yonatan166 Author

Level 1

6 points

Multicam tracks are aligned, but playback not in sync

The two multicam tracks are visually aligned (the audio waveform looks aligned) but on playback it is not

This is coming from the fcpxml import

What might be the issue?

This is the fcpxml start (just the resources tag where the multicam is declared)

<mc-angle

angleID="0"

name="Angle 1">

<asset-clip

ref="r28b546c4-c779-4c6a-839c-962d13cd13d2"

offset="0/24000s"

duration="8480472/24000s"

start="0/360000s"

audioRole="dialogue"

<adjust-transform scale="1 1" anchor="0 0" position="0 0" />

</asset-clip>

<asset-clip

ref="r4f90e4cb-57bf-4780-986a-270de21c1f77"

offset="8480472/24000s"

duration="8288280/24000s"

start="0/360000s"

audioRole="dialogue"

<adjust-transform scale="1 1" anchor="0 0" position="0 0" />

</asset-clip>

</mc-angle>

<mc-angle

angleID="1"

name="Angle 2">

<asset-clip

ref="ra3d4fdb9-4ac3-4742-8b23-21680dedbeb9"

offset="162162/24000s"

duration="16469760/24000s"

start="0/48000s"

audioRole="dialogue"

<adjust-transform scale="1 1" anchor="0 0" position="0 0" />

</asset-clip>

</mc-angle>

</multicam>

</media>

<asset

id="r28b546c4-c779-4c6a-839c-962d13cd13d2"

start="0/360000s"

duration="127207080/360000s"

hasVideo="1"

hasAudio="1"

format="rf-28b546c4-c779-4c6a-839c-962d13cd13d2"

audioSources="1"

audioChannels="2"

audioRate="48000"

<media-rep kind="original-media" src="file:///Users/yonatanbd/Downloads/P1000152-001.MP4"/>

</asset>

<format

id="rf-28b546c4-c779-4c6a-839c-962d13cd13d2"

height="2160"

width="3840"

frameDuration="1001/24000s"

name="FFVideoFormat3840x2160p2398"/>

<asset

id="r4f90e4cb-57bf-4780-986a-270de21c1f77"

start="0/360000s"

duration="124324200/360000s"

hasVideo="1"

hasAudio="1"

format="rf-4f90e4cb-57bf-4780-986a-270de21c1f77"

audioSources="1"

audioChannels="2"

audioRate="48000"

<media-rep kind="original-media" src="file:///Users/yonatanbd/Downloads/P1000153-002.MP4"/>

</asset>

<format

id="rf-4f90e4cb-57bf-4780-986a-270de21c1f77"

height="2160"

width="3840"

frameDuration="1001/24000s"

name="FFVideoFormat3840x2160p2398"/>

<asset

id="ra3d4fdb9-4ac3-4742-8b23-21680dedbeb9"

start="0/48000s"

duration="32939519/48000s"

hasVideo="0"

hasAudio="1"

audioSources="1"

audioChannels="2"

audioRate="48000"

<media-rep kind="original-media" src="file:///Users/yonatanbd/Downloads/Aan%20het%20opnemen%20(57).wav"/>

</asset>

<format

id="rf-a3d4fdb9-4ac3-4742-8b23-21680dedbeb9"

height="1080"

width="1920"

frameDuration="1/60s"

name="FFVideoFormat1080p60"/>

</resources>

MacBook Pro 14″, macOS 15.5

Posted on Jul 1, 2025 7:38 AM

Top-ranking reply

terryb

Level 4

3,334 points

Posted on Jul 9, 2025 12:19 PM

I ran the Gling fcpxml (v1.9) and the FCP-generated fcpxml (v1.13) through Claude and described the audio sync problem with instructions to inspect them for meaningful differences. Of course Claude can be wrong and/or BS us, but here's the response:

"After carefully analyzing both FCPXML files, I've identified the root cause of the 18-frame audio sync issue. The problem lies in how the 3rd party tool calculated the audio offset and timeline synchronization.

The main issues are:

Missing Gap Compensation: The working Info.fcpxml uses a gap element (<gap name="Gap" offset="0s" start="86400314/24000s" duration="18018/24000s"/>) to properly align the audio timeline before the audio clip starts. The broken Moose 29.fcpxml completely omits this gap.

Incorrect Offset Calculation:

- Info.fcpxml: Uses 36036/48000s = 0.75075s for audio offset
- Moose 29.fcpxml: Uses 17017/24000s = 0.709041667s for audio offset
- The difference combined with the missing gap creates a timing shortfall of approximately 0.79 seconds, which equals about 19 frames at 24fps - very close to your reported 18 frames.

Sample Rate Inconsistency: The Moose 29.fcpxml uses a 24000Hz denominator for the audio offset timing, but the audio is actually 48kHz. This fundamental mismatch in how time is calculated for audio vs video elements causes the synchronization to break.

The fix would be for the 3rd party tool to:

Include the proper gap element before the audio clip
Calculate the audio offset using the correct sample rate (48kHz)
Properly account for the timeline synchronization needed between video and audio-only angles

This explains why the audio waveforms appear incorrectly positioned and why the playhead doesn't snap properly - the timeline calculations are fundamentally flawed in the generated FCPXML."

I then asked Claude to double-check the audio asset definitions between the two fcpxmls in case they are referencing the audio differently.

"The audio asset definitions are functionally identical in terms of timing - both start at 3600 seconds and have the same duration (1998848/48000s). The synchronization issue is NOT caused by differences in the asset definitions themselves.

This confirms that the problem lies entirely in how the multicam angles are constructed and synchronized, specifically:

The missing gap element in the Moose 29.fcpxml
The incorrect offset calculation (17017/24000s vs 36036/48000s)
The improper timeline alignment between video and audio-only angles

The asset definitions are correct - it's the multicam implementation that's broken in the 3rd party generated file."

View in context

17 replies

Jul 9, 2025 11:10 AM in response to terryb

Thank you for looking into this. I'm interested to see what you find when you have the time to look into it more.

terryb

Level 4

3,334 points

Jul 15, 2025 1:21 PM in response to reynoldsm89

That's great news! Glad it worked out.

This thread has been closed by the system or the community team. You may vote for any posts you find helpful, or search the Community for additional answers.

Multicam tracks are aligned, but playback not in sync