ffmpeg concat give Non-monotonous DTS in output stream

Question

Env: Windows7, ffmpeg 4.2.2

copy the first 10 seconds from long.mp4 to UNO.mp4
copy sec 120 to sec 130 from long.mp4 to DUE.mp4    

create List.txt with

file 'C:\Users\Admin\Videos\UNO.mp4'
file 'C:\Users\Admin\Videos\DUE.mp4'

here the ffprobe output of the two files:

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'UNO.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf58.29.100
  Duration: 00:00:10.04, start: 0.000000, bitrate: 671 kb/s
    Stream #0:0(und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p(tv, smpte170m/smpte170m/bt709), 640x360 [SAR 1:1 DAR 16:9], 536 kb/s, 25 fps, 25 tbr, 12800 tbn, 50 tbc (default)
    Metadata:
      handler_name    : ISO Media file produced by Google Inc. Created on: 09/18/2020.
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 128 kb/s (default)
    Metadata:
      handler_name    : ISO Media file produced by Google Inc. Created on: 09/18/2020.

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'DUE.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf58.29.100
  Duration: 00:00:11.96, start: 0.000000, bitrate: 657 kb/s
    Stream #0:0(und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p(tv, smpte170m/smpte170m/bt709), 640x360 [SAR 1:1 DAR 16:9], 524 kb/s, 25 fps, 25 tbr, 12800 tbn, 50 tbc (default)
    Metadata:
      handler_name    : ISO Media file produced by Google Inc. Created on: 09/18/2020.
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 127 kb/s (default)
    Metadata:
      handler_name    : ISO Media file produced by Google Inc. Created on: 09/18/2020.

use the two files as input to

ffmpeg -f concat -safe 0 -i "C:\Users\Admin\Videos\List.txt" -c copy "C:\Users\Admin\Videos\mergedVideo.mp4"   

it outputs:

mov,mp4,m4a,3gp,3g2,mj2 @ 00be7f80] Auto-inserting h264_mp4toannexb bitstream filter
nput #0, concat, from 'C:\Users\Admin\Videos\List.txt':
 Duration: N/A, start: 0.000000, bitrate: 825 kb/s
   Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 624x352 [SAR 1:1 DAR 39:22], 697 kb/s, 25 fps, 25
tbr, 90k tbn, 50 tbc
   Metadata:
     handler_name    : VideoHandler
   Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 128 kb/s
   Metadata:
     handler_name    : SoundHandler
utput #0, mp4, to 'C:\Users\Admin\Videos\mergedVideo.mp4':
 Metadata:
   encoder         : Lavf58.29.100
   Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 624x352 [SAR 1:1 DAR 39:22], q=2-31, 697 kb/s, 25
fps, 25 tbr, 90k tbn, 90k tbc
   Metadata:
     handler_name    : VideoHandler
   Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 128 kb/s
   Metadata:
     handler_name    : SoundHandler
tream mapping:
 Stream #0:0 -> #0:0 (copy)
 Stream #0:1 -> #0:1 (copy)

then a long series of :

mp4 @ 00c43c40] Non-monotonous DTS in output stream 0:0; previous: 899370, current: 720000; changing to 899371. This may result in incorrect timestamps in the output file.

at end as last messages :

rame=  552 fps=528 q=-1.0 Lsize=    2220kB time=00:00:20.01 bitrate= 908.4kbits/s speed=19.2x
ideo:1873kB audio:329kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.827539% 

When i play the output mergedVideo.mp4 with ffplay, it does not gove errors it is only a time stamp problem, or some frames or some audio are lost ? in the latter case, is there a parameter to correct the problem, or i have to re-encode ? (to further test, i tried with avidemux an it concats UNO.mp4 and DUE.mp4 without re-encoding)

Thanks in advance to all for the support Maurizio

Answer 1

First of all, unless you call FFmpeg with the -xerror option (which you didn't) "Non-monotonous DTS..." message is not an error. It's just a warning. It is just informing what it's doing for you, and thankfully its automatic adjustment indeed worked for your case base on your description.

It is making the adjustment because you are concatenate-and-copying streams, of which data frame carries timestamp (unlike WAV or MPEG streams). The concat demuxer likely cannot adjust the timestamp in the stream data on the fly (to avoid the warnings/corrections from the main ffmpeg program).

The bottom line. If the warning messages must go, change the log level to ignore warning: -loglevel error.

Reference ffmpeg.c Lines 817-834