by Eric Bintner
Getting the Number of Beats per second
This can be done in two ways: if you know the the tempo of the music or if you need to estimate the tempo of the music.
If you know the the tempo of the music.
If you have used a sequencer or sequencing program (image 1) (Reason, Pro Tools, Cubase, Acid Pro, Digital Performer...) then this is quite simply a math equation.
image 1
(image 2 ) .
This is the number of beats per second.. Dividing this by it's time signature (4, usually) reveals the number of measures per second. This number can be used in The Wiggler in AE to locate series of points on a null layer (part II) for visual synchronization.
113bpm / 60 sec. = 1.883 bps
1.883 / 4 = 0.4708 measures per second
image 2
This is a little more difficult and rarly perfect for the duration for a song, but often accurate enough and handy for shorter segments in which you are looking for more structure and synchronous actions. Finding the interval can be achieved at least three ways:
1. By counting out measures and timing it on your watch. For 16 measures, the number of beats (64 in 4/4) divided by the time it takes to count out 16 measures equals the number of beats per second. So, if 16 measures of 4/4 took 34 seconds -- the bps is 1.882. Divide this number by 4 to reveal the measures per second. Also you can count the number of beats in one minute and use the math of part A instead.
64 beats / 34 sec. = 1.882 bps
1.882 / 4 = 0.4706 measures per second
2. Using a Wave editor such as Cool Edit/Adobe Audition you can find the distance between Five peaks for one measure (or the distance between 2 peaks for the interval of one beat). This is posible by simply highlighting exactly the distance between these points (image 3) and looking to see how long that time is (image 4). Divide the number of beats by the length of time to determine the bpm.
4 beats / 2.121 = 1.886
1.886 / 4 = 0.4715 measures per second
image 3
image 4
Beats or Measures?
The number we calculated is used to define a series of points in a null layer describing the interval of the beats or measures. Decide wheather you want a series of points to describe beat or measure intervals. Measure intervals help describe changes in song structure as four points can easily be dintinguished and are commonly descriptive of one part in a song. Point intervals describing beats are often too cluttered with points to be helpful, but can be effective in intense or high action moments.
| Create a Null Layer and apply the interval via The Wiggler In AE, go to Layer, New, Null Object (image 5). Open The Wiggler from the Window menu. Pick any attrubute, such as position, from the null layer (and be sure not to parent anything to the layer) and create two points, one at the begining one at or past the end. Type the measures per second or bps in the frequency query in The Wiggler then click Apply (image 6). |
 image 5 |
image 6
Zoom in close to the begining of the audio waveform in AE. Likely, you will have to listen for the 1 count and slide the null layer such that the points are alligned (image 7). If the music came from a MIDI based sourse you may be lucky and find that they're already lined up. Also, if you set up the first point (before you use the wiggler) at the first 1 count in the audio, all the other points will follow. Don't worry the wiggler is easy to delete and redo.
image 7
If you just want your Video/Audio composition to feel like one cohesive production (with an emphasis on the music), use the grid of points as a guideline not a rule. In other words if you are too strict to the pace of the audio/video sequencing it will seem robotic; if your pacing goes on tangents or goes too many places without root in the audio, the audio and video will feel as though they were different and separate.
A video/audio sequence can look seemless without a single cut on beat.
If your video pacing is close to the beat, be sure to know the music well -- know where that measure of five is and how long the reverb lasts (and, of course, be able to depict this through video rhetoric).
I've noticed that if I have a waveform open in After Effects and in Cool Edit, AE doesn't have to think when I view the waveform. Pretty cool.