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• How to mux *only* TTML subtitles into fmp4 file(s)

  25 juin 2021, par Chris

  As of WWDC 2017, HLS on Apple devices has supported TTML (specifically IMSC1)

  


  Subtitles are made available to me in a single XML file, which I wish to slice/package so that it can be distributed alongside the video assets, i.e. as fmp4 fragments containing only the subtitle stream.

  


  (slicing is not strictly required - correct muxing into a single file would be acceptable)

  


  While it feels like something that ffmpeg or mp4box should be capable of doing, documentation for correctly achieving that is proving elusive. Examples, including this one which would perform slicing and m3u8 generation, all assume subtitles are being added to a video asset, rather than packaged to be delivered in parallel.

  


  Any attempt to approximate that, having removed video-specific options only results in errors :

  


  ffmpeg -i subitles.ttml \
    -hls_time 60 \
    -hls_playlist_type vod \
    -hls_segment_type fmp4 \
    -hls_segment_filename "segment%d.m4s" \
    index.m3u8
    
subtitles.ttml: Invalid data found when processing input


  


  Whether this is because the TTML is invalid (although I don't believe that's likely), or because FFmpeg is trying to read it as if it was video, I cannot say.

  


• How to generate pcm audio file by setting 20 bit depth ?

  6 juillet 2015, par vip007

  No option using ffmpeg

  I am using ffmpeg to generate audio data. I am using following command

  ffmpeg.exe -i in.wav -ar 44100  -acodec pcm_s16le  -ac 1 out.wav

  but there is no option to convert to 20 bit depth pcm audio. (something like pcm_s20le).

  I want to perform some operations on apple codec (e.g. encoding pcm audio data to alac).

  On ALAC source : http://alac.macosforge.org/trac/browser/trunk/ReadMe.txt , it has been mentioned like this ;

  Apple Lossless supports the following features
  1. Bit depths 16, 20, 24 and 32 bits.

  Media Foundation on windows throwing exception for 20 bit depth

  I tried sample encoding and transcoding application mentioned on MSDN https://code.msdn.microsoft.com/windowsapps/media-transcode-sample-f7ba5269

  with following configuration but it failed

  spTypeOut->SetUINT32(MF_MT_AUDIO_BITS_PER_SAMPLE, 20);  

  NOTE : but i put 16 instead of 20 ; it created smoothly.

  Is there any way to generate such 20 bit PCM audio ? What is so special about 20 bit depth that it is not being considered on ffmpeg and media foundation but on apple codec ?

• ffmpeg messing up audio when copying PCM streams

  2 juin 2022, par lioumens

  I'm using ffmpeg to compress videos, but when trying to copy the audio stream from one video file using the same codec and container, the audio is completely corrupted and unusable.

  


  The video I'm trying to compress has stream data :

  


  $ ffprobe -hide_banner -i testing.mov
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'testing.mov':
  Metadata:
    major_brand     : qt
    minor_version   : 0
    compatible_brands: qt
    creation_time   : 2019-04-25T05:37:35.000000Z
    com.apple.quicktime.make: Apple
    com.apple.quicktime.model: MacBookPro15,2
    com.apple.quicktime.software: Mac OS X 10.14.2 (18C54)
    com.apple.quicktime.creationdate: 2019-04-25T00:28:40-0500
  Duration: 00:06:35.75, start: 0.000000, bitrate: 66117 kb/s
  Stream #0:0[0x1](und): Video: prores (Standard) (apcn / 0x6E637061), yuv422p10le(tv, smpte170m/bt709/bt709, progressive), 1280x720, 65923 kb/s, 29.01 fps, 29.01 tbr, 30k tbn (default)
    Metadata:
      creation_time   : 2019-04-25T05:37:35.000000Z
      handler_name    : Core Media Video
      vendor_id       : appl
      encoder         : Apple ProRes 422
  Stream #0:1[0x2](und): Audio: pcm_s16be (lpcm / 0x6D63706C), 8000 Hz, 1 channels, s16, 128 kb/s (default)
    Metadata:
      creation_time   : 2019-04-25T05:37:35.000000Z
      handler_name    : Core Media Audio
      vendor_id       : [0][0][0][0]


  


  Just working with the audio, I tried to copy the audio into a separate file with ffmpeg -i testing.mov -vn -channel_layout mono -c:a copy tmp.mov, which results in a video file with stream data :

  


  $ ffprobe -hide_banner -i tmp.mov
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'tmp.mov':
  Metadata:
    major_brand     : qt
    minor_version   : 512
    compatible_brands: qt
    encoder         : Lavf59.16.100
  Duration: 00:06:35.26, start: 0.018000, bitrate: 128 kb/s
  Stream #0:0[0x1]: Audio: pcm_s16be (twos / 0x736F7774), 8000 Hz, mono, s16, 128 kb/s (default)
    Metadata:
      handler_name    : Core Media Audio
      vendor_id       : [0][0][0][0]


  


  It seems the only difference when copying the audio is the original says pcm_s16be (lpcm / 0x6D63706C) while the resulting audio has pcm_s16be (twos / 0x736F7774). My understanding is that twos is Apple Quicktime's FOURCC identifier for signed 16 bit big-endian., which is correct and shouldn't affect the audio.

  


  It seems another forum user had a similar issue, but they were using a hex editor to directly manipulate bits, which I'm not comfortable doing. Is there a fix from within ffmpeg ?

  


  EDIT :
I've uploaded a sample of the clip that has this problem.