ffmpeg Changes Frame Rate to Variable?

[koberulz]

General
Format                                   : Matroska
Format version                           : Version 4
File size                                : 4.65 GiB
Duration                                 : 1 h 59 min
Overall bit rate                         : 5 588 kb/s
Frame rate                               : 59.940 FPS
Writing application                      : Lavf61.7.100
Writing library                          : Lavf61.7.100
ErrorDetectionType                       : Per level 1

Video
ID                                       : 1
Format                                   : AVC
Format/Info                              : Advanced Video Codec
Format profile                           : High@L3.2
Format settings                          : CABAC / 4 Ref Frames
Format settings, CABAC                   : Yes
Format settings, Reference frames        : 4 frames
Codec ID                                 : V_MPEG4/ISO/AVC
Duration                                 : 1 h 59 min
Bit rate mode                            : Constant
Nominal bit rate                         : 6 872 kb/s
Width                                    : 1 280 pixels
Height                                   : 720 pixels
Display aspect ratio                     : 16:9
Frame rate mode                          : Constant
Frame rate                               : 59.940 (60000/1001) FPS
Color space                              : YUV
Chroma subsampling                       : 4:2:0
Bit depth                                : 8 bits
Scan type                                : Progressive
Bits/(Pixel*Frame)                       : 0.124
Time code of first frame                 : 05:00:01;16
Writing library                          : AVC Coding
Default                                  : Yes
Forced                                   : No
Color range                              : Limited
VENDOR_ID                                : [0][0][0][0]

Audio
ID                                       : 2
Format                                   : AAC LC
Format/Info                              : Advanced Audio Codec Low Complexity
Codec ID                                 : A_AAC-2
Duration                                 : 1 h 59 min
Channel(s)                               : 2 channels
Channel layout                           : L R
Sampling rate                            : 48.0 kHz
Frame rate                               : 46.875 FPS (1024 SPF)
Compression mode                         : Lossy
Language                                 : English
Default                                  : Yes
Forced                                   : No
VENDOR_ID                                : [0][0][0][0]

ffmpeg -i source.mkv -c:a copy -c:v copy output.mp4

General
Format                                   : MPEG-4
Format profile                           : Base Media
Codec ID                                 : isom (isom/iso2/avc1/mp41)
File size                                : 4.66 GiB
Duration                                 : 1 h 59 min
Overall bit rate mode                    : Constant
Overall bit rate                         : 5 598 kb/s
Frame rate                               : 53.389 FPS
Tagged date                              : 2026-02-16 14:37:30 UTC
Writing application                      : Lavf61.9.102

Video
ID                                       : 1
Format                                   : AVC
Format/Info                              : Advanced Video Codec
Format profile                           : High@L3.2
Format settings                          : CABAC / 4 Ref Frames
Format settings, CABAC                   : Yes
Format settings, Reference frames        : 4 frames
Codec ID                                 : avc1
Codec ID/Info                            : Advanced Video Coding
Duration                                 : 1 h 59 min
Bit rate mode                            : Constant
Bit rate                                 : 5 471 kb/s
Nominal bit rate                         : 6 872 kb/s
Width                                    : 1 280 pixels
Height                                   : 720 pixels
Display aspect ratio                     : 16:9
Frame rate mode                          : Variable
Frame rate                               : 53.389 FPS
Minimum frame rate                       : 0.006 FPS
Maximum frame rate                       : 62.500 FPS
Original frame rate                      : 59.940 (60000/1001) FPS
Color space                              : YUV
Chroma subsampling                       : 4:2:0
Bit depth                                : 8 bits
Scan type                                : Progressive
Bits/(Pixel*Frame)                       : 0.111
Time code of first frame                 : 05:00:01;16
Stream size                              : 4.55 GiB (98%)
Codec configuration box                  : avcC

Audio
ID                                       : 2
Format                                   : AAC LC
Format/Info                              : Advanced Audio Codec Low Complexity
Codec ID                                 : mp4a-40-2
Duration                                 : 1 h 59 min
Bit rate mode                            : Constant
Bit rate                                 : 112 kb/s
Channel(s)                               : 2 channels
Channel layout                           : L R
Sampling rate                            : 48.0 kHz
Frame rate                               : 46.875 FPS (1024 SPF)
Compression mode                         : Lossy
Stream size                              : 95.1 MiB (2%)
Language                                 : English
Default                                  : Yes
Alternate group                          : 1

Text #1
ID                                       : 1-CC1
Format                                   : EIA-608
Muxing mode                              : SCTE 128 / DTVCC Transport
Muxing mode, more info                   : Muxed in Video #1
Duration                                 : 1 h 59 min
Start time (commands)                    : 5124095 h
Bit rate mode                            : Constant
Stream size                              : 0.00 Byte (0%)

Text #2
ID                                       : 1-CC3
Format                                   : EIA-608
Muxing mode                              : SCTE 128 / DTVCC Transport
Muxing mode, more info                   : Muxed in Video #1
Duration                                 : 1 h 59 min
Start time (commands)                    : 5124095 h
Bit rate mode                            : Constant
Stream size                              : 0.00 Byte (0%)

Where is the text coming from? Why does the frame rate change?

[ProWo]

Why does the frame rate change?

This happens for the mp4/mov container only.
The reason is, that the timebase is not exactly divisible by the frame rate.
You can fix this with clever FFmpeg-GUI.
Load your mp4, extract streams, check the video stream only, click extract, set raw stream.
Load the created .h264 raw stream, you'are asked for the framerate, enter 59.94
The file is streamcopied to a new mp4 file and automatically loaded.
Click multiplex, drag your original mp4 into, on the left deselect it's videostream only (all other streams should be selected), set mp4 as container and click multiplex.
Done.

[Vidd]

Your source is mkv. MKV muxing is not lossless and strips some values that ffmpeg needs to mux back to MP4. Since ffmpeg does not have the information, it has to calculate it causing the variable framerate. It comes out variable due to the different time base calculations.

If you have access to VideoReDo, perform a Quick Stream Fix on the mkv with mp4 output and it will be constant framerate.

Or you can add this to the ffmpeg command:

-fps_mode cfr -r 60000/1001

but you may need to rebuild the raw elementary streams.

See here:
https://forum.videohelp.com/threads/405114-ffmpeg-MKV-to-MP4-without-rencode-makes-out...-rate-variable



The text is closed captions. They are embeded in the video and were always there. You just did not see it in the mkv container.

[hello_hello]

Try opening your MP4 with MKVToolNix GUI and remuxing it as an MKV. You'll probably find MediaInfo no longer says it's VFR. If so, it's just a timebase issue for MP4s and nothing to worry about.

[koberulz]

Or you can add this to the ffmpeg command:

-fps_mode cfr -r 60000/1001

That gets me the same VFR output. Which makes even less sense.

Try opening your MP4 with MKVToolNix GUI and remuxing it as an MKV. You'll probably find MediaInfo no longer says it's VFR. If so, it's just a timebase issue for MP4s and nothing to worry about.

I want an MP4, not an MKV. I'm trying to pull the clip into Adobe Premiere, which doesn't support MKV or, I'm assuming, VFR, as the sync is completely screwed and parts of it play out in slow motion.

[hello_hello]

I suggested remuxing as an MKV simply as a test, as MKVToolNix saves statistic tags to the MKV that MediaInfo can read, rather than having to guess. Although MediaInfo reporting this for your MKV seems somewhat odd:

Time code of first frame: 05:00:01;16

I'd try remuxing the original MKV as another MKV with MKVToolNixGUI, while specifying a default FPS for the video stream of 60000/1001p. Also check the "fix bitstream timing info" option. If the new MKV plays okay and the audio is in sync, try remuxing it as an MP4 instead.

[koberulz]

If the new MKV plays okay and the audio is in sync, try remuxing it as an MP4 instead.

Well, the MP4 plays fine in VLC at the moment, it's just Premiere that can't handle it. And obviously I can't check MKV sync with Premiere, because that's the reason I'm doing the conversion in the first place.

[koberulz]

None of the above suggestions got me a CFR file. clever-ffmpeg and demuxing to .264 then remuxing both got me a VFR file with the frame rate varying between 59.940 and 59.943, which is at least small enough that I'm not seeing sync issues in Adobe.

[ProWo]

None of the above suggestions got me a CFR file. clever-ffmpeg and demuxing to .264 then remuxing both got me a VFR file with the frame rate varying between 59.940 and 59.943, which is at least small enough that I'm not seeing sync issues in Adobe.

There is another possibility to fix your vfr mp4 to cfr in clever FFmpeg-GUI.
Load your mp4, click lossless changes, check the "Fix time stamps" option.
If here your video is shown as 23.976 fps then click fix.
The new created mp4 video should be constant framerate.


[Attachment 91246 - Click to enlarge]

[hello_hello]

If the new MKV plays okay and the audio is in sync, try remuxing it as an MP4 instead.

Well, the MP4 plays fine in VLC at the moment, it's just Premiere that can't handle it. And obviously I can't check MKV sync with Premiere, because that's the reason I'm doing the conversion in the first place.

If it's truly variable frame rate and you change it to CFR, obviously the audio will go out of sync no matter what container type it's in or what program you use to play it. If it stays in sync, I'd be guessing you can remux the new CFR MKV as an MP4 instead to see how Premier copes with it.