Properly Cutting MKVs by Keyframes Using FFmpeg

[simon744]

Hi, everyone! I have an MKV file that I want to cut a small portion from, using ffmpeg. Here's the MediaInfo log of it:

Video
ID                                       : 1
Format                                   : AVC
Format/Info                              : Advanced Video Codec
Format profile                           : High 4:2:2@L4.1
Format settings                          : CABAC / 2 Ref Frames
Format settings, CABAC                   : Yes
Format settings, Reference frames        : 2 frames
Format settings, GOP                     : M=1, N=36
Codec ID                                 : V_MPEG4/ISO/AVC
Duration                                 : 2 h 47 min
Bit rate mode                            : Constant
Bit rate                                 : 18.9 Mb/s
Nominal bit rate                         : 20.9 Mb/s
Width                                    : 1 920 pixels
Height                                   : 1 080 pixels
Display aspect ratio                     : 16:9
Frame rate mode                          : Variable
Frame rate                               : 59.940 (60000/1001) FPS
Original frame rate                      : 29.970 (30000/1001) FPS
Color space                              : YUV
Chroma subsampling                       : 4:2:2
Bit depth                                : 8 bits
Scan type                                : Interlaced
Scan type, store method                  : Separated fields
Scan order                               : Top Field First
Bits/(Pixel*Frame)                       : 0.152
Stream size                              : 22.1 GiB (83%)
Default                                  : Yes
Forced                                   : No

Audio
ID                                       : 2
Format                                   : DTS XLL
Format/Info                              : Digital Theater Systems
Commercial name                          : DTS-HD Master Audio
Codec ID                                 : A_DTS
Duration                                 : 2 h 47 min
Bit rate mode                            : Variable
Bit rate                                 : 3 725 kb/s
Channel(s)                               : 6 channels
Channel layout                           : C L R Ls Rs LFE
Sampling rate                            : 48.0 kHz
Frame rate                               : 93.750 FPS (512 SPF)
Bit depth                                : 24 bits
Compression mode                         : Lossless
Stream size                              : 4.37 GiB (16%)
Language                                 : English
Default                                  : Yes
Forced                                   : No

And here's the command that I'm using:

ffmpeg -i "C:\FFmpeg\bin\input.mkv" -ss 01:47:49 -to 01:54:52 -codec copy output.mkv

However, after cutting, ffmpeg changes the framerate value from the original 59.940 (60000/1001) FPS to 1 428.238 FPS. It adds a full second delay to the audio, and it also changes the Bits/(Pixel*Frame) value from 0.152 to 0.006.

Can somebody please help me figure out the right command to do this properly without messing up everything. Thanks, in advance!

[ProWo]

You don't know, where the keyframes are.
You can cut with streamcopy on keyframes only.
Cutting with the ffmpeg command line is extremely tricky, even if you know the exact keyframes.
Use avidemux or clever FFmpeg-GUI for this, the complicated calculations are done by the program.

[simon744]

You don't know, where the keyframes are.
You can cut with streamcopy on keyframes only.
Cutting with the ffmpeg command line is extremely tricky, even if you know the exact keyframes.
Use avidemux or clever FFmpeg-GUI for this, the complicated calculations are done by the program.

Thanks so much, but the thing is that keyframes are exactly what am trying to cut with. I get the exact same result with Clever FFmpeg-GUI that I get with FFmpeg's CLI. Except the file outputted by FFmpeg has worse audio delay (1s+ against -25ms) than the one by Clever FFmpeg-GUI. Here's the MediaInfo log:

Video
ID                                       : 1
Format                                   : AVC
Format/Info                              : Advanced Video Codec
Format profile                           : High 4:2:2@L4.1
Format settings                          : CABAC / 2 Ref Frames
Format settings, CABAC                   : Yes
Format settings, Reference frames        : 2 frames
Format settings, GOP                     : M=1, N=36
Codec ID                                 : V_MPEG4/ISO/AVC
Duration                                 : 7 min 3 s
Bit rate mode                            : Constant
Bit rate                                 : 18.9 Mb/s
Nominal bit rate                         : 20.9 Mb/s
Width                                    : 1 920 pixels
Height                                   : 1 080 pixels
Display aspect ratio                     : 16:9
Frame rate mode                          : Variable
Frame rate                               : 1 428.238 FPS
Original frame rate                      : 29.970 (30000/1001) FPS
Color space                              : YUV
Chroma subsampling                       : 4:2:2
Bit depth                                : 8 bits
Scan type                                : Interlaced
Scan type, store method                  : Separated fields (2 fields per block)
Scan order                               : Top Field First
Bits/(Pixel*Frame)                       : 0.006
Stream size                              : 22.1 GiB
Default                                  : Yes
Forced                                   : No

Audio
ID                                       : 2
Format                                   : DTS XLL
Format/Info                              : Digital Theater Systems
Commercial name                          : DTS-HD Master Audio
Codec ID                                 : A_DTS
Duration                                 : 7 min 3 s
Bit rate mode                            : Variable
Bit rate                                 : 3 725 kb/s
Channel(s)                               : 6 channels
Channel layout                           : C L R Ls Rs LFE
Sampling rate                            : 48.0 kHz
Frame rate                               : 93.750 FPS (512 SPF)
Bit depth                                : 24 bits
Compression mode                         : Lossless
Delay relative to video                  : -25 ms
Stream size                              : 4.37 GiB
Language                                 : English
Default                                  : Yes
Forced                                   : No

The frame rate is still displayed incorrectly, so is the Bits/(Pixel*Frame) value...it's supposed to be 0.152. The duration for the audio and video tracks too is incorrect here. Even if I'm cutting out a large file to around 4~ minutes, by this method, the output should have the metadata of the original file from which it was cut. I then ran the following command to correct the frame rate:

ffmpeg -i input.mkv -r 59.94 -vcodec copy output.mkv

Not only does this not adjust the frame rate, but it also transcodes the DTS-HD MA track down to Opus. Adding "-acodec copy" to the command changes nothing.

[ProWo]

Your source file seems to have incorrect framerate info.
I think it is 29.970 fps, not 59.940 fps.
try the following:
Load it in clever Ffmpeg-GUI, click main, click extract streams, select the videostream only, click extract, set export raw stream, not container.
Then click source, drag & drop the new raw file, enter 29.970 in the fps field.
click multiplex, drag & drop the original source, set the blue videostream and the imported audiostream and click muliplex.
Finally repeat the cut with the new generated file.

[simon744]

Your source file seems to have incorrect framerate info.
I think it is 29.970 fps, not 59.940 fps.

Well, I think that might actually be the field rate, not frame, since the source is interlaced. MediaInfo just happens to usually refer to fields in such cases.

try the following:
Load it in clever Ffmpeg-GUI, click main, click extract streams, select the videostream only, click extract, set export raw stream, not container.
Then click source, drag & drop the new raw file, enter 29.970 in the fps field.
click multiplex, drag & drop the original source, set the blue videostream and the imported audiostream and click muliplex.
Finally repeat the cut with the new generated file.

Thanks so much, but doesn't that sound like a little too much work for this? I would've given it a shot, but I don't have 22+ GBs of free hard disk space in my possession at the moment.