Speeding up sound for slo-mo
- Thread starter Gogu
- Start date
Steve House
Veteran
What are you trying to keep in sync - music, dance rhythms, speech, or what?
I will be filming someone who plays the cello. I want to speed up the piece he is playing by an amount so that if I film him with 50 or 60 frames/sec and then see what i've shot at 25 frames/sec his movements will look natural and in sync with the piece which will now sound "normal" at 25 frames.
Thanks
Thanks
Steve House
Veteran
Impossible. First of all, there is no such thing as "recording speed" exactly analogous to film/video "frames per second." You can slow down a recording on playback but you need to think in terms of a percentage of the original recording speed rather than some absolute speed measure. So the math is easy, let's say you film him at 50 FPS and play it back at 25, a 50% reduction in speed. That means the sound recording will also need to be slowed by 50%. His movements will be half as fast as normal. If he moves the bow four times a second in real life, the film will seem as if he is moving it twice a second. Thus the tempo of the music will be half as fast. He plays an "A" at 440 Hz. On playback, what took 1 second now takes 2. The recorded 440 cycles play back in 2 seconds instead of 1 and so the frequency of the note becomes 220 Hz. The playback tempo of the music will be slowed by half and the pitch dropped.
You may be able to get part of the way there by shooting double system, recording him as he plays and using a time stretch tool or plugin in your NLE to stretch the music to fit the new time without shifting the pitch. But the tempo change can't be avoided so it will sound anything but "normal."
Not impossible. In fact, fairly simple.
Correct me if I'm wrong, Gogu, but you're talking in terms of taping the cellist as he "mimes" along with a pre-recorded piece, yes? This is a common trick in music videos. I've done it a few times with great results. I'm not the best when it comes to math, but the basic idea you need to keep in mind is inverse proportion... or in this case, percentage.
Let's take the easy rout here: shooting 50fps for 25fps playback. You are shooting for 25fps in the end. A 50fps clip will be slowed down to half-speed (50%) when placed in a 25fps time line (or conformed to 25fps). Therefore, the music track will need to be played back a double speed (200%) during production. Time compression is the best tool here, as it generally won't change the pitch of the playback. For example, a music recording that runs 3:50 will need to be time-compressed to run 1:55.
Now, these percentages may vary by tiny fractions of a percent based on your individual camera (which is...?), as it may not be capturing a perfectly even 50fps, or 25fps. But it's a starting point, and you just need to run some test shoots to see how accurate the percentages are.
EDIT: Barry Green posted a very handy guide a while back, that can be found HERE. Again, please note that the percentages given are starting points and are subject to your individual camera.
Correct me if I'm wrong, Gogu, but you're talking in terms of taping the cellist as he "mimes" along with a pre-recorded piece, yes? This is a common trick in music videos. I've done it a few times with great results. I'm not the best when it comes to math, but the basic idea you need to keep in mind is inverse proportion... or in this case, percentage.
Let's take the easy rout here: shooting 50fps for 25fps playback. You are shooting for 25fps in the end. A 50fps clip will be slowed down to half-speed (50%) when placed in a 25fps time line (or conformed to 25fps). Therefore, the music track will need to be played back a double speed (200%) during production. Time compression is the best tool here, as it generally won't change the pitch of the playback. For example, a music recording that runs 3:50 will need to be time-compressed to run 1:55.
Now, these percentages may vary by tiny fractions of a percent based on your individual camera (which is...?), as it may not be capturing a perfectly even 50fps, or 25fps. But it's a starting point, and you just need to run some test shoots to see how accurate the percentages are.
EDIT: Barry Green posted a very handy guide a while back, that can be found HERE. Again, please note that the percentages given are starting points and are subject to your individual camera.
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thank you both,
Yes C2V I want to do the music video trick
I've seen it done before but i wasn't sure but how much I should speed up the sound. I thought I''d just be streching it by 50% as 50fps is 2 times longer than 25fps.
I want to film the slomo sewunces with a canon 7D. this is the first time i'm shooting with this camera so if you know of some problems that I might encounter regarding what i'm trying to do here please tell me.
what about 60fps ? how much time streching would be required ? more than 200% I presume...
i'm not very good at this math thing.
EDIT: I reread more carefully your post now and I understand how the 200% thingie works. We have to play the music faster for a slower motion effect.
2nd EDIT: I have a friend who will do this time streching in Nuendo. As he showed me Nuendo has a time streching function which compresses or expands the music. So I should compress it 50% to achieve this 200% faster playback for 25fps right ?
Yes C2V I want to do the music video trick
I've seen it done before but i wasn't sure but how much I should speed up the sound. I thought I''d just be streching it by 50% as 50fps is 2 times longer than 25fps.I want to film the slomo sewunces with a canon 7D. this is the first time i'm shooting with this camera so if you know of some problems that I might encounter regarding what i'm trying to do here please tell me.
what about 60fps ? how much time streching would be required ? more than 200% I presume...
i'm not very good at this math thing.EDIT: I reread more carefully your post now and I understand how the 200% thingie works. We have to play the music faster for a slower motion effect.
2nd EDIT: I have a friend who will do this time streching in Nuendo. As he showed me Nuendo has a time streching function which compresses or expands the music. So I should compress it 50% to achieve this 200% faster playback for 25fps right ?
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Steve House
Veteran
If the recording is played back at double speed and the cellist mimes to it, the cellist will be doing all his movements at double speed. When played back at 50% of the shooting speed, doesn't the "slow-mo" make his movements become normal speed for him once again? Background action going on at normal speed during shooting would be slowed to slow-mo on playback but I interpretted the OP's case as wanting the cellist to be moving at half-speed while the music sounded normal. Having him mime to double-speed music won't do that since his movements will also be at double speed along with the music.
unclebob6958
Well-known member
If I remember right the speed was pushed up 25% for this one...
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That's the goal, yes.
It works quite well, actually. It's shooting in overcrank, so even though the timing of the slowed movements will match the original music track, the look of the footage will have the smooth motion of slow-mo.
You've seen this done before, I can almost guarantee it.
Noiz2
Sound Ninja
Actually sample rate is very analogous to frame rate. It is quite easy to record at 96k and playback at 48K and get a very smooth 50% slowdown. With the right device it is possible to record at all kinds of nonstandard sample rates. You do have to playback at a standard SR, but that is pretty much exactly what you deal with in film and video.
As to what the OP want's to do it doesn't make much sense to me either. You can't have things playback at different speeds and stay in sync. On the flip side if you speed everything up and then slow it down it will stay in sync BUT in the end everything is back at normal speed. So I'm not sure what the OP expects to accomplish.
@Noiz: Yes it will bring the movement speed back to normal but picture this. A singer walking towards the camera her mouth moving in sync with the lyrics. By recording at 60 fps or any other framerate that will look as slow motion when playing back at 25 fps her mouth will move at normal speed but the her movement walking towards the camera will be slowed down. It is a pretty cool effect.
As to my original question it still stands. If anyone is familiar with Nuendo using the time compressor by what percentage to I have to compress a music piece's length for 50fps and 60fps. I think that for 50fps the corect setting is 50% compression as this will speed up the music by 200% but I don't know how much it should be for 60fps.
hope this makes sense.
thanks.
As to my original question it still stands. If anyone is familiar with Nuendo using the time compressor by what percentage to I have to compress a music piece's length for 50fps and 60fps. I think that for 50fps the corect setting is 50% compression as this will speed up the music by 200% but I don't know how much it should be for 60fps.
hope this makes sense.
thanks.
Steve House
Veteran
True, if you can record at one sample rate and then playback at a lower sample rate without it getting converted in the process.
Steve House
Veteran
So your cellist's arms, hands, and body will end up moving as normal but the rest of her movements will be at half-speed?
60/25 = 240% speedup. 100*(1-(25/60)) = 58.33% compression, resulting in the compressed file ending up 41.66% the length of the original.
Try this ... record "The brown cow jumped over the moon." Now play it back twice as fast and pantomine saying the phrase in lipsync with the recording. Try it for the length of a song. Now try it playing a cello, pantomiming the bowing and fingering at double speed. It just doesn't seem very practical to me.
We're talking about a music video here. And this is a tried and true process. As UncleBob pointed out, Sting and the Police used it long ago for their "Wrapped Around Your Finger" video. It has been done many, many times since then.
We aren't talking about recording audio at a faster speed and then slowing it down. In fact, we aren't talking about audio recording at all. This is really a video-related effect when it comes down to it. The goal of the effect is to overcrank, so that when the footage is slowed down, it is still in sync with the original audio recording... time-compressed audio is only for reference and nothing that is done to the audio to speed it up will ever actually be used in the end.
In order to make this effect work, you need to figure out how much of a slow-motion percentage you want and calculate the percentage of speed increase required to achieve it. So, let's say that we want a 50% speed reduction. That means that we need to shoot video (or film) at twice the frame rate, which also means that the musician will need to be playing/lip-syncing along with a music track that is also playing at twice the speed, or is time-compressed 50% (200% speed = 50% length).
So speaking in generic terms of 24fps as the project time base, a 50% slow-mo effect would require source footage shot at 48fps. For a 25fps time base, it requires 50fps source footage.
The effect is pretty cool, and is really effective if there are other elements in the frame to accentuate the slow-mo effect. Wind blowing the musician's hair/clothing, leaves falling from trees, people and/or traffic in the back ground... all these things can enhance the look. But no matter what, slow-mo has a very distinct look to it, and this overcranking can achieve that look while maintaining lip-sync. Yes, it can be a bit taxing on the musician(s) to perform a full-length song at double speed, but no one ever said production was easy. If you want an effect, you're going to have to challenge your resources. Ask a stunt man.
A quick Google search even turned up a tutorial to achieve the "Wrapped Around Your Finger" effect using iMovie.
And the inverse of this, undercranking, would shoot at a lower frame rate with the music track slowed down. Then, when the footage is played at the nominal project time base, everything looks like fast-motion but is still in sync.
Gogu, I wish you the best in this. Please, if you can, post the results when you're done!
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Noiz2
Sound Ninja
Gotcha, I see how this would work. I don't think you want to slow down by 50% though. You should do some picture tests since that is a BIG speed change. You just need to divide one length by the other to get the % change.
C2V, wording like the above is what confused those of us who haven't seen this before. Not "everything" is in fast motion. The whole point is to have selective fast (or slow) motion. ie in what your talking about the lips are moving in real time but the body is in fast motion. The OP wants the arms and playing in normal motion with the rest in slow motion. It's the selectiveness that I missed. I'll go look at the clips because it does sound like a cool effect.
Cheers
SK
Paul Hudson
Vacuum Tube Member
They used to do this often back in the early days of music video (late 80's). Of course all the playback was analog. Let us know how you come out.