[FFmpeg-user] no pts value

[Gabri Nurtinaz Shally]

i try to transcode only the audio but got this.

ffmpeg -i "input.avi" -c:v copy -c:a libvorbis -b:a 64k "output.mkv"
ffmpeg version N-35509-gfd36cbf-Sherpya, Copyright (c) 2000-2011 the FFmpeg
deve
lopers
  built on Dec  7 2011 06:17:11 with gcc 4.6.2
  libavutil    51. 30. 0 / 51. 30. 0
  libavcodec   53. 41. 0 / 53. 41. 0
  libavformat  53. 24. 0 / 53. 24. 0
  libavdevice  53.  4. 0 / 53.  4. 0
  libavfilter   2. 51. 0 /  2. 51. 0
  libswscale    2.  1. 0 /  2.  1. 0
  libpostproc  51.  2. 0 / 51.  2. 0
[avi @ 01db9c80] max_analyze_duration 5000000 reached at 5016000
Input #0, avi, from 'input.avi':
  Metadata:
    encoder         : MEncoder Sherpya-SVN-r34401-4.6.2
  Duration: 00:24:54.36, start: 0.000000, bitrate: 589 kb/s
    Stream #0:0: Video: h264 (High) (h264 / 0x34363268), yuv420p, 848x480
[SAR 4
31:424 DAR 431:240], SAR 160:159 DAR 16:9, 23.98 fps, 23.98 tbr, 23.98 tbn,
47.9
5 tbc
    Stream #0:1: Audio: mp3 (U[0][0][0] / 0x0055), 48000 Hz, stereo, s16,
128 kb
/s
Output #0, matroska, to 'output.mkv':
  Metadata:
    encoder         : Lavf53.24.0
    Stream #0:0: Video: h264 (h264 / 0x34363268), yuv420p, 848x480 [SAR
160:159
DAR 16:9], q=2-31, 23.98 fps, 1k tbn, 23.98 tbc
    Stream #0:1: Audio: vorbis, 48000 Hz, stereo, s16, 64 kb/s
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
  Stream #0:1 -> #0:1 (mp3 -> libvorbis)
Press [q] to stop, [?] for help
[matroska @ 02767ac0] Can't write packet with unknown timestamp
av_interleaved_write_frame(): Invalid argument

i try change the output format to mp4 and works, but with a lot of "no pts
value" message, may be all the time...

what should i do?
i know it is mencoder fault for not creating good avi file. but, is there
workaround? to forcing mencoder to add pts value for example ('-'a)