Beat sync listens to the song on your timeline, finds the hits, and then edits to them for you: cutting your video on every beat, dropping a popping dot on each one, or adding a quick zoom punch. It is plain signal analysis that runs in your browser, so the same song always gives the same beats.
Click Templates in the action bar and open the Beats tab. It has one analyze button and three actions. The actions stay greyed out until beats have been detected.
Beat sync reads from an audio clip on an audio track, so add your music first.
Bitgrain analyzes the first audio track that has clips on it, and on that track it picks the longest clip. If you have music and voice on separate tracks, keep the music on the higher track in the list.
Note: Beat sync reads audio clips only. The sound inside a video clip is not analyzed. If your music came in as part of a video, detach the audio to its own track first.
Click Analyze audio track. While it works, the status reads "Listening for beats…". When it finishes you see how many beats were found, for example "84 beats detected. Pick an action."
If it says "No audio clip found on the timeline. Add one first.", there is no audio clip on any audio track yet.
Only beats inside the part of the song your clip actually uses are kept, and they are placed at their real positions on the timeline. If you move or trim the song afterwards, analyze again so the beats line up with its new position.
Bitgrain decodes the song and measures its loudness in short slices (about 23 ms at 44.1 kHz). A beat is marked wherever the loudness jumps sharply compared to the average of the whole song, and is the biggest jump in its neighborhood. Beats are never closer together than 0.18 seconds.
In practice this catches kicks, snares, strong plucks and other sharp attacks. Songs with punchy drums give tight, regular results. Ambient pads, slow swells and heavily compressed mixes give fewer or less regular beats.
Each action changes your timeline directly and closes the Templates panel when something was added. Every one of them can be undone with Cmd / Ctrl + Z.
"Cuts the active video clip at every detected beat."
For every beat, Bitgrain finds the video clip playing at that moment and splits it right on the beat. Nothing is moved or deleted, so the video plays exactly as before, but it is now made of many short clips whose edges sit on the music.
This is the starting point for a fast beat-cut edit:
Beats that fall within 0.1 seconds of a clip's start or end are skipped, since there is nothing to cut there.
"Drops a popping dot on the text track for every beat."
Adds one text clip per beat to the first text track (created if you have none). Each is a large white dot in the middle of the frame, 0.6 seconds long, using the Pop In animation with a short fade out.
They are ordinary text clips, so you can make them your own. Select one, change the text to a word or a symbol such as a star or a slash, and restyle it. To restyle many at once, restyle one, then delete the rest and duplicate your styled one onto the beats you care about.
"Adds a quick scale-up effect clip on every beat."
Adds a 0.18 second Zoom & Transform clip on the first effect track (created if needed) at every beat, each one scaling the picture to 1.06x. The result is a small punch-in on every hit, the classic music video pulse.
To make the punch stronger or softer, select one of the clips and change Zoom / Scale in the inspector. Each clip is separate, so change the ones on the downbeats for accents.
M on the beats you care about most, then slice by hand with B for full control. See markers.