[FFmpeg-user] Why does fps == 1 for a webm videos?

[Misha Penkov]

Hello,

I have a webm video that is played back correctly with ffplay.  However,
its fps is incorrect:

ffprobe version N-51169-gcedf276 Copyright (c) 2007-2013 the FFmpeg
developers
  built on Mar 21 2013 18:12:33 with gcc 4.6 (Ubuntu/Linaro 4.6.3-1ubuntu5)
  configuration: --enable-gpl --enable-libass --enable-libfaac
--enable-libfdk-aac --enable-libmp3lame --enable-libopencore-amrnb
--enable-libopencore-amrwb --enable-libspeex --enable-librtmp
--enable-libtheora --enable-libvorbis --enable-libvpx --enable-x11grab
--enable-libx264 --enable-nonfree --enable-version3 --enable-shared
--enable-pic
  libavutil      52. 22.100 / 52. 22.100
  libavcodec     55.  1.100 / 55.  1.100
  libavformat    55.  0.100 / 55.  0.100
  libavdevice    55.  0.100 / 55.  0.100
  libavfilter     3. 48.100 /  3. 48.100
  libswscale      2.  2.100 /  2.  2.100
  libswresample   0. 17.102 /  0. 17.102
  libpostproc    52.  2.100 / 52.  2.100
Input #0, matroska,webm, from '8szDFB30azA.webm':
  Duration: 00:15:18.62, start: 0.000000, bitrate: 546 kb/s
    Stream #0:0(eng): Video: vp8, yuv420p, 640x360, SAR 1:1 DAR 16:9, 1k
fps, 1k tbr, 1k tbn, 1k tbc (default)
    Stream #0:1: Audio: vorbis, 44100 Hz, mono, fltp (default)

While it doesn't affect playback, it creates problems when I convert the
video using ffmpeg.  Specifically, the converted video is much longer than
expected.

What can I do to fix this?

Kind regards,
Michael