How to synchronize videos with external audios
Recording audio with a external audio recorder allows to get the best audio quality, it also allows you to use polar patterns to reduce
unwanted background noise, but, when not using expensive gear with timecode synchronization, the camera's audio and the external
audio need to be synchronized in post production.
In the simplest scenario, we get one video per one external audio starting at different times, but things can get more complicated, for
example we can keep the video rolling and record several external audios.
Manually synchronizing the camera’s audio with the external recorder is a tedious process, one of the methods is using a clapboard or
making a loud sound at the beginning of the recording, this will create a spike in both audios, and then in the editor we can locate and
align the spikes in the waveforms.
AV Sync doesn’t need clapping or timecode, it analyses the audios to sync the video with the external recorder audio.
AV Sync synchronizes videos and audios in the easiest way and supports automatically the most common situations, for example:
•
one video and one external audio
•
one video and several external audios
•
one external audio and several videos
•
multiple camera videos with external audios
To start synchronizing, just import or drop in the media panel all the media to sync, the imported media appears with the video and audio information:
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What is the audio drift
Synchronize automatically videos with external audios
with drift correction
When recording external audio, specially in long recordings, audio drift may appear, the audio drift is not related to the video frame rate
or the audio sample rate.
The drift is caused by the different timing of the devices, producing different audio lengths recorded by the camera and by the external
recorder.
For example, we record 1000 seconds of video and external audio, this is, the video camera records 1000 seconds but the external audio
recorder records 1003 seconds of audio.
Then in post production, when trying to match the internal and external audio track in the editor, after aligning the audio start
waveforms, we can see that the end is drifted and out of sync, the audio and video sync changes over the duration of the recording:
AV Sync detects and corrects the audio drift automatically, just activate the drift detection and synchronize the media.
After the sync we can see in the external audio information if drift has been detected:
When exporting the synchronized videos or audios, the drift will be automatically corrected.
For detecting and correcting audio drift, activate the “Correct audio drift” button, if not activated, the audio drift will not be detected and corrected.
Now just press the button “Synchronize” to start the sync.
After the synchronization is finished, the synchronized videos with the external audios will appear in the synchronized panel:
In the synchronized panel we can check the video sync information, for example:
The video v00001.mp4 syncs with the external audio REC00002.wav.
The external audio starts at 00:00:24 and the duration is 00:04:29.
And there is a detected audio drift of 0.338 seconds.
In this case, the video syncs with several external audios:
The video v00003.mp4 syncs with the external audios REC00003_1.wav, REC00003_2.wav and REC00003_3.wav.
When there are several audios synced, the scrollbar appears in the right side , move the scrollbar to check all the synced audios.
As we can see, the external audios only cover part of the video length.
The created synced audios always have the same length as the video, in the case the synced audio length is shorter than the video, silence will be added
to match the video length.
1: Import the videos and external audios
2: Synchronize the media
3: export the synchronized videos or audios
The synchronized media can be exported to the media list, or synced audio files can be created.
Export the synchronized audios to audio files
To create synced audio wav files, select “Export synchronized audio to audio files”.
The created audios will have the same filename as the synced video, with the suffix “_sync”, to change the suffix select the “Set suffix” button.
The destination folder for the audios can be the same folder as the video, the same folder as the non-synced audio, or it can be selected by checking
the button “Selected folder”
Press the button “Export to audio files” to create the audio files.
In case a video syncs with multiple external audios, a synced audio file will be created for each external audio.
Export the synchronized audios to media list
“Export synchronized videos to media list”, exports the videos with all synced audio tracks to neoFootage media list.
If you want to remove the video internal audio, check the button “Remove internal audio”.
Press the button “Export to media list” to start the synchronization.
After the processing, the videos with the created synced audios will appear in the media list:
In this case the internal audio track appears as “disabled” cause the button “Remove internal audio” was checked.
The audio tracks, including internal audio track, can be enabled or disabled by click over the video with the right mouse button and selecting
the menu “Select audio tracks / external audio tracks”.
Now we can for example cut the unneeded parts of the video in the trim dialog, or apply any processing setting.
Also, in the audio tab we can apply a filter or convert the audio parameters , for example convert the sample rate.
Tips and tricks
Checking the synchronized videos
After synchronization, the videos with the synchronized audio
can be checked in the play/trim dialog.
Just export the video to media list, open the video in the
play/trim dialog, and select the external audio track.
Recording and synchronizing external audios
Acrovid AV Sync tutorial
Download media samples for testing
These are several videos and external audios for testing purposes.
Media samples are taken from the excellent 1968 film “The night of the living dead”, actually in public domain.
AV Sync tutorial
Faster synchronization
When synchronizing multiple videos with multiple audios, all the
videos must be correlated with all audios, so the processing is
slower, for example:
- Case 1:
- video_a_1 with external audio_a_1, audio_a_2
The video_1 is correlated with both audios.
- Case 2:
- video_A_1 with external audio_A_1, audio_A_2
- video_B_1 with external audio_B_1, audio_B_2
- video_C_1 with external audio_C_1, audio_C_2
In this case, every video is checked with all 6 audios, but only
correlated with 2 audios, so the process is slower.
For faster processing, import only one video with their associated
external audios.
For hands-off processing, import all the videos all audios.
