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• GOP size does not correlate with actual latency

  28 mai 2019, par Alex

  As far as I know, GOP size should correlate with observable video delay (latency). For example, if GOP size is 2, then delay in video should be near two seconds and so on, at least with CBR. But, when I set GOP size to 2, publish stream to ingest server, consume this stream and measure latency, it is between 0.8-1.2 seconds, not 2+ seconds as excepted. Increasing GOP size leads to same results : with GOP 4 latency is near 2.5 seconds, not 4 seconds.

  How I measure this latency : stream working stopwatch from web-camera using OBS to ingest server and calculate difference between stopwatch value and value displayed in stream consumed from ingest. For greater measurement accuracy, I make a photo with stopwatch and actual image from ingest in one field of view.

  My OBS settings is here :

  

  Can you suggest, why do I get such results and how relevant is my statement about correlation between GOP size and video latency ? Maybe, H264 settings like "zerolatency" makes some magic ?

  Thanks.

• avformat/tee : Fix TeeSlave.bsfs pointer array size

  3 mai 2016, par Jan Sebechlebsky

  avformat/tee : Fix TeeSlave.bsfs pointer array size
  TeeSlave.bsfs is array of pointers to AVBitStreamFilterContext,
  
  so element size should be really size of a pointer, not size
  
  of TeeSlave structure.
  Reviewed-by : Nicolas George <george@nsup.org>
  
  Signed-off-by : Jan Sebechlebsky <sebechlebskyjan@gmail.com>
  
  Signed-off-by : Marton Balint <cus@passwd.hu>
  

  • [DH] libavformat/tee.c

• FFmpeg - confused with the concept of audio frame size and its calculation

  12 mars 2019, par Jinx

  I’m learning video and audio codecs with FFmpeg. I’m strgulling to understand frame size and some other concepts.

  Frame size
  This is the size in bytes of each frame. This can vary a lot : if each sample
  is 8 bits, and we’re handling mono sound, the frame size is one byte.
  Similarly in 6 channel audio with 64 bit floating point samples, the frame size is 48 bytes (PCM Terminology and Concepts)

  As described above, if each sample is 8 bits and there are 6 channels, then the frame size will be 48 bytes. The result from my code was 96 (16 bits * 6 channels). On the other side, the result from the call stream->codecpar->frame_size was 1024. Why were they different ? Is the frame size 1024 for 6 channels or just each channel ?

  main.cpp :

  else if (stream->codecpar->codec_type == AVMEDIA_TYPE_AUDIO) {
  
      std::cout << "audio sample rate: " << stream->codecpar->sample_rate << std::endl;
  
      std::cout << "audio bits: " << stream->codecpar->bits_per_coded_sample << std::endl;
  
      std::cout << "audio channels: " << stream->codecpar->channels << std::endl;
  
      std::cout << "audio frame size: " << stream->codecpar->frame_size << std::endl; 
  
      std::cout << "audio frame size: (16bits * 6 channels) = " << stream->codecpar->channels * stream->codecpar->bits_per_coded_sample << std::endl; 
  
      audio_stream_index = i;
  
  }

  console :

  audio sample rate: 48000
  
  audio bits: 16
  
  audio channels: 6
  
  audio frame size: 1024
  
  audio frame size: (16bits * 6 channels) = 96