slow-mo to sync with music

[Ali Jafri]

Shooting a music video this weekend and wanted some nice slow motion footage that syncs with the song. If I shoot 50 or even 100 fps (PAL) how much would I need to speed up the audio at the shoot for reference so that everything syncs perfectly when its put on a timeline and used as slow motion?

Is there a formula for this? Or do I just speed the song up by 200% for 50fps, 300% for 100 fps, and so on?

I am renting a Sony FS700 camera on the day of the shoot so won't have time to experiment. Hoping someone here would help. Thanks!

[Greg Miller]

Since 100 fps is 4 x 25 fps, I'd think you'd want to run the music at 400% of the original speed, not 300%.

[Ali Jafri]

Thank you Greg. Yes, I'm afraid my math skills leave a lot to be desired.

Since we're still on the subject, do I absolutely have to match the audio to the frame rate? At 400% the music is nothing but a garbled mess. Why can't I speed up the audio till wherever its still able to be performed and then try and match the video speed to fit? Would I encounter any flickering or other artifacts in the matched video? I'm thinking since its a music video we won't have long drawn out scenes anyways.

[Greg Miller]

I think you're asking, for example, if the music is still "understandable" at, let us say, 257%, then can you run the camera at 2.57 x 25 = 64.25 FPS? In theory that would stay in sync. The deciding factor is: what is the camera capable of? Can you dial in an infinite resolution of frame rates? Taking a quick glance at the online specs, it appears that in 50Hz mode, you have a choice of 100, 200, 400, or 800 FPS (and only in short bursts). If that's the case, and if you want motion within the scene to sync up with the music, then you'd have to play your music track (while shooting) at 4x, 8x, 16x, or 32x normal speed. And yes, that is going to become quite unintelligible, probably even difficult to pick out the beat.

I didn't see any other frame rates, or variable frame rates, mentioned in the online brochure. Maybe someone else here has experience with this specific camera. (You might have better luck looking in a different forum.)

Anyway, if the video speed ratio is not pretty close to the audio speed ratio, you will quickly get out of sync.

For example, if you want a 10-second shot, you start with 10 seconds of real-time audio. Let's say you increase the audio speed to 300%. Then the audio clip, as you play it back (while shooting video) would last 10 / 3 = 3.33 seconds. So let's say you shoot 3.33 seconds of video at 100 FPS, you will end up with 333 frames of video. When you play that back at 25 FPS, it will last 13.32 seconds, so you will be out of sync by over 3 seconds, just on this short clip! As you can see, the ratios need to match pretty closely.

Of course at 400% the lyrics will be mostly unintelligible, but then again, nobody could move their mouth fast enough to lip-sync at 400%. At 800 FPS (32x real speed) ... fuhgeddaboutit.

[Gary Nattrass]

I think you will find that 2X or 50fps will be about as much a musician can handle to keep sync.

This film by The Police was shot at 50fps and I have found that gives enough slo mo for effect.

The Police - Wrapped Around Your Finger - YouTube

For info this was shot on film with a nagra playback so it was easy to run the 1/4" tape at 15ips rather than 7.5ips to match the 50fps on the camera.

Music video's tend to be fairly loose sync anyway so I doubt if everything was even crystal locked for this example.

[Greg Miller]

It might have been OK as a trampoline ad, but everyone's constant jumping up and down looks pretty silly to me. Just sayin' ...