Fifty frames per second with half of them redundant.

[Ralph Corderoy]

Hi,

I'm used to an hour of iPlayer video needing about 1 GiB.  In the past,
this doubled for a while because the frame rate doubled from 25 to 50
per second.  But stepping through the frames, say with mpv(1)'s ‘.’,
showed the first frame of a pair will be a scene update and the second
is a very minor adjustment of the pixels.  So the camera shot might move
in frames 1, 3, 5, ... and nothing much happen in frames 2, 4, 6...
A waste for the BBC and me.  Then downloads went back to the normal
1 GiB/hour.

Recently, like the last few weeks, it's doubled again with the same
cause.  Take PID m001x0zq.  ffprobe(1) shows

    Stream #0:0(und): Video:
        h264 (High) (avc1 / 0x31637661),
        yuv420p(tv, bt709),
        1280x720 [SAR 1:1 DAR 16:9],
        5058 kb/s,
        50 fps,
        50 tbr,
        90k tbn,
        100 tbc (default)

    Stream #0:1(eng): Audio:
        aac (LC) (mp4a / 0x6134706D),
        48000 Hz,
        stereo,
        fltp,
        128 kb/s (default)

    Stream #0:2: Video:
        mjpeg,
        yuvj420p(pc, bt470bg/unknown/unknown),
        192x108 [SAR 72:72 DAR 16:9],
        90k tbr,
        90k tbn,
        90k tbc

Does anyone have insights as to why this happens and what causes the BBC
to return to normal?

Is there anything I can do to force a sane 25 fps without dropping
quality?

-- 
Cheers, Ralph.