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• avformat/oggenc : Avoid allocating and copying when writing page data

  3 mai 2020, par Andreas Rheinhardt

  avformat/oggenc : Avoid allocating and copying when writing page data
  When the Ogg muxer writes a page, it has to do three things : It needs to
  
  write a page header, then it has to actually copy the page data and then
  
  it has to calculate and write a CRC checksum of both header as well as
  
  data at a certain position in the page header.
  To do this, the muxer used a dynamic buffer for both writing as well as
  
  calculating the checksum via an AVIOContext's feature to automatically
  
  calculate checksums on the data it writes. This entails an allocation of
  
  an AVIOContext, of the opaque specific to dynamic buffers and of the
  
  buffer itself (which may be reallocated multiple times) as well as
  
  memcopying the data (first into the AVIOContext's small write buffer,
  
  then into the dynamic buffer's big buffer).
  This commit changes this : The page header is no longer written into a
  
  dynamic buffer any more ; instead the (small) page header is written into
  
  a small buffer on the stack. The CRC is then calculated directly via
  
  av_crc() on both the page header as well as the page data. Then both the
  
  page header and the page data are written.
  Finally, ogg_write_page() can now no longer fail, so it has been
  
  modified to return nothing ; this also fixed a bug in the only caller of
  
  this function : It didn't check the return value.
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
  

  • [DH] libavformat/oggenc.c

• Piping PCM data from FFMPEG to another process with Python subprocess

  13 février 2017, par Pete Bleackley

  I am trying to transcribe a podcast. To do so, I am decoding the mp3 stream with FFMPEG, and piping the resulting PCM output to the speech recognition component. My code looks like this.

  mp3=subprocess.Popen(['ffmpeg','-i',audio_url,
  
                                '-f','s16le','-ac','1','-ar','16000','pipe:0'],
  
                                stdout=subprocess.PIPE)
  
  sphinx=subprocess.Popen(['java','-jar','transcriber.jar'],
  
                                  stdin=mp3.stdout,
  
                                  stdout=subprocess.PIPE)

  Where audio_url is the url of the mp3 file.

  When I try to run this, it hangs. It appears that feeding the decoded PCM data through the pipe has deadlocked. What can I do to fix this ? The size of the output data is likely to be too big for subprocess.Popen.communicate to be an option, and explicitly calling mp3.stdout.close() has had no effect.

• Piping PCM data from FFMPEG to another process with Python subprocess

  1er mars 2019, par Pete Bleackley

  I am trying to transcribe a podcast. To do so, I am decoding the mp3 stream with FFMPEG, and piping the resulting PCM output to the speech recognition component. My code looks like this.

  mp3=subprocess.Popen(['ffmpeg','-i',audio_url,
  
                                '-f','s16le','-ac','1','-ar','16000','pipe:0'],
  
                                stdout=subprocess.PIPE)
  
  sphinx=subprocess.Popen(['java','-jar','transcriber.jar'],
  
                                  stdin=mp3.stdout,
  
                                  stdout=subprocess.PIPE)

  Where audio_url is the url of the mp3 file.

  When I try to run this, it hangs. It appears that feeding the decoded PCM data through the pipe has deadlocked. What can I do to fix this ? The size of the output data is likely to be too big for subprocess.Popen.communicate to be an option, and explicitly calling mp3.stdout.close() has had no effect.