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  • At the 1939 world's fair in New York, the exciting new tech was the live television broadcast.

  • Roosevelt became the first president to address the nation live on TV

  • But for years leading up to this event, engineers have been working on one particular technical problem:

  • How to ensure the audio and video remain perfectly synced during the live broadcast ?

  • Without this, words and lip movements wouldn't match up, which would be annoying and distracting for viewers.

  • So, how did they do it ?

  • Well actually, they didn't,

  • instead they discovered something pretty incredible:

  • We are not very good at discerning whether audio and video are in sync

  • For example, I intentionally delayed the audio of this entire monolog

  • nearly a 1/10th of a second

  • and did you notice?

  • I'll clap to make it more obvious.

  • The engineers also found that there's an asymmetry in our tolerance for this misalignment,

  • we don't really notice if a sound lags video by up to 125 milliseconds

  • but we can tell something is wrong if it's leading the video by more than 45 milliseconds.

  • And to understand why, take a look at this:

  • Here I am bouncing a basketball as I walk away from the camera.

  • The sight and the sound of the bounces match up perfectly;

  • but as I walk away, you know the sound will be increasingly delayed due to the extra time it takes the sound

  • to reach the camera, but the sounds still appears synced.

  • This is because your brain is not reporting to you each instance exactly as it happens, but rather a short interval

  • of time reorganised to make sense, so in this case

  • your brain automatically aligns the sound with

  • the sight of the bounce.

  • At least, up to a point.

  • Once i'm over 30 meters away, the sound is now delayed by over 100 milliseconds

  • and your brain no longer integrates the information from your eyes and ears.

  • Here, let me play the actual sound of the bounce together with the sound as received by the camera.

  • This explains why sound can lag video by more than it can lead.

  • I mean imagine you were at a basketball game, and because of how far away you're sitting, the sound is delayed.

  • Your brain can handle that.

  • but if the sound precedes the sight of an event

  • that would look really odd

  • because that's something that would never happen in nature.

  • This is why the broadcast guidelines for acceptable audio and video mismatches

  • are skewed in favour of audio lagging behind the video,

  • our brains are good at aligning audio with the vision that preceded it.

  • We can actually exploit our audio-syncing-capabilities to produce some strange results

  • for example, we created this computer program

  • where when you press the space bar a light appears on the screen.

  • But not immediately,

  • there is an 80ms delay between the button push and the light coming on.

  • In a study participants who familiarised themselves with a similar program

  • came to believe that the light turned on immediately after they pushed the button,

  • just as our brains sychronized the sight and sound of the basketball bounce

  • Press the space key once to begin

  • This is just the section where you get the idea of what it does,

  • so you push the spacebar

  • Now watch what happens when you remove the delay.

  • That last one came up without me even pressing anything!

  • You didn't press anything and it just flashed up there?

  • Right!

  • Some participants were convinced that the light

  • came on before they pushed the button.

  • They believed that something else caused the light to come on,

  • even though it was their action that made it happen.

At the 1939 world's fair in New York, the exciting new tech was the live television broadcast.

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B1 中級

現在とは一体何なのか? (What Exactly is the Present?)

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    林宜悉 に公開 2021 年 01 月 14 日
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