Cut video to the beat: Sume has no beat finder, so set the starts

YouTube's audio page lists a find-beats tool; Sume has no beat detector in its docs. If you know the track's tempo, you can still cut to the beat: one beat is 60 divided by the BPM in seconds, so at 120 BPM a bar of four beats is 2 seconds. Put each video slot's start on a multiple of that number and Timeline will cut there.

YouTube's tool list is from Add & edit audio, read 2026-10-03. The slot rules are in the Timeline 1.0 doc.

How do I compute the starts?

Pick the BPM, beats per cut and the track length. Starts are n * beats_per_cut * 60 / bpm. A slot must be at least 0.2 seconds, video[0].start must be 0 and starts must increase, so a very fast tempo with one beat per cut can fail the 0.2-second floor only above 300 BPM; at normal tempos it is fine.

import json

BPM = 120
BEATS_PER_CUT = 4
TRACK_SECONDS = 24
CLIPS = [f"https://media.sume.com/artifacts/artf_demo/c{i}.mp4" for i in range(1, 4)]

cut = BEATS_PER_CUT * 60 / BPM
slots = []
t = 0.0
i = 0
while t < TRACK_SECONDS:
    d = min(cut, TRACK_SECONDS - t)
    slots.append({"source_url": CLIPS[i % len(CLIPS)], "start": round(t, 3), "duration": round(d, 3)})
    t += cut
    i += 1
print(json.dumps({"video": slots}, indent=2))

What about music with no known tempo?

The Music Router and Music 1.0 have no tempo parameter in their docs, but you can ask for a tempo in the prompt, for example "120 BPM". Treat the result as approximate: generated audio may drift from a perfect grid, so check the first and last cuts by ear before you commit the batch. Music generation is $0.125 per generation.

How tight is a cut?

Cuts land on the frame grid of the output. Default output fps is not fixed unless you set 24, 25, 30 or 60, so set fps once for a series. At 30 fps a frame is 0.033 seconds.

What Sume does not do

No beat analysis, no waveform view and no auto-cut to music. The tempo has to come from you.