[FFmpeg-user] ffmpeg recording RTSP stream with audio out of sync

[Calvert, Andy]

Hi,
I am having a problem with the video / audio sync when recording from an RTSP stream with ffmpeg.

This is the script I am using (only doing a 10 second clip for test):
-channel_layout mono -i "rtsp://172.16.210.50/RTP-Unicast_pMediaProfile4" -c:v copy -bsf:a aac_adtstoasc -y
-t 10 -movflags faststart "streamtest_high_%datetimef%.mp4"

Which seems to capture the source without any errors:

C:\STREAMTEST>c:\ffmpeg\ffmpeg -channel_layout mono -i "rtsp://172.16.210.50/RTP
-Unicast_pMediaProfile4" -c:v copy -bsf:a aac_adtstoasc -y -t 10 -movflags fasts
tart "streamtest_high_30012015_102911.mp4"
ffmpeg version N-69406-ga73c411 Copyright (c) 2000-2015 the FFmpeg developers
  built on Jan 29 2015 22:02:28 with gcc 4.9.2 (GCC)
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-av
isynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enab
le-iconv --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --
enable-libfreetype --enable-libgme --enable-libgsm --enable-libilbc --enable-lib
modplug --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrw
b --enable-libopenjpeg --enable-libopus --enable-librtmp --enable-libschroedinge
r --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --en
able-libvidstab --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-libvorbis
--enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-
libx265 --enable-libxavs --enable-libxvid --enable-lzma --enable-decklink --enab
le-zlib
  libavutil      54. 18.100 / 54. 18.100
  libavcodec     56. 21.101 / 56. 21.101
  libavformat    56. 19.100 / 56. 19.100
  libavdevice    56.  4.100 / 56.  4.100
  libavfilter     5.  9.101 /  5.  9.101
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  1.100 /  1.  1.100
  libpostproc    53.  3.100 / 53.  3.100
Input #0, rtsp, from 'rtsp://172.16.210.50/RTP-Unicast_pMediaProfile4':
  Metadata:
    title           : Session streamed by Oslo
    comment         : RTP-Unicast_pMediaProfile4
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #0:0: Audio: pcm_mulaw, 8000 Hz, mono, s16, 64 kb/s
    Stream #0:1: Video: h264 (Baseline), yuv420p, 1280x720, 25 fps, 25 tbr, 90k
tbn, 50 tbc
Output #0, mp4, to 'streamtest_high_30012015_102911.mp4':
  Metadata:
    title           : Session streamed by Oslo
    comment         : RTP-Unicast_pMediaProfile4
    encoder         : Lavf56.19.100
    Stream #0:0: Video: h264 ([33][0][0][0] / 0x0021), yuv420p, 1280x720, q=2-31
, 25 fps, 25 tbr, 90k tbn, 90k tbc
    Stream #0:1: Audio: aac (libvo_aacenc) ([64][0][0][0] / 0x0040), 8000 Hz, mo
no, s16, 128 kb/s
    Metadata:
      encoder         : Lavc56.21.101 libvo_aacenc
Stream mapping:
  Stream #0:1 -> #0:0 (copy)
  Stream #0:0 -> #0:1 (pcm_mulaw (native) -> aac (libvo_aacenc))
Press [q] to stop, [?] for help
frame=   43 fps=0.0 q=-1.0 size=     566kB time=00:00:02.58 bitrate=1795.0kbits/
frame=   56 fps= 53 q=-1.0 size=     863kB time=00:00:03.10 bitrate=2277.3kbits/
frame=   68 fps= 43 q=-1.0 size=    1138kB time=00:00:03.59 bitrate=2596.3kbits/
frame=   81 fps= 39 q=-1.0 size=    1403kB time=00:00:04.10 bitrate=2803.0kbits/
frame=   95 fps= 36 q=-1.0 size=    1646kB time=00:00:04.66 bitrate=2893.5kbits/
frame=  108 fps= 34 q=-1.0 size=    1900kB time=00:00:05.18 bitrate=3001.6kbits/
frame=  120 fps= 33 q=-1.0 size=    2159kB time=00:00:05.66 bitrate=3122.7kbits/
frame=  134 fps= 32 q=-1.0 size=    2325kB time=00:00:06.22 bitrate=3060.6kbits/
frame=  147 fps= 31 q=-1.0 size=    2583kB time=00:00:06.73 bitrate=3139.3kbits/
frame=  159 fps= 31 q=-1.0 size=    2829kB time=00:00:07.21 bitrate=3210.4kbits/
frame=  172 fps= 30 q=-1.0 size=    3163kB time=00:00:07.74 bitrate=3346.2kbits/
frame=  186 fps= 30 q=-1.0 size=    3378kB time=00:00:08.30 bitrate=3332.5kbits/
frame=  199 fps= 29 q=-1.0 size=    3598kB time=00:00:08.82 bitrate=3341.1kbits/
frame=  211 fps= 29 q=-1.0 size=    3902kB time=00:00:09.30 bitrate=3436.8kbits/
frame=  224 fps= 29 q=-1.0 size=    4181kB time=00:00:09.81 bitrate=3489.1kbits/
[mp4 @ 04dfb960] Starting second pass: moving the moov atom to the beginning of
the file
frame=  228 fps= 28 q=-1.0 Lsize=    4402kB time=00:00:10.05 bitrate=3587.7kbits
/s
video:4382kB audio:13kB subtitle:0kB other streams:0kB global headers:0kB muxing
overhead: 0.138498%

The result seems leave the video and audio out of sync in Windows Media Player (which unfortunately is the application I need to use for this), where as VLC seems to be able to cope with this and rectifies the problem.
I thought that it maybe a buffering problem, that ffmpeg is writing the audio track as soon as it receives it, which must be before the video track. I had some success with the -ss command by delaying the recording by x seconds, which seemed to work on some systems but not others.

Does anyone have any idea what is causing this, and provide any suggestions?
Many thanks in advance