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I am struggling to understand the relation between playback speed versus real time, for example, on YouTube.

If there is 10 minute duration video. If I listen it on 1.5 then as per my understanding 1.5 mean 150% there for it should take 100%(10 minute) - 50%(5 minute) = 50%(5 minute). I know that is wrong interpretation. But another side I can listen 15 minute content in 10 minutes on 1.5 speed. then why first one is false because first one is calculated 10/1.5 = 6 minute 40 seconds. I am very bad at math and another thing I want to became a programmer, but as you see I am stuck with these simple things. What is your suggestion I will put effort into understanding things or choose another field because I think I have a weak IQ or I am not smart or intelligent enough to continue with math.

Rodrigo de Azevedo
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salamak
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    Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. – Community Aug 13 '22 at 15:26
  • playback speed 1.5 mean 150% (one + half ) or something else – salamak Aug 13 '22 at 15:29

2 Answers2

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See. $2$ times playback speed means the video is being played twice as fast as the recording. So if you recorded the video at $30$ frames per second, the video is being played back at $30\times 2=60$ frames per second. So if your recording was $10$ seconds i.e. $300$ frames long, it would complete playback in $\frac{300}{60}=5$ seconds

Similarly $1.5$ times means that the video is being played back at $1.5$ times the speed of the original recording. That means a $30$ FPS video is played back at $30 \times 1.5=45$ frames per second. So $10$ seconds ($300$ frames) of the original recording get played back in $\frac{300}{45}=6.6666$ seconds

you can apply this concept to any playback speed as long as it is positive.

as for the IQ question, i am not qualified enough to answer that.

Gurmukh Singh
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  • for example your 15 min video, lets assume for the sake of simplicity was at 2 frames per second. so it had $15 \times 60 \times 2=1800$ frames. at playback speed 1.5, it would play back at $2\times 1.5=3$frames per second. $1800/3=600$seconds which is actually 10 minutes only. Hence Verified. – Gurmukh Singh Aug 13 '22 at 16:02
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From physics, we have $$\text{speed} = \frac{\text{distance}}{\text{time}}.$$

In the context of playing back a video, just think of distance as the length of the video. If you watch the entire video twice on different speeds, you're still watching the same video, so the length of the video hasn't changed (it is constant). So rearranging, we have

$$\text{playback speed} \times \text{playback time} = \text{constant}.$$

Now we see that if playback speed has been increased (e.g. multiplied by $1.5$), then playback time must be divided by the same factor (e.g. divided by $1.5$).

This is consistent with what you observed: a $15$-minute video, when played at $1.5$ speed, takes $15/1.5=10$ minutes to play.

Benjamin Wang
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