a video brainteaser: frames and beats matching

[sidereal-studios]

Fun with math...

I'm trying to align a metric montage video production with a particular BPM in the music score in order for it to align.

Here's the situation: the montage edit comprises very short and precise shot lengths that are repeated in a cyclical way. The shot lengths are 5 frames, 15 frames, 10 frames, 5 frames, repeated over and over. This comprises 35 frames total.

Now what I'd like to do is compose music that accentuates the metric montage. Since video works at 30 fps and there are 35 frames before the cycle begins again, it's tough to determine a proper BPM that will hit on the downbeat at the very start of the 35-frame cycle. Basically, what I'm looking for is an exact BPM number to compose to.

Making matters even more tricky, I'd like to create a polyrythmic contrapuntal soundtrack to the video montage. For example, if the video montage cycles around 3 times (105 frames total), I'd like the soundtrack to have completed 4 measures total. This way I've created a 3 (video) against 4 (music) polyrythmic counterpoint in which the two conjoin in regularly distributed intervals. What would be a BPM for such an endeavor?

Any sync issues I should look out for? Obviously there will likely be some slip since BPMs are never exact.

I've always been horrible at math, which is why I'm asking for help. Hooray for brainteasers!

[ScottMan]

Math schmath, use CUE. It'll do the math for you. In your breakdown notes, type in the tc numbers or feet and frames that you want to hit. If I remember correctly, you have three choices of importance level. Bring your timings into the BPM search window and select the BPM that hits most of your major and/or minor hits. It's been awhile since I've used Cue, but it offers several ways to figure this out. You may find it necessary to change time signatures to fit it perfectly, but it's doable. You didn't mention your set up, so I have no idea if Cue will work with your system.
Check it out:
http://www.olypen.com/rickj/cue.html

[Don Barto]

That pattern, for me, suggests a 7/8 figure with an eigth note lasting 5 frames: eight note, dotted quarter, quarter, eight note, would be the most obvious possibility. Contrasting such a quirky little rhythm with something very simple and very duple would probably create the tension you suggest with your polyrhymic idea -- or just something that moves along very relaxed and gracefully against the 7/8.

[sidereal-studios]

Thanks guys... Looks like Cue works on my system, and there's a 2-week demo mode (on faith, but which I'll honor).

In a slight change, I think I'm going to juxtapose 3 cycles (105 frames) against 1 measure (not 4 measures, which would create a scary fast tempo). One measure is 4 beats. Using BPM (4 beats per measure: 105 / 4 = 26.25, but that still doesn't give me BPM). Ugh. I'm lost. Trying to write up an equation for BPM this way is not my forte.

I will give Cue a shot.

[sidereal-studios]

Actually, for anyone who cares a light bulb came on and I figured it out. I drew time lines for cycles, frames, seconds, and beats on a grid and found the lowest common denominator where they met up. It was eaiser than trying to fit 4 beats into 3.5 seconds. The LCD where the montage (frame) cycles and time cycles meet up is 210 frames (6 cycles @ 35 frames; 7 seconds @ 30 fps). I went to Pro Tools and found 210 frames on the timecode ruler, set the main counter to bars and beats and eventually found that 210 frames equals 137 BPM. What this means is that for every 7 seconds of actual time, the montage will cycle 6 times and they will both meet up on the downbeat of every measure. There's bound to be some slip, but I can TC/E it since it's looped music.

Whew.

[TLmix]

Try this.. if my math is correct.

there are 1800 frames per second

divide 1800/ 105 = 17.14285714 bars per second
multiply by 4 = 68.571 bpm

[TLmix]

correction.. I meant 1800 frames per min/// 1.74..... bars per min..

not second.. sorry

[sidereal-studios]

aha! that is good to know. thanks.

[tomcat]

please post a clip of this montage when you are finished!

[ekuehnl]

For future reference, you could do this in about 10 seconds in Pro Tools. (Assuming you can use one time signature for the cue.)

Go to the first frame. Use edit>identify beat to punch in the starting bar and beat.

Go to the last frame. Use edit>idenfity beat to punch in the ending bar and beat.

Viola! Take a look at the tempo track. The tempo is calculated for you.

If you want to change time signatures during the cue it gets a bit more complicated. You could always calculate an initial tempo in 4/4 and then tweak from there.

Try it and see if it works for you.

-Eric