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I am using a Raspberry Pi 3B+ to record video from a ActiA31 security camera (connected vie a power-over-ethernet router). The camera is set to record at 29.97 fps. The command I am using is

sudo /usr/local/bin/ffmpeg -i rtsp://username:pwd@IPaddress -t 10 -vcodec copy -acodec copy outputfile.mp4

The video does record, but the metadata of the recorded video shows as slightly different framerate (fps). The output is below(using ffprobe):

Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuvj420p(pc, bt709), 2048x1536, 12074 kb/s, 30.02 fps, 29.97 tbr, 90k tbn, 180k tbc (default)

Why is the fps different than the tbr?

I've copied the output from the original ffmpeg command here in hopes that it will help solve the question. I've tried using the option '-use_wallclock_as_timestamps 1 ' but it didn't result in anything different. I am using the video for velocity analysis and need the framerate to be exactly whatever the camera is recording at, hence I can't set my own framerate. I am also getting some 'Invalid NAL' errors but those seem to be pretty common from what I've seen on stackexchange.

pi@raspberrypi:~/lspiv/scripts $ ffmpeg version N-94564-gaac382e9e5 Copyright (c) 2000-2019 the FFmpeg developers
  built with gcc 8 (Raspbian 8.3.0-6+rpi1)
  configuration: --arch=armel --target-os=linux --enable-gpl --enable-libx264 --enable-nonfree
  libavutil      56. 33.100 / 56. 33.100
  libavcodec     58. 55.100 / 58. 55.100
  libavformat    58. 30.100 / 58. 30.100
  libavdevice    58.  9.100 / 58.  9.100
  libavfilter     7. 58.100 /  7. 58.100
  libswscale      5.  6.100 /  5.  6.100
  libswresample   3.  6.100 /  3.  6.100
  libpostproc    55.  6.100 / 55.  6.100
[h264 @ 0x1e31790] Invalid NAL unit 0, skipping.
[h264 @ 0x1e31790] Invalid NAL unit 8, skipping.
    Last message repeated 5 times
[h264 @ 0x1e329e0] Invalid NAL unit 0, skipping.
[h264 @ 0x1e329e0] Invalid NAL unit 8, skipping.
    Last message repeated 1 times
[h264 @ 0x1e329e0] Invalid NAL unit 0, skipping.
[h264 @ 0x1e329e0] Invalid NAL unit 8, skipping.
[rtsp @ 0x1e2e390] decoding for stream 1 failed
Input #0, rtsp, from 'rtsp://username:pwd@IPaddress':
  Metadata:
    title           : Video Server Session
  Duration: N/A, start: 0.033344, bitrate: N/A
    Stream #0:0: Video: h264 (High), yuvj420p(pc, bt709, progressive), 2048x1536, 59.94 tbr, 90k tbn, 180k tbc
    Stream #0:1: Video: h264 (High), yuvj420p(pc, bt709, progressive), 320x240, 90k tbr, 90k tbn, 180k tbc
Output #0, mp4, to '/home/pi/lspiv/video_ops/saved_videos/19860501_20200226_112619.mp4':
  Metadata:
    title           : Video Server Session
    encoder         : Lavf58.30.100
    Stream #0:0: Video: h264 (High) (avc1 / 0x31637661), yuvj420p(pc, bt709, progressive), 2048x1536, q=2-31, 59.94 tbr, 90k tbn, 90k tbc
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
Press [q] to stop, [?] for help
[mp4 @ 0x1e90670] Timestamps are unset in a packet for stream 0. This is deprecated and will stop working in the future. Fix your code to set the timestamps properly
[mp4 @ 0x1e90670] Non-monotonous DTS in output stream 0:0; previous: 0, current: 0; changing to 1. This may result in incorrect timestamps in the output file.
frame=  301 fps= 63 q=-1.0 Lsize=     412kB time=00:00:09.99 bitrate= 337.9kbits/s speed=2.09x
video:409kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.871962%
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  • There will some jitter in the period of the frames and the fps indicates the average framerate. – Gyan Feb 26 '20 at 18:37
  • @Gyan is there any way to fix this? – Bomhof Feb 27 '20 at 13:32
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    There's nothing really to fix. ffmpeg is calculating fps based on timestamps of frames, and those timestamps originate at the camera. You can check if your camera can be forced to output at exact intervals only but I doubt it. Or you could retime frames - will require a re-encode. I'd suggest to just use timestamps instead of inferring it from fps. – Gyan Feb 27 '20 at 15:38
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Did you try to use the input option -r?

ffmpeg -r 30 -i rtsp://...

or for 29,97

ffmpeg -r 30000/1001 -i rtsp://...
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  • I tried this and it didn't have any effect on the fps as determined by ffprobe – Bomhof Mar 3 '20 at 16:59

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