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• Trying to execute a SPLICE effect with libsox

  4 décembre 2012, par cube

  When two input files and one output file is required for an effect such as the splice effect, how should the sox_open_read() method be called ?

  A typical libsox routine would look something like :

  in=sox_open_read(file_path_in, NULL, NULL, NULL);
  
      out=sox_open_write(file_path_out, &in->signal, NULL, NULL, NULL, NULL);
  
  chain = sox_create_effects_chain(&in->encoding, &out->encoding);
  
  
  
  e = sox_create_effect(sox_find_effect("input"));
  
  args[0] = (char *)in, assert(sox_effect_options(e, 1, args) == SOX_SUCCESS);
  
  
  
  assert(sox_add_effect(chain, e, &in->signal, &in->signal) == SOX_SUCCESS);
  
  
  
      e = sox_create_effect(sox_find_effect("speed"));
  
      args[0] = 5,
  
      assert(sox_effect_options(e, 1, args) == SOX_SUCCESS);
  
      assert(sox_add_effect(chain, e, &in->signal, &in->signal) == SOX_SUCCESS);

  However, I want to use the splice effect which requires 2 input files to combine into one output.

  Any help would be appreciated.

• MP4 video slightly asynchronous - but only on initial play

  3 mai 2012, par TheSHEEEP

  Here is our procedure :

  1. We create live MP4 videos using ffmpeg (H264/AAC), both as API and as command line tool.*
  2. qt-faststart is used to move the metadata to the beginning of the file to enable fast playback in our Flash client.
  3. Video is renamed to *.m4v.
  4. User uses our Flash client to download the video and starts playing immediatly due to the metadata being at the beginning.

  Edit :
  *First, a MPG video is created with the ffmpeg API, this is then copied together with an intro and an outro video (you can simply chain MPG videos). The resulting MPG is then converted to MP4 using the following command :

  ffmpeg -i inputvideo.mpg -sameq -b 3000000 -vcodec libx264 -acodec libvo_aacenc -ac 2 -g 60 -y outputvideo.mp4

  Basically, this does work.

  The problem :

  We noticed that during the initial playback (the user hasn't clicked anything yet, video still downloading or not does not make a difference), the audio is playing asychnronous, noticeable if you look out for it.

  But as soon as you click around in the video (one time is enough, no matter where you click), the audio becomes more synchronous (still not perfect !) for the rest of the video.

  This behaviour is the same, no matter if you view it via our Flash player or Windows Media Player. Is there anything we can do from our side (in ffmpeg or qt-faststart) to make it synchronous from the start ?

• The Fastest Way To Learn Assembly Language

  4 septembre 2011, par Multimedia Mike — Programming

  I saw an old StackOverflow thread linked from Hacker News asking how to whether it’s worthwhile to learn assembly language and how to go about doing so. I’d like to take a stab at the last question.

  The fastest way to learn an assembly language is to reverse engineer something. Seriously, start with something that you know (like a C program that you wrote yourself) and take it apart. The good news is that assembly language is very simple and you will get a lot of practice in a short amount of time with RE.

  So here’s how you do it :

  • Take a simple program in C and build it with your tool chain, whether GNU gcc on Linux, Xcode on Mac, or MSVC on Windows. Also, make sure to turn on debugging symbols during compilation (this will help annotate the disassembly).
  • On Linux, use objdump : objdump -d program_binary
  • On Mac, use otool : otool -tV program_binary
  • On Windows : I admit, I’m a bit fuzzy on this one– I’m quite certain there’s a standard MSVC tool that prints the assembly listing.

  Anyway, look at the disassembled code and find the main() function. Work from there. Whatever the first instruction is, look it up on Google. You’ll likely find various CPU manuals that will explain the simple operation of the instruction. Look up the next unfamiliar instruction, then the next. Trust me, you’ll become an ASM expert in no time.

  Good luck !