I'm downloading iPlayer videos but the only way to get the best picture quality of 1280 x 720p 5 Mbps is to force 50fps in the downloader. Otherwise you get 960 x 540 1.6 Mbps. I could remove the frames with Avisynth but I would lose quality re-encoding. Is there a way to remove the frames with a muxer or something else without re-encoding?
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Why do you want to do this? I assume your answer will be "to get a smaller file size." However, you are mistaken that this will produce a smaller file size. File size is determine by one, and only one, metric: the encoding bitrate. Therefore, whether you remove frames or not, you would have to re-encode at a lower bitrate in order to get a smaller file size.
Also, removing all duplicate frames and then changing the playback to 25 fps will not make the video look any different when you play it. In fact, there will be 0.00% difference.
To understand why this is so, consider this thought. Take a 25 fps progressive video and add forty duplicate frames for each original frame in order to get to 1,000 fps. Play this back at 1,000 fps so that the speed of the motion is identical to the original 25 fps video. The 1,000 fps version will look 100% identical to the original 25 fps because there is no difference between showing the original frame for 1/40 of the original time period, followed by thirty-nine duplicates each shows for 1/40 of the original duration, and simply showing the original frame for the entire 1/25 of a second. For those who remember movie film, you might initially think what I say is wrong because, when watching a movie there is an interruption between frames of film when the projector shutter closes and the film is advanced to the next frame. There is no such mechanism with video however, and therefore you have no moment between frames when the screen goes blank.
Bottom line: there will be zero difference in what you see when looking at the 50 fps version you downloaded and the 25 fps original, assuming that the 50 fps version was created by simply duplicating each and every original video frame.
OK thanks. Just wanted to check. Only wanted to remove the duplicate frames in case you could see the duplicate frames in playback.
AVISynth script, and the results will probably never be the same as the original.