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I'm shooting at 60fps, the result is video playing back normal speed but displaying 60fps.

I then have stretched the clip so that i get 30fps playback speed with a slow motion clip twice as long as the original.

The camera is recording the original video at 50mbps bit rate, my question is this.

If both clips have the same number of total frames, yet one is serving half as many of them for every second, will the bitrate have reduced roughly by half as well?

I ask this because i have rendered both version of the clip. one technically twice as long but with the same number of frames, i rendered them both with the same bit rate as the original source, one of course turned out twice the size of the other.

I would like the slow motion version to be roughly the same file-size as the original, i figure with the same number of frames it should be more or less the same size.

But because i encoded them both at the source files bit rate one is twice as large.

Can i reduce the bit rate for the slow motion one without losing quality?

Many thanks for your time and expertise should you answer!

P.S i'm new (as a user) to stack exchange i first posted this in the superuser section, since exploring i found this dedicated area so thought it best to put it here!

Regards, Cal

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In theory, if the number of keyframes and their placement were about the same, as decided by the encoder, then yes, you can reduce the bitrate to half and retain the same quality. In practice, the encoder may be configured to place a keyframe every N seconds, in which case, you should use a bitrate a bit higher than half the original bitrate, say, around 55%.

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  • thanks for your response, logic would appear to prevail! I must now learn how to implement this knowledge effectively. Regarding your answer: what do you mean by Keyframe every N Seconds? Do you have any suggestions for determining the number of key frames and their placement in the original footage? perhaps i should post this as a separate question. Thanks again!
    – Cal
    Jul 23, 2016 at 13:58
  • Which software and export settings?
    – Gyan
    Jul 23, 2016 at 13:59
  • Using After Effects, the source file is 60fps MP4 at 50mbps, i'm using Adobe Media Encoder and attempting to encode with as little quality loss as possible: H.264 target bitrate 48mbps (max 60) VRB, 2 pass. I have not checked the advanced settings check box and defined a custom key frame distance, though the number 72 is in there already but i'm not sure if that is what is being applied.
    – Cal
    Jul 23, 2016 at 16:48
  • Looks like the keyframe distance is in terms of frames, not time, so you can just reduce the bitrate to 24mbps.
    – Gyan
    Jul 23, 2016 at 16:52

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