Help me understand framerate disrepency

[jamesonuk]

I have two Reolink RCL-820As setup in Frigate and I noticed a difference in the reported fps between the feed direct from the camera an the feed via go2RTC.

I have started doing some digging and when I grabbed the steam using ffmpeg

ffmpeg -i rtsp://user:pass@camera:554/ -use_wallclock_as_timestamps 1 -c copy /ff/camera.mkv

Then I get this for the direct camera feed (there are some issues in there but the metadata does show up as 15fps which is what I had changed it to for testing.

ffmpeg camera output

[rtsp @ 0x1d8c000] max delay reached. need to consume packet
[rtsp @ 0x1d8c000] RTP: missed 24 packets
[h264 @ 0x1d90e00] error while decoding MB 106 35, bytestream -5
[h264 @ 0x1d90e00] concealing 8743 DC, 8743 AC, 8743 MV errors in P frame
[rtsp @ 0x1d8c000] max delay reached. need to consume packet
[rtsp @ 0x1d8c000] RTP: missed 21 packets
[h264 @ 0x1d90e00] error while decoding MB 23 44, bytestream -7
[h264 @ 0x1d90e00] concealing 7386 DC, 7386 AC, 7386 MV errors in P frame
[rtsp @ 0x1d8c000] max delay reached. need to consume packet
[rtsp @ 0x1d8c000] RTP: missed 5 packets
[h264 @ 0x1d90e00] concealing 3885 DC, 3885 AC, 3885 MV errors in P frame
[rtsp @ 0x1d8c000] max delay reached. need to consume packet
[rtsp @ 0x1d8c000] RTP: missed 47 packets
Input #0, rtsp, from 'rtsp://user:password@camera:554/':
  Metadata:
    title           : Session streamed by "preview"
    comment         : reolink rtsp stream
  Duration: N/A, start: 0.000313, bitrate: N/A
    Stream #0:0: Video: h264, yuv420p(progressive), 2560x1440, 15 fps, 15 tbr, 90k tbn, 30 tbc
    Stream #0:1: Audio: aac, 16000 Hz, mono, fltp
Output #0, matroska, to '/ff/camera.mkv':
  Metadata:
    title           : Session streamed by "preview"
    comment         : reolink rtsp stream
    encoder         : Lavf58.20.100
    Stream #0:0: Video: h264 (H264 / 0x34363248), yuv420p(progressive), 2560x1440, q=2-31, 15 fps, 15 tbr, 1k tbn, 90k tbc
    Stream #0:1: Audio: aac ([255][0][0][0] / 0x00FF), 16000 Hz, mono, fltp
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
  Stream #0:1 -> #0:1 (copy)
Press [q] to stop, [?] for help
[matroska @ 0x1df3800] Non-monotonous DTS in output stream 0:0; previous: 386, current: 78; changing to 386. This may result in incorrect timestamps in the output file.
[matroska @ 0x1df3800] Non-monotonous DTS in output stream 0:0; previous: 386, current: 198; changing to 386. This may result in incorrect timestamps in the output file.
[matroska @ 0x1df3800] Non-monotonous DTS in output stream 0:0; previous: 386, current: 220; changing to 386. This may result in incorrect timestamps in the output file.
[matroska @ 0x1df3800] Non-monotonous DTS in output stream 0:0; previous: 386, current: 263; changing to 386. This may result in incorrect timestamps in the output file.
[matroska @ 0x1df3800] Non-monotonous DTS in output stream 0:0; previous: 386, current: 358; changing to 386. This may result in incorrect timestamps in the output file.
[rtsp @ 0x1d8c000] max delay reached. need to consume packetbitrate=7357.6kbits/s speed=1.59x
[rtsp @ 0x1d8c000] RTP: missed 2 packets
[rtsp @ 0x1d8c000] max delay reached. need to consume packetbitrate=6666.6kbits/s speed=1.26x
[rtsp @ 0x1d8c000] RTP: missed 4 packets
frame=  933 fps= 15 q=-1.0 Lsize=   64338kB time=00:01:02.96 bitrate=8370.7kbits/s speed=1.04x
video:63834kB audio:488kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.025104%

ffmpeg go2RTC output

Input #0, rtsp, from 'rtsp://192.168.40.201:8554/hall_camera_frigate':
  Metadata:
    title           : go2rtc/1.9.2
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: h264, yuv420p(progressive), 2560x1440, 90k tbr, 90k tbn, 180k tbc
    Stream #0:1: Audio: aac, 16000 Hz, mono, fltp
Output #0, matroska, to '/ff/go.mkv':
  Metadata:
    title           : go2rtc/1.9.2
    encoder         : Lavf58.20.100
    Stream #0:0: Video: h264 (H264 / 0x34363248), yuv420p(progressive), 2560x1440, q=2-31, 90k tbr, 1k tbn, 90k tbc
    Stream #0:1: Audio: aac ([255][0][0][0] / 0x00FF), 16000 Hz, mono, fltp
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
  Stream #0:1 -> #0:1 (copy)
Press [q] to stop, [?] for help
frame=  954 fps= 15 q=-1.0 Lsize=   65705kB time=00:01:04.93 bitrate=8288.7kbits/s speed=1.05x

which does seem to suggest the feed is 15fps but the metadata appears to have bee lost (not sure what tbr, tbn, tbc mean)

If I open up the go2RTC stream in VLC it is reporting the frame rate as 29.97 (but if I play the mkv output from above there is no fps at all)

Now the go2RTC feed appears to play better than the direct camera feed and I am trying to work out what (if anything) go2RTC is doing to change the feed (or is this just an issue with what VLC is showing as the metadata ?)