Last week, in my “Secret Tips of Final Cut Pro X,” I wrote that we should not trust the audio in compound clips. This week, I want to explore that subject in more detail, because compound clips are working correctly, but not necessarily in a way that you would expect.
NOTE: If compound clips are new to you, please read this article to gain a basic understanding.
To get started, there are four key ideas you need to remember about compound clips:
Compound clips can have different image sizes, frame rates, audio tracks, render settings than your project. They don’t NEED to be different, but the key point is that they CAN be different.
NOTE: This is exactly the same behavior as Nests in Final Cut Pro 7.
Compound clips are projects which are stored in the Browser. As such, compound clips can contain common elements that you would like to use between projects.
However, it is these last two points which are the focus of this article: compound clips configure themselves based upon the first clip you edit into them. And, unlike projects, compound clips have output mono audio.
The reason this is important is that mono audio when added to a stereo project, changes its levels.
STEREO TO STEREO
Assuming you don’t change any levels, if you put a stereo audio clip in a compound clip, the audio levels in the compound clip will match the audio levels when you insert the compound clip into a project.
Here, for example, is a stereo clip placed into a compound clip. The level is -12 dB.
Here’s that same compound clip placed into a project. Note that the audio levels match in both examples: -12 dB.
If we raise the audio levels +3 dB in the compound clip, the audio levels increase by +3 dB in the project, automatically, without us needing to adjust levels in the project.
So far, this is exactly what we would expect – audio levels match between compound clips and the projects into which they are placed.
MONO TO STEREO
However, things get weird when we add a mono clip to a compound clip, then put that compound clip into a stereo project.
NOTE: Remember, compound clips can be mono OR stereo, while projects are only stereo.
To understand this, we need to learn about the Pan Rule, which states:
Pan Law, or Pan Rule, is an audio recording and mixing principle that states that any signal of equal amplitude and phase that is played in both channels of a stereo system will increase in loudness up to 6.02 dBSPL, provided there is perfect response in the loudspeaker system and perfect acoustics in the room. [Wikipedia]
What this means in English is that a mono clip placed into a stereo clip needs to be reduced by -6 dB to provide the perception of similar audio levels between listening to the mono clip and listening to the stereo clip. Here’s what this means in practice.
Here’s a mono audio clip placed into a compound clip. Again, the levels are at -12 dB. However, notice that the output is mono. This one change makes a BIG difference.
When this mono compound clip is placed into a stereo project, notice what’s happened to the audio levels: Final Cut lowered them by -6 dB on each channel!
Final Cut is not broken, instead, it is applying the Pan Rule to a mono clip which is now spread across a stereo output.
Just as a test, I changed the project settings to Surround and the mono clip played perfectly on the Center channel at -12 dB.
So, if all the audio you are mixing is stereo, levels behave as you expect.
But, if you are editing mono clips in a compound clip, then adding it to a stereo project, expect your audio levels to drop -6 dB per channel. This is weird, but normal. The easiest way to prevent this confusion is to make sure all your compound clips are stereo.
Here’s an article that goes into this in more detail.