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This video was filmed without sound.
Is it possible to use only these images to
reconstruct the sound? that is can you hear pictures?
in this video I'm gonna
try to demonstrate that it's possible to get sound from pictures but it's not
gonna be easy so I'm gonna need some help.
this episode was sponsored by
LastPass which allowed me to fly to the Bay Area to meet with one of my science
heroes so now on with the video.
How're you doing?
I'm sorry my place of my concern is like threw all
this crap over here.
So this is the experiment that I came up with. -It's like
a crumpled up ball of tinfoil? -yeah like if I had a more powerful camera and I
had like the right lens then you know we could do something that looks more like
you're spying on somebody. But we should be able to demonstrate that you can
recover like a rhythm or a sound from you know whatever camera you have.
Now you might think it would be easy to record sound in video because after all
sound is just vibrations so the air is vibrating back and forth and everything
it hits should vibrate back and forth too. So you'd think all we need to do is
video that motion and plot displacement versus time and then recover the sound.
But it's not that simple because for one thing I mean these sound vibrations are
incredibly tiny. They move objects only about one micrometer and even if you're
super zoomed in that is way less than a pixel we're talking a hundredth or a
thousandth of a pixel. -We're not seeing something that is at one pixel move to
an adjacent pixel. You're seeing one pixel get slightly darker and the
next pixel get slightly brighter. -What objects work best for recovering sound? -So
the things that work best are things that have a lot of damping but are also
very light so that they move very readily with changes in the air pressure
so what are some good examples? -well like a bag of chips.
you know the initial
experiments were very like contrived in a way. You know we had these objects on
optical benches we were blasting light at them
the sounds were like as loud as we could make them.
Mary had a little lamb, little lamb, little lamb
I figured we'd try to do a rhythm.
This is...
shave and a haircut
let's put that camera on a tripod
- oh yeah that's I mean that'd be great
All right, let's give it a shot,
this is the actual clip I recorded and I want you to
notice two things: first, you can't really see much motion; and second, there are
plenty of pixels getting dimmer and brighter because of image noise. I mean
it's not a pristine, perfect image so how do you tell the difference between
pixels getting brighter and dimmer due to tiny movements versus noise?
Essentially you look for edges in the image and then you say well if the object
moves by some fraction of a fraction of a pixel in one particular direction
then pixels on one side of that edge will get a little bit brighter pixels
the other side of that edge will get a little bit darker. And so basically what
we do is we sum together all the ones that are supposed to get brighter and
subtract all the ones that are supposed to get darker and then that gives us sort of
one number right and if you track that number over time then that gives you an
estimate of the displacement over time.
This is time but in samples and
then this is position.
-hmm so what do you do now?
well we're gonna try to do some
filtering on that.
it's clipping. You don't say
I mean it's not much... it's not much but you can
recognize two of the beats.
This is what we've recovered from a hundred and
eighty frames per second, which isn't really a whole lot within the range of
like audible frequencies so that's why you know kind of the most we can hope
for here is a bit of a rhythm
Now of course the main limiting factor
is framerate because we can hear sounds from 20 Hertz to 20,000 Hertz but most
cameras only shoot 30 frames per second so they miss virtually all of these sound frequencies.
Imagine this is the motion created by a 30 Hertz sound if
you try to capture this with a camera at 30 frames per second you would end up
seeing the object always in the same position because it's at the same point
in the wave cycle. So your perception would be that the object is not moving at all
So in order to measure a frequency of sound you actually need to
sample at least twice that frequency which is why a lot of music is sampled
at 44 or 48 kilohertz - that's more than twice as much as the highest sound we can hear.
At any rate if you want to get a something more intelligible you're
going to need some higher frame rate camera.
so we just went to the camera store and picked up a new camera that should be
able to shoot a thousand frames per second or thereabouts.
Is that gonna be enough?
it'll be enough for something.
Hehehe I love that confidence
This is one modulation away from dubstep right here.
Just a little bit more of a wump wump
and then we have the next big track.
I've set it to a thousand frames per second
now we're talking -yeah
okay
okay so you have the footage there and
you're cropping in a bit, tell me about that.
well we're running this on my
laptop as opposed to the servers that I had back at MIT and that is gonna mean
that if we run it on the full video of my laptop will crash -okay
so we're gonna crop it and try it on that - I can see a little bit of motion
-yeah I mean I think
that one question is whether that's like -resonant motion
yeah well in this case
would be kind of like the equivalent of a rocking chair like if the if the foil
has a like a rocking mode then that's actually not gonna give us a lot of
sound information -mm-hmm
Can you tell whether this is gonna work or not?
I am optimistic.
I think because I know what I'm listening for I can hear it in there but
yeah you've gotta be careful though really?
that you're well just gonna be careful
that you're not like confirmation bias sure.
let's try that
that's about 60 Hertz that seems a little much
okay we're gonna try one more time. We have basically put the piece of foil on
top of the speaker, we're dialing up the volume to... 11. -Well I mean it's a shower
speaker so
Cool So what do you think of that image there?
it's beautiful, it's gorgeous.
it has been like a couple years since I've looked at this code so I suspect I
might have forgotten something and I'm using it wrong but I want to point
something out here, which is that here's what we recovered and here is here is
the piano roll of the signal that we sent.
This looks like duh duh to me and
that's the duhhh yeah yeah
that's it that is the...
---dih duh dud din duhhhh din dih - yeah okay so let's
see if we can actually get it to hear that
-We can see it, we just can't hear it
I think I know what it is.
What is it?
I don't think my laptop can play those frequencies.
hold on a second let me get some headphones
We need real speakers or something.
What do you hear Abe?
hold on.
I hear shave and a haircut two bits
here, listen this.
All right. -here, I'll hold it.
thank you.
yeah.
and... pitch shift
[recovered sound of Shave and a haircut, two bits]
yep there you go, visual microphone. [laughter]
this was a basic proof
of concept but Abe showed that with more powerful equipment he could recover
human speech from outside soundproof glass.
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