PT mixer headroom severely reduced to 20dB over 0dBFS. Bug?

[Farhoof]

--> UPDATE on page 6 #56, it was a hidden "feature".

Original post below.
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I'm running into an issue here. It seems the Protools mixer digital headroom has been severely reduced to max 20dB over zero. Is anyone able to confirm?

Steps to take:

- Create a stereo track, out put to your main out (turn the speaker volume down!)
- Use the signal generator to render a clip with an 80Hz sine at 0dB peak.
- Set the volume of the track to -30dB (or lower).
- Increase the clip gain of the clip to +20dB.

- Playback the sound (mind your speaker level). The internal mixer should process a signal +20dB over 0dBFS, but is then lowered by 30dB before the track output so the end result is safe at -10dB peak. Playback should be normal.

- Now increase the clip gain to +21dB and playback. The sine is now distorted even though it outputs at a safe -9dB. You can lower the output to whatever you want, this is not an interface or clip gain issue, the distortion seems to occur in the Protools mixer.

The same issue occurs using any other method when you go past +20dB within the Protools mixer, like having 2x +12dB trim on a track.

Delving deeper into this, it looks like any sample OVER +20dB is processed as -INF. So it's not digital clipping where anything over +20 is just clipped at +20.

This issue DOES occur in:
Protools Ultimate 2020.12
Protools Ultimate 2019.12
Protools Ultimate 2019.6

This DOES NOT occur in:
Protools Ultimate 2018.12
Protools Ultimate 2018.10
Protools Ultimate 12.8.2
Afaik any version earlier all the way down to PT 11.


Question 1:
Is anyone able to test this using Protools Ultimate 2021.3.1? I cannot run this since it needs an HDX update and I don't want to do that right now.

Question 2:
Is anyone able to confirm this using either PT Ultimate 2020.12 / 2019.12 / 2019.6?

This issue started somewhere between PT 2018.12 and PT 2019.6 so it's been here for a while.
And before you start lecturing me about the importance of gain staging ; no this doesn't affect me on my normal work, I've encountered this while testing new limiters and pushing to the max. It needs to be resolved either way, Protools is using a 64bit floating point mixer and this wasn't an issue before PT 2019.6.

/Edit
Note: I'm running HDX where the mixer is always dsp. This could be a HDX only issue. Not hardware bound though, as different PT version give different results. Please let me know what you are running when testing.