Eli5 why does fast forwarded video sound find but slowed down sounds echoey?
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Because if you actually slowed down the sound, people would get very deep voices. Stretching the sound but retaining the frequencies instead makes it more intelligible but also echoey. Fast forward just shortens everything, which makes it less echoey, but that often isn’t noticeable since peope tend to try recording with very little echo anyway.
Because when you fast forward the video, the sounds will play faster, which leads to increased pitch for everything (think of cassette tape or VHS fast forward).
To avoid that, the player chops up the sounds, pick sections up at a given interval and splice them up, in the end it plays normal sounds but skips forward, no new sounds are generated and no sections are played more than once.
The same cannot be done when slowing it down, because you would have to fill in the blanks between torn apart intervals. The player will either have to replay the same section repeatedly then proceed to the next section then do the same, or sample the sound sections and try to “invent” what would it sound like in between.
The first method creates literal echoes and the second one still sounds like echoes because the player can only guess so far, the “invented” fillings in between still sounds pretty much like the original samples, making them sound echoey.
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