MP4 file has video speed up part of the way through but audio does not

[the_steve_randolph]

I gave you some hints and you answer with questions instead of working off the hints.
I am out

You tell me to do stuff with incomplete instructions. I try and get an error message that I posted, not knowing if I should proceed or not. So, I don't know what I am doing and I am not learning anything. If that offends you, so be it.

[davexnet]

Open the source in Avidemux. Set the output format (lower left) to mp4 muxer.
Use the A and B buttons at the bottom to mark off a short section, one or two minutes.
Save, and verify that the file plays back properly. This will be the reference file for untrunc.

Open untrunc and give it the reference file and the bad file, it will create a third, the "fixed" file.

[Nilzzon]

TMPGEnc MPEG Smart Renderer

[the_steve_randolph]

Open the source in Avidemux. Set the output format (lower left) to mp4 muxer.
Use the A and B buttons at the bottom to mark off a short section, one or two minutes.
Save, and verify that the file plays back properly. This will be the reference file for untrunc.

Open untrunc and give it the reference file and the bad file, it will create a third, the "fixed" file.

First, in post #23 I post the error message when I first open the MP4. Nobody has addressed it, so, I don't know if Avidemux can deal with the file or if I should click on "proceed". Also, nobody has addressed by Avidemux is recommended or required to create the reference file. So, I used this command to create a 2 minute reference file:

ffmpeg out.mp4 -t 00:02:00.000 -c copy to reference.mp4"

Then, I ran untrunc-gui. The first time, I ran it without any changes in the settings tab and it generated this error:

Assertion failed!
Program:  untrunc-gui.exe
File:  src/atom.cpp, Line 481
Expression:  end - start = = new_length
<Abort>  <Retry>  <Ignore>

I clicked "ignore" and it finished without any other error messages. untrunc does not seem to have the ability to generate a log file and trying to highlight and copy the displayed log does not work. Windows and VLC show the length of the fixed MP4 to be 2:52:05, which is 11 seconds longer than the original MP4. The untrunc-gui log screen shows "Duration of mp4a: 2h 51 min 54s 536ms" and "Duration of avc1: 2h 52min 5s 736ms". When I play it in VLC, it shows a length of 2:52:05 and everything is good until 1:42:22 when both the video and the audio stops. This is the MediaInfo for that MP4:

General
Complete name                            : C:out_fixed.mp4
Format                                   : MPEG-4
Format profile                           : Base Media
Codec ID                                 : isom (isom/iso2/avc1/mp41)
File size                                : 4.66 GiB
Duration                                 : 2 h 52 min
Overall bit rate                         : 3 877 kb/s
Writing application                      : Lavf58.78.100
IsTruncated                              : Yes

Video
ID                                       : 1
Format                                   : AVC
Format/Info                              : Advanced Video Codec
Format profile                           : High@L4
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                                 : 2 h 52 min
Bit rate                                 : 6 402 kb/s
Maximum bit rate                         : 20.1 Mb/s
Width                                    : 1 920 pixels
Height                                   : 1 080 pixels
Display aspect ratio                     : 16:9
Frame rate mode                          : Variable
Frame rate                               : 29.937 FPS
Minimum frame rate                       : 5.994 FPS
Maximum frame rate                       : 120.000 FPS
Original frame rate                      : 29.970 (30000/1001) FPS
Color space                              : YUV
Chroma subsampling                       : 4:2:0
Bit depth                                : 8 bits
Scan type                                : Progressive
Bits/(Pixel*Frame)                       : 0.103
Stream size                              : 7.70 GiB
Codec configuration box                  : avcC

Audio
ID                                       : 2
Format                                   : AAC LC
Format/Info                              : Advanced Audio Codec Low Complexity
Codec ID                                 : mp4a-40-2
Duration                                 : 2 h 51 min
Bit rate mode                            : Constant
Bit rate                                 : 93.6 kb/s
Channel(s)                               : 2 channels
Channel layout                           : L R
Sampling rate                            : 44.1 kHz
Frame rate                               : 43.066 FPS (1024 SPF)
Compression mode                         : Lossy
Stream size                              : 118 MiB (2%)
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                                 : 2 h 52 min
Bit rate mode                            : Constant
Stream size                              : 0.00 Byte (0%)
CaptionServiceName                       : CC1

Text #2
ID                                       : 1-CC3
Format                                   : EIA-608
Muxing mode                              : SCTE 128 / DTVCC Transport
Muxing mode, more info                   : Muxed in Video #1
Duration                                 : 2 h 52 min
Bit rate mode                            : Constant
Stream size                              : 0.00 Byte (0%)
CaptionServiceName                       : CC3

Next, I ran untrunc-gui again, but, this time in the "Settings" tab, I click "on":

• "skip unknown (-s)"
• "stretch video to match audio (-sv)"
• "keep unknown sequences (-k)"

Windows also shows this MP4 to be 2:52:05 long with the same "Duration" information as above. When I view this MP4 in VLC, it shows the length of 2:52:05 and plays all of the way to the end. But, the audio is out of sync with the video throughout. It seems that the 11 second difference is spread out the entire length of the viewing - gradually getting worse. Here is the MediaInfo if that MP4:

General
Complete name                            : C:out.mp4_fixed-s1-k-sv.mp4
Format                                   : MPEG-4
Format profile                           : Base Media
Codec ID                                 : isom (isom/iso2/avc1/mp41)
File size                                : 7.82 GiB
Duration                                 : 2 h 52 min
Overall bit rate                         : 6 507 kb/s
Writing application                      : Lavf58.78.100

Video
ID                                       : 1
Format                                   : AVC
Format/Info                              : Advanced Video Codec
Format profile                           : High@L4
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                                 : 2 h 52 min
Bit rate                                 : 6 402 kb/s
Maximum bit rate                         : 20.1 Mb/s
Width                                    : 1 920 pixels
Height                                   : 1 080 pixels
Display aspect ratio                     : 16:9
Frame rate mode                          : Variable
Frame rate                               : 29.937 FPS
Minimum frame rate                       : 5.994 FPS
Maximum frame rate                       : 120.000 FPS
Original frame rate                      : 29.970 (30000/1001) FPS
Color space                              : YUV
Chroma subsampling                       : 4:2:0
Bit depth                                : 8 bits
Scan type                                : Progressive
Bits/(Pixel*Frame)                       : 0.103
Stream size                              : 7.70 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                                 : 2 h 51 min
Bit rate mode                            : Constant
Bit rate                                 : 93.6 kb/s
Channel(s)                               : 2 channels
Channel layout                           : L R
Sampling rate                            : 44.1 kHz
Frame rate                               : 43.066 FPS (1024 SPF)
Compression mode                         : Lossy
Stream size                              : 118 MiB (1%)
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                                 : 2 h 52 min
Bit rate mode                            : Constant
Stream size                              : 0.00 Byte (0%)
CaptionServiceName                       : CC1

Text #2
ID                                       : 1-CC3
Format                                   : EIA-608
Muxing mode                              : SCTE 128 / DTVCC Transport
Muxing mode, more info                   : Muxed in Video #1
Duration                                 : 2 h 52 min
Bit rate mode                            : Constant
Stream size                              : 0.00 Byte (0%)
CaptionServiceName                       : CC3

So, it looks like untrunc is able to fix the mismatched sync, but, it is not exact. I am not sure what to try next.

[the_steve_randolph]

TMPGEnc MPEG Smart Renderer

And, what am I supposed to do with this app? The description does not mention the problem I am dealing with.

[the_steve_randolph]

OK, it looks like I have a solution. After running untrunc, I end up with an MP4 where the video (via info from MediaInfo) is 2:51:54 long (the original length) and the audio is 2:52:05 long. A quick look in VLC seems to indicate that the 11 second difference on syncing the audio and video is evenly spread out. Doing a bit a math, the video is 0.998915331556 times as long as the audio. So, I decided to try the following command:

ffmpeg" -i out_of_sync.mp4 -filter_complex "[0:v]setpts=1.0*PTS[v];[0:a]atempo=0.998915331556[a]" -map "[v]" -map "[a]" -crf 16 out_fix.mp4

This is supposed to change the speed/rate of the audio. (The "-crf 16" is just trying to retain the original quality and the "16" can be adjusted to whatever your original quality is.) The resulting MP4 seems to have the audio and video in sync!

I will post a final post for the thread that recaps the thread and solution for any future readers.

[the_steve_randolph]

**************************************
This is a recap of the thread issue and the solution
**************************************

I have an H264 MP4 that I downloaded from a streaming site via the Firefox add-on "Video DownloadHelper". Windows and VLC tells me that it is 2:51:54 long. At the 1:57:07 mark, the video track speeds up and then stops before the end, but, the audio track continues at normal speed to the end. VLC shows the frame rate to be 94.126760 (yeah, that's weird!). I downloaded that file at least 5 times and the problem starts at the same timestamp each time. When I stream the video in Firefox, everything works fine all of the way through to the end.

It was suggested that I use untrunc to correct the problem - an app that I have never used before. It needs a "reference file" that has all of the attributes/setting that you want for the resulting corrected file. It was suggested that I use Avidemux (another app I have never used) to create this reference file, but, instead I used this ffmpeg command to create a 2 minute long file from the beginning of the MP4:

ffmpeg out.mp4 -t 00:02:00.000 -c copy to reference.mp4"

Then, I ran untrunc-gui without clicking "on" any of the checkboxes in the "Settings" tab", but, it did not run properly (see post #34 for that information). Next, I ran untrunc-gui again, but, this time in the "Settings" tab, I click "on":

• "skip unknown (-s)"
• "stretch video to match audio (-sv)"
• "keep unknown sequences (-k)"

This ran without any errors and generated a "fixed" MP4 that was 2:52:05 in both Windows and VLC - 11 seconds longer than the original. Viewing it in VLC, everything starts off fine, but, gradually the video and audio tracks were out of sync by 11 seconds by the end. MediaInfo confirmed that the video and audio tracks were different lengths. I remembered the ffmpeg "-filter_complex & setpts" switch where you can change the speed of either the audio, video, or, both. Since MediaInfo told me that the audio was longer than the original length of the file, I decided that I needed to speed up the audio, or, in other words, reduce/compress the length of the audio. So (warning: MATH!), I divided the original 2:51:54 length (10314.536 seconds) by the fixed 2:52:05 length (10325.736 seconds) to come up with a factor of 0.998915331556. Next, I ran this command:

ffmpeg" -i out_of_sync.mp4 -filter_complex "[0:v]setpts=1.0*PTS[v];[0:a]atempo=0.998915331556[a]" -map "[v]" -map "[a]" -crf 16 out_new.mp4

This command does not change the speed of the video track, changes the audio track speed, and, tries to retain the original overall quality (via "-crf 16") since the file is being re-encoded. This resulted in an MP4 where it is the original length, and, the audio and video tracks are in sync. SUCCESS!

The end result is that I was able to fix the MP4. But, I did not learn what might have caused this problem, what app to use to discover where/how/why the problem occurred, or, how it was fixed (meaning, I know that untrunc fixed it but I don't know what it changed/fixed). Simply, I just wanted to know exactly what was broken, but, I guess sometimes it is not worth knowing (for instance: "just reboot the computer to fix that") when you have a tool that can fix it.

[Budman1]

Most of the times I have encountered this is when I have inadvertantly joined 2 segments with different formats, particularly the AUDIO. Joining these videos gives different lengths for audio and video. Other mismatches cause similar problems but almost all that I have encounteredd have been from joining 2 segments of un-similar formats.

Joining 44.1k sample rate with 48k sample rate with ffmpeg delivers the following mediainfo.

Video
ID : 1
Format : AVC
Format/Info : Advanced Video Codec
Format profile : High@L3.1
Format settings : CABAC / 3 Ref Frames
Format settings, CABAC : Yes
Format settings, Reference frames : 3 frames
Codec ID : avc1
Codec ID/Info : Advanced Video Coding
Duration : 15 min 54 s
Source duration : 15 min 54 s
Bit rate : 792 kb/s
Width : 854 pixels
Height : 480 pixels


Audio
ID : 2
Format : AAC LC
Format/Info : Advanced Audio Codec Low Complexity
Codec ID : mp4a-40-2
Duration : 17 min 18 s
Bit rate mode : Constant
Bit rate : 101 kb/s
Channel(s) : 2 channels
Channel layout : L R
Sampling rate : 44.1 kHz
Frame rate : 43.066 FPS (1024 SPF)
Compression mode : Lossy