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I am using ProMovie Recorder to record high quality videos using iPhone X, and I noticed a weird thing today.

Here's what I did:

  1. I recorded a 4K 60fps video for 6 seconds (HEVC encoding).
  2. I recorded a 4K 30fps video for 6 seconds (HEVC encoding).
  3. Both the above videos captured the same physical content.
  4. Both videos were captured with the same bit-rate, ~118 Mbps.

Here's what I found:

  1. The two videos I captured (with different fps) are of the same file size - 84MB. How? Why?

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Question

A higher frame rate video with same bit-rate as that of a video with lower frame rate should be double the file size, shouldn't it?

Why isn't there a difference in file size? More the number of frames will lead to more the file size, since we're storing extra 30 frames for every second, right? Or is the underlying HEVC encoding optimises the file size for higher frame rate videos?

Or, can slight differences in exposure cause significant file size difference?

Please correct/clarify.

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    Bitrate is set and measured per second of media duration, not per some number of frames. So, the fps doesn't matter. – Gyan Jul 18 at 15:48
  • Thanks. So it won’t make a difference even if the frame rate is like, 1000fps? – badhanganesh Jul 18 at 16:10
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    In theory, no. In practice, it depends on how well the encoder's rate-control works. At 1000 fps, an encoder in normal circumstances would force many keyframes, and it would be hard to keep to the limit. But notionally, yes, the fps doesn't matter. – Gyan Jul 18 at 16:15
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As it seems both videos have a bitrate of 118 (117) megabits per second. This means - regardless off any other parameter - your video will take up 118 mb for each recorded second.

But as you already noted, a video with double the FPS should have double the data, so if you originaly exported the 4k60 video with 118, you can export the 4k30 video with 57mb/s and gain about the same quality. There's more to it like diffrent codecs optimized for diffrent types of videos or Interframe Compression, so you can compress high FPS a bit easier. It is recommendable to go slightly above the half.

Keep in mind, that many consumer cameras tend to have a low bitrate for high FPS as the camera is not able to record more.

  • So is it okay to consider that 118 mbps is divided into 60 for 60 FPS and 30 for 30 FPS? Can I assume that 60 FPS video has lower per-frame quality than 30 FPS video? Is this assumption right? – badhanganesh Jul 19 at 9:24
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    a video with double the FPS should have double the data: that's not exactly true, because when using codecs that support inter-frame compression doubling the frames while keeping the same number of I-frames only produces little difference on the output size – mcont Jul 19 at 13:06
  • @BadhanGanesh Kinda yes, but as mcont already noted, there's a bit more toonit like Interframe compression which works a bit better with higher FPS. It is diffrent from codec to codec, but generally, you will get about half as much + a bit more data from half the FPS. – Timothy Lukas H. Jul 20 at 0:49

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