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• Can you think of a reason why windows might not enable audio if noone is logged in ?

  3 juillet 2017, par Caius Jard

  I’m having a bizarre problem with some virtual servers created to record podcasts. They run on amazon AWS as windows server 2012 instances and a small c# app tells FFMPEG to do the heavy lifting of capturing from the virtual screen and reading from the virtual sound card (Virtual Audio Cable : https://en.wikipedia.org/wiki/Virtual_Audio_Cable) via DirectShow filters

  The problem I have is if I leave the machine to do its stuff unattended, the recordings are sometimes silent. If I log in via VNC and watch it doing its stuff the audio is recorded just fine. All other aspects of the test op are the same, and the virtual machine is shut down between successive recordings so each one should theoretically be a clean slate. The app runs under a logged in session (hence the use of VNC rather than RDP)

  I’m now wondering if there is some optimisation of the windows sound engine whereby it doesn’t bother playing audio if it thinks noone is listening. The confusing thing to me is that not every virtual machine suffers these problems ; some of them record fine (and they’re all created from the same seed virtual hard disk image) in unattended mode

  I’m asking this question with the aim of getting together a list of things I can check/look into/debug.. I don’t have much knowledge of how MME/DirectSound/WASAPI work internally...

• Accurate cut of mp4 file using FFmpeg

  16 février 2020, par user12129980

  I want to cut mp4 video to seperate videos by seconds
  in the example :

  00:00:00 - 00:00:01
  
  00:00:01 - 00:00:02
  
  00:00:02 - 00:00:10.01

  This is the commands I used :

  ffmpeg.exe -i 10sec.mp4 -ss 00:00:00 -t 00:00:01 -async 1 -strict -2 -c copy 0002.mp4
  
  ffmpeg.exe -i 10sec.mp4 -ss 00:00:01 -t 00:00:02 -async 1 -strict -2 -c copy 0003.mp4
  
  ffmpeg.exe -i 10sec.mp4 -ss 00:00:02 -t 00:00:10.01 -async 1 -strict -2 -c copy 0004.mp4

  And the files I got are :
  1.3 second long
  2.6 second long
  8.5 second long

  So obviously the cutting it’s not accurate...

  I understand it’s hes something to do with the key frame, so I want to decompress the original file (mpeg with very high quality) to be not compress at all (so every frame will be key frame) and then do the cutting on it (so now the cutting will be accurate)

  The reason accuracy is so important to me is because after the cutting I must rejoin the slices to the single movie, and currently I just have replication in the movie I create from the slices.

  I tried the .avi version :

  ..\ffmpeg.exe -i 10sec.mp4 -vcodec rawvideo -acodec copy uncompressed.mp4

  it’s obviously failed duo to :

   [mp4 @ 091f1300] Could not find tag for codec rawvideo in stream #0, codec not currently supported in container
  
   Could not write header for output file #0 (incorrect codec parameters ?): Invalid argument
  
   Error initializing output stream 0:0 --

  So my q. is, how can I accurate slice a file using ffmpeg (I have the pkt_pts as well as the time I want to slice at, so if it’s possible to accurate cut by pkt_pts it also will helkp me) ?

• avcodec/aacenc_is : replace pow(x, 0.75) by x/sqrtf(sqrtf(x))

  14 janvier 2016, par Claudio Freire

  avcodec/aacenc_is : replace pow(x, 0.75) by x/sqrtf(sqrtf(x))
  This is quite an accurate approximation ; testing shows 2ulp error in
  
  the floating point result. Tested with FATE.
  Alternatively, if one wants "full accuracy", one can use powf, or sqrt
  
  instead of sqrtf. With powf, one gets 1 ulp error (theoretically should be 0, as
  
  0.75 is exactly representable) on GNU libm, with sqrt, 0 ulp error.
  Signed-off-by : Ganesh Ajjanagadde <gajjanagadde@gmail.com>
  Abstracted into pos_pow34 utility function
  Signed-off-by : Claudio Freire <klaussfreire@gmail.com>
  

  • [DH] libavcodec/aacenc_is.c
  • [DH] libavcodec/aacenc_utils.h