One of the problems with tightly cutting video to music is that our ears are a lot better at timing than our eyes, and unless you are lucky and pick music where the beats fall perfectly on frame boundaries this can lead to less than perfect edits. Read on to learn more about these perfect beats per minute speeds for video editing, and how to tweak music so it can be cut to perfectly, every time.
To fool our eyes into perceiving motion as continuous, we need to see at least 24 frames per second (let’s leave the inevitable arguments about the benefit of higher frame rates for another day and accept that as a ball-park figure for now). Of course our ears don’t see frames, they detect changes in air pressure, but they do this as a much, much faster rate. To convince our ears that a sound is natural, we need to feed them somewhere in the region of 41,400 different air pressures per second. To put it simply, our ears are about 2000 times more sensitive about timing than our eyes.
This might appear to be one of those meaningless QI facts, but actually it does have a serious application for video editors. When you understand that your video editing software can only deal with 25ths of a second (or 30ths, or something in that ball-park), while sound editing software is 2000 times more accurate, it becomes clear that video editors are being given a very blunt instrument to perform a high-precision task. Trying to edit video to fit perfectly with sound is almost always a clumsy process, as the beat of the music will (1999 times out of 2000) not fall exactly in the place where the frame changes. If we work the other way round, adding sound effects to our video (or using sound recorded at the same time as our video), this isn’t a problem, as our video editing software will position sounds (or the cuts between them) exactly at the start of a new frame, which is why documentary footage or fast-cut sequences often appear ‘tighter’ than music videos.
So far so good, but what can we do about this problem? Well, in recent years most high-end sound editing packages have included the ability to time-stretch sound (the audio equivalent of After Effects time remapping), and we can use this to our advantage if we throw a bit of maths into the mix. Have you ever noticed that a lot of 1980s dance videos look really tightly cut? That’s because the dance music scene in the 1980s tended to turn out a lot of tracks that ran at the same speed, 120 beats per minute, and that’s a rather nice number when it comes to 25 frames per second video editing. 120 beats per minute is 2 beats per second. This means that if you cut video into chunks that are 25, 50, 75 or any other multiple of 25 frames long, your edits will always fall exactly on the even numbered beats (which is where the clunky 4 beat per bar rhythms of ’80s dance music always put the kick and snare). 120 bpm isn’t perfect though, if we want to cut on the odd numbered beats, we have to compromise and alternate between 12 frame and 13 frame chunks. But what if we speeded the music up a fraction, so instead of the beats being 12.5 frames apart, they were exactly 12 frames apart? This would make the video editing really easy, and for those frenetic action sequences, we could cut on half beats (6 frames) or quarter beats (3 frames) too! Here’s where the maths comes in…
At 25 frames per second, 120 bpm has a beat every 12.5 frames. If we have a beat every 12 frames, how do we work out what that means in beats per minute? Well, let’s start by thinking in frames per minute rather than frames per second. Multiplying 25 frames by 60 seconds gives us 1500 frames per minute. divide that by 12 to find out how many beats we can fit into those 1500 frames and we get 125 beats per minute.
So, if we have music that runs at exactly 125 beats per minute, it will be really easy to edit video perfectly in time with it. The bad news is that while we can squish music around in a digital editor, not everything will sound good at 125 bpm. Unless we want drum and bass to sound like the musicians are in a coma or Enya to sound like she’s being fed into a shredder, we need to trade off ease of editing for respect for the music, and find other speeds like 120 bpm that are good for video editing, but not perfect. Time for some more maths.
If we cut every 11 frames, that gives us 136.36 bpm (if you’ve ever looked at library music by Hybrid, who make most of those aggressive high-tech beats that top gear play when Jeremy is thrashing a sports car, most of it is at exactly that speed). This is also usefully close to 140 bpm, which is the default speed for dubstep.
If we cut every 10 frames, that gives us 150 bpm, which is the lower reaches of drum’n’bass, or the peak of 90s rave.
If we cut every 9 frames, that gives 166.67 bpm, slap band in the middle of drum and bass territory.
If we cut every 8 frames, we get 187.5 bpm, the upper end of drum and bass, and the lower end of gabber (does anyone else remember gabber?). This is pretty much as fast as any even remotely mainstream music style ever gets. From about 140 bpm upwards, music has a tendency to occasionally drop down to exactly half speed now and then. This is common in dubstep tracks that make a switch from menacing to aggressive, and in drum and bass that goes from jazzy to fast. Cutting every 8 frames on the beat is visually identical to cutting every 8 frames on the half-beat of a piece of music that’s half the speed. S0 our 8 frames at 187.5 bpm can be thought of as 2 beats at half the speed, or to look at it another way…
16 frames per beat is 93.75 bpm, in slow ballad territory.
15 frames per beat is 100 bpm, where we’ll often find ballads and hip-hop
14 frames per beat is 107.14 bpm, more hip-hop and ballads.
13 frames per beat is 115.38 bpm, which is the slow end of pop/rock, and brings us right back round to…
12 frames per beat at 120 bpm, back at the 80s dance music we started with.
So, throw your audio into an editor (I use Apple’s “Logic”, which is full-featured but pricey, a cheaper alternative would be Algoriddim’s “Djay”, which is £13.99 on the Apple app store), tweak the speed to fit the nearest bpm from the list above and off you go, perfect cuts to the beat every time! There are a few ‘gotchas’ to be aware of though. Music from before about 1982 won’t be made with a drum machine, so the speed will drift, as will a lot of rock music that uses a trick the Beatles used to do almost instinctively, slowing down a fraction for verses and speeding up for choruses. You might also be working with a format like film or NTSC that runs at a different speed – all is not lost though, just multiply the frames per second by 60 to get frames per minute, then divide by a number of frames to get the beats per minute.