Misplaced chapters for mp3 files (ID3v2 Chapter Frame Addendum)

[sandreas]

When i use ffmpeg to export ffmetadata of an mp3 file with chapter and then reimport the same ffmetadata, the chapters timestamps are different.

1. download the media file ​https://send.firefox.com/download/efb4d882f95b99bd/#gbNb7ZUVFxrVWDSobvZvYg

2. create a file ffmeta.txt with the following contents:

   ;FFMETADATA1
   title=01 - Preface.
   
   album=History of a Six Weeks' Tour
   
   genre=Speech
   
   artist=Percy Bysshe Shelley
   
   track=1
   
   encoder=Lavf58.29.100
   
   [CHAPTER]
   TIMEBASE=1/1000
   START=0
   END=196437
   title=01 - Preface.
   [CHAPTER]
   TIMEBASE=1/1000
   START=196437
   END=381591
   title=02 - Author's Introduction.
   [CHAPTER]
   TIMEBASE=1/1000
   START=381591
   END=1712027
   title=03 - France.
   [CHAPTER]
   TIMEBASE=1/1000
   START=1712027
   END=2653177
   title=04 - Switzerland.
   [CHAPTER]
   TIMEBASE=1/1000
   START=2653177
   END=3279154
   title=05 - Germany.
   [CHAPTER]
   TIMEBASE=1/1000
   START=3279154
   END=3640978
   title=06 - Holland.
   [CHAPTER]
   TIMEBASE=1/1000
   START=3640978
   END=4214143
   title=07 - Letters Written During a
   [CHAPTER]
   TIMEBASE=1/1000
   START=4214143
   END=4657969
   title=08 - Letters Written During a
   [CHAPTER]
   TIMEBASE=1/1000
   START=4657969
   END=6200600
   title=09 - Letters Written During a
   [CHAPTER]
   TIMEBASE=1/1000
   START=6200600
   END=7782426
   title=10 - Letters Written During a
   [CHAPTER]
   TIMEBASE=1/1000
   START=7782426
   END=8352603
   title=11 - Mont Blanc. Lines Written
   

3. import the ffmeta.txt chapters by:

   'ffmpeg' '-hide_banner' '-i' 'merged.mp3' '-i' 'ffmeta.txt' '-map_metadata' '1' 'merged.new.mp3'
   

4. export a new ffmeta-after-import.txt from the file just imported:

   'ffmpeg' '-hide_banner' '-i' 'merged.new.mp3' '-f' 'ffmetadata' 'ffmeta-after-import.txt'
   

5. diff ffmeta.txt and ffmeta-after-import.txt

Because the chapters did not change between import and export, i would expect, that the chapters are at the same time, but the times are different. ffmpeg seems to change something here while importing chapters, whatever the reason is...

[mkver]

1. Your file is gone; you should have uploaded it ​here or attached it to this ticket in case of small samples.
2. A quick test of mine shows that the chapters are offset by an amount corresponding to encoder delay. In fact, if one has an mp3 file with chapters and a xing frame indicating encoder delay, then each round of remuxing (without reencoding) offsets the chapters by this encoder delay. The demuxer just signals the encoder delay via side data and the AVStream's start_time. It is ffmpeg that offsets the chapters to (presumably) somehow compensate for this. This is definitely a bug (regardless of the missing sample).

[sandreas]

Replying to mkver:

1. Your file is gone; you should have uploaded it ​here or attached it to this ticket in case of small samples.
2. A quick test of mine shows that the chapters are offset by an amount corresponding to encoder delay. In fact, if one has an mp3 file with chapters and a xing frame indicating encoder delay, then each round of remuxing (without reencoding) offsets the chapters by this encoder delay. The demuxer just signals the encoder delay via side data and the AVStream's start_time. It is ffmpeg that offsets the chapters to (presumably) somehow compensate for this. This is definitely a bug (regardless of the missing sample).

Thank yo for looking into this. The file is 33MB, so attachment did not work. The size is intended, with smaller files the issue is not fully reproducible. I uploaded the file to Videolan with reference project ffmpeg and gitlab ticket #8728, but it did not give me a reference to post here... As you already pointed out, it happens with the most mp3 files that have chapters and should be easily reproducible using a merged version of ​https://librivox.org/history-of-a-six-weeks-tour-by-percy-bysshe-shelley/ (I used ​https://github.com/sandreas/m4b-tool/ with m4b-tool merge audio_book_folder/ -o merged.mp3 --audio-channels 1 --audio-format mp3 --audio-bitrate 64k, but it can also be done with raw ffmpeg, if you import the chapters correctly).

[mkver]

Your file is available ​here.