[Libav-user] Why container and codec has different time base?

[Morduhaev, Igor (igorm@stats.com)]

I used ffmpeg to get video info. The output is
  Duration: 00:05:57.00, start: 0.000000, bitrate: 611 kb/s Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 808x610, 609 kb/s, 25 fps, 25 tbr, 12800 tbn, 50 tbc
The time base is used to somehow(this is also my another question) calculate when to decode and show the frame, right? So whose time base is used, container (12800) or codec (50)?
The another question is why tbn=12800 and not 90000?

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