https://bugs.kde.org/show_bug.cgi?id=516398
Bug ID: 516398
Summary: Prevent the user that volume won't match the timeline
when Downmixing to mono
Classification: Applications
Product: kdenlive
Version First 25.12.2
Reported In:
Platform: Microsoft Windows
OS: Microsoft Windows
Status: REPORTED
Severity: major
Priority: NOR
Component: Rendering & Export
Assignee: [email protected]
Reporter: [email protected]
Target Milestone: ---
First of all, I apologize if "major" isn't the real status, because as I will
explain, it's not a major bug of kdenlive itself, however it is in my opinion a
major problem.
The problem:
Since Kdenlive is based on FFMpeg, it performs the same volume normalization on
mono tracks when they are inserted into a 2-channel project, which lowers the
volume by ×0.707, basically -3dB. Therefore, any mono audio clip we insert into
a Kdenlive timeline set to 2-channels when creating the project, will play at
-3dB in the timeline. If you also insert stereo audio clips, their volume won't
be changed however, so the balance you hear in the timeline does not reflect
the true volumes, which can cause problems later.
There are two cases:
- If the project is rendered in mono, all mono clips will be +3dB compared to
the timeline (which restores their true volume, so +0dB, but this can be
misleading for the user since they chose volumes based on timeline playback).
Stereo clips, however, will either sound the same as in the timeline, or too
loud (+3dB compared to their true volume), I didn't figure out yet why, but it
doesn't really matter regarding the following case.
- If the project is rendered in stereo, all mono clips will sound exactly like
in the timeline (-3dB compared to the source), and the stereo clips will also
sound the same as in the timeline.
Therefore, if we make a project that is supposed to be in mono at the very end,
but the timeline contains mono and fake stereo audio clips, we shouldn’t render
it to mono, otherwise the volume balance from the timeline will be broken.
Solution:
Kdenlive should absolutely warn the user when rendering a 2-channel timeline
using a 1-channel preset.
We worked on a project that has several episodes, and we'll need to re-render
everything because we weren't aware of this. In my opinion, the random user
shouldn't be misleaded by such tricky things without any warnings, because it
can badly affect a lot of projects.
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