually from 121 bpm to 115 bmp in 6 fine steps (120 bpm, 119 bpm, 118 bpm, 117 bpm, 116 bpm and 115 bpm).

But this caused some sort of subliminal but still noticeable crackling noises (in the project as well as in the exported audio file) at the points with the tempo change markers.

I think I've already found the cause behind this problem, but maybe some of you can contribute some further information, experience or maybe even a solution to even circumvent the nasty sound artefacts.


The problem sounds like this:

 

It's strange that the crackling noises in the exported audio files are always in this little area with the tempo automations, but from export to export or from replay to replay the noises are slightly different (in one exported audio file the crackling noises might be less loud and much more around the first tempo change markers, but in another exported audio file the noises are louder and more around the final tempo markers).

There are no hints that it could be due to some hardware issues.
And my PC is also pretty powerful (i-6700 processor system - 4 cores with 3,4 GHz base frequency as well as 8 threads in total, 32 GB DDR-4 RAM, still far over 1 TB of free HDD space and a good Steinberg UR44 audio interface).

I also checked the settings in my DAW, already used the highest ASIO buffer size and tried out all kinds of settings in the resampling, bouncing and dithering section - but no difference regarding the crackling noises in the area with the tempo change markers.

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The thing is, that these crackling noises seem to be only caused by the source signal.
The aux reverb sends played alone didn't contain any unwanted noises, as you can hear in this audio sample:

 

And after focusing more on the source signals of the two instruments (harp playing the melody and acoustic guitar playing the chords), I found out that only one of the two instruments causes this problem with the clicks or crackling noises - the harp:

 

The acoustic guitar, on the other hand, is completely free of crackling noises during the final tempo automations, as you can hear in the following audio sample:

 

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So could this be a VSTi-based issue (even with a VSTi that was created for this DAW?), or is it rather a problem caused by something completely different?

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As it seems, this phenomenon of crackling noises during tempo automations seems to annoy users of all kinds of DAWs and samplers:

https://forum.ableton.com/viewtopic.php?t=248259#p1815867

https://forum.image-line.com/viewtopic.php?t=306707

https://harderstate.com/viewtopic.php?t=13757

https://forums.cockos.com/showthread.php?t=174397

https://www.logicprohelp.com/forums/topic/122298-clicks-and-pops-during-tempo-changes/

https://forum.synapse-audio.com/viewtopic.php?p=118754&sid=9f368a4bf687d83282f07bf8d1ff33a1

https://vi-control.net/community/threads/kontakt-delay-clicking-with-tempo-changes.18614/

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And it was right in the latter links that I came across the point that these clicking noises during the tempo automations can be caused by delay effects.

Indeed, this makes sense, which is why in the end only the harp is affected by these unwanted sound artifacts.

In contrast to the acoustic guitar, I have activated an internal delay directly in the VSTi interface of the harp.

I also use a slight delay for the acoustic guitar - but this is added via a separate aux send together with some convolution reverb.
And as a separate aux send, the delay effect doesn't seem to cause any problems during the tempo automations, no matter how much I turn up the delay in all its settings.

So if I deactivate the internal delay in the harp VSTi interface, the crackling noises are gone as well, as you can hear in the last audio sample:

 

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Why it is that direct delay effects in the source signal track cause problems or crackling noises with tempo automations, but delay effects that are added via aux sends apparently work without problems at the critical points with tempo change markers, is completely beyond my technical or digital imagination.

Perhaps someone can gently explain this to the readers, even if they are not the presidents of the local math and physics clubs or some die-hard computer scientists with a degree in military or space technology.

This is a bit frustrating, especially since I can't really reproduce the original delay from the VSTi with a comparable delay as an aux send.
It somehow always sounds different (even with similar settings), which could perhaps be due to the signal chain in the digital process sequence, which apparently produces a different sound with the VSTi's internal delay.

I have also just found out that even with an aux send delay for the harp, if I turn it up properly, it can also produce crackling noises - much less crackling noises than with the VSTi's internal delay, but still perceivable crackling noises.

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So if this ends up in the final question "Rather delay effects or rather tempo automations?", I would tend to go with the delay effects instead of the few unremarkable tempo automations for the final chords in this case.

Maybe I find a circumvention to slow down the last chords by shifting and stretching the last MIDI notes a bit out of the timing pattern.


But if you have a better solution for this problem, please let me know. ))

 

 

Edited 20 hours ago by Master Mi