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• gstreamer records

  13 octobre 2012, par Eric

  I use the following command to record audio and video from my webcam

  gst-launch-0.10 v4l2src ! video/x-raw-yuv,width=640,height=480,framerate=30/1 ! \
  
               tee name=t_vid ! queue ! videoflip method=horizontal-flip ! \
  
               xvimagesink sync=false t_vid. ! queue ! \
  
               videorate ! video/x-raw-yuv,framerate=30/1 ! queue ! mux. \
  
               autoaudiosrc ! audiorate ! audio/x-raw-int,rate=48000,channels=1,depth=16 ! queue ! \
  
               audioconvert ! queue ! mux. avimux name=mux ! \
  
               filesink location=video.avi

  And the result is correct in term of synchronicity between the flows. However the avi file is very big since that's uncompressed data...
  Could you advice me howto reduce the size of the records. Note that I after recording I split audio and video in separate files for processing. It is crucial to keep the synchronicity.

  * Edit *

  I tried to use ffmpeg to compress the avi files using this command :

  ffmpeg -i video.avi -vcodec msmpeg4v2 output.avi

  But it seems that bitrate is invalid (N/A since its raw data ?)
  Here is the output :

  Input #0, avi, from 'video.avi':
  
  Duration: 00:00:00.00, start: 0.000000, bitrate: -2147483 kb/s
  
    Stream #0.0: Video: rawvideo, yuv420p, 640x480, 30 tbr, 30 tbn, 30 tbc
  
    Stream #0.1: Audio: pcm_s16le, 48000 Hz, 1 channels, s16, 768 kb/s
  
  [buffer @ 0xef57e0] w:640 h:480 pixfmt:yuv420p
  
  Incompatible sample format 's16' for codec 'ac3', auto-selecting format 'flt'
  
  [ac3 @ 0xedece0] channel_layout not specified
  
  [ac3 @ 0xedece0] No channel layout specified. The encoder will guess the layout, but it     might be incorrect.
  
  [ac3 @ 0xedece0] invalid bit rate
  
  Output #0, avi, to 'output.avi':
  
    Stream #0.0: Video: msmpeg4v2, yuv420p, 640x480, q=2-31, 200 kb/s, 90k tbn, 30 tbc
  
    Stream #0.1: Audio: ac3, 48000 Hz, mono, flt, 200 kb/s
  
  Stream mapping:
  
    Stream #0.0 -> #0.0
  
    Stream #0.1 -> #0.1
  
  Error while opening encoder for output stream #0.1 - maybe incorrect parameters such as bit_rate, rate, width or height

  Thanks for helping.

• ffmpeg - preserve time base and pts of all frames during transcode

  18 mars 2021, par jdizzle

  Context :

  


  I have an application that produces mp4s with HEVC encoding. I want to convert them to AVC for use in browser-based displaying. A very crucial part of how I want to use this is to preserve exact PTS times, as this is how we correlate the frames to other data streams not included in the video.

  


  Question :

  


  How do I make ffmpeg preserve this information across the transcode ? All the obvious flags seem to have no effect and ffmpeg just does whatever it wants.

  


  $ ffprobe -show_streams original.mp4 2>/dev/null | grep time_base
codec_time_base=16666667/500000000
time_base=1/1000


  


  Here is my convert command :

  


  $ ffmpeg -i original.mp4 -copyts -copytb 0 test.mp4


  


  And its result :

  


  $ ffprobe -show_streams test.mp4 2>/dev/null | grep time_base
codec_time_base=1/60
time_base=1/15360


  


  I would expect the time_bases to match. The PTS of the frames also don't match when doing ffprobe -show_frames

  


  EDIT :
@Gyan suggested using -video_track_timescale, but that didn't get the exact behavior I was looking for :

  


  $ sdiff <(ffprobe -show_frames test.mp4  | grep pkt_pts_time) <(ffprobe -show_frames original.mp4 | grep pkt_pts_time)
pkt_pts_time=0.000000                           pkt_pts_time=0.000000
pkt_pts_time=0.033000                           pkt_pts_time=0.033000
pkt_pts_time=0.067000                         | pkt_pts_time=0.066000
pkt_pts_time=0.100000                           pkt_pts_time=0.100000
pkt_pts_time=0.133000                           pkt_pts_time=0.133000
pkt_pts_time=0.167000                         | pkt_pts_time=0.166000
pkt_pts_time=0.200000                           pkt_pts_time=0.200000
pkt_pts_time=0.233000                           pkt_pts_time=0.233000
pkt_pts_time=0.267000                         | pkt_pts_time=0.266000
pkt_pts_time=0.300000                           pkt_pts_time=0.300000
pkt_pts_time=0.333000                           pkt_pts_time=0.333000
pkt_pts_time=0.367000                         | pkt_pts_time=0.366000
pkt_pts_time=0.400000                           pkt_pts_time=0.400000
pkt_pts_time=0.433000                           pkt_pts_time=0.433000
pkt_pts_time=0.467000                           pkt_pts_time=0.467000
pkt_pts_time=0.500000                           pkt_pts_time=0.500000
pkt_pts_time=0.533000                         | pkt_pts_time=0.532000
pkt_pts_time=0.567000                         | pkt_pts_time=0.565000
pkt_pts_time=0.600000                         | pkt_pts_time=0.598000
pkt_pts_time=0.633000                         | pkt_pts_time=0.631000
pkt_pts_time=0.667000                         | pkt_pts_time=0.665000
pkt_pts_time=0.700000                         | pkt_pts_time=0.698000
...


  


• Best Settings for H264_Nvenc To Minimize Latency ? (FFMPEG)

  1er novembre 2022, par M. Ying

  I'm currently using Libavcodec to encode video frames using H.264. Because I'm streaming this video frames after they're encoded, minimizing latency is crucial to me. My current settings are :

  



  avcodec_register_all();
codec = avcodec_find_encoder(AV_CODEC_ID_H264);

// allocate and set ffmpeg context
context = avcodec_alloc_context3(encoder->codec);
context->bit_rate = bitrate;
context->width = out_width;
context->height = out_height;
context->time_base.num = 1;
context->time_base.den = 30;
context->gop_size = 1; // send SPS/PPS headers every packet
context->max_b_frames = 0;
context->pix_fmt = AV_PIX_FMT_YUV420P;

// set encoder parameters to max performance
av_opt_set(context->priv_data, "preset", "ultrafast", 0);
av_opt_set(context->priv_data, "tune", "zerolatency", 0);

// open capture encoder
avcodec_open2(context, codec, NULL);


  



  These settings work well, but I am trying to switch to hardware-based encoding to take the workload off my CPU. I currently have an NVIDIA GPU, so I tried using the following settings with h264_nvenc :

  



  codec = avcodec_find_encoder_by_name("h264_nvenc");

// allocate and set ffmpeg context
context = avcodec_alloc_context3(encoder->codec);
context->dct_algo = FF_DCT_FASTINT;
context->bit_rate = bitrate;
context->width = out_width;
context->height = out_height;
context->time_base.num = 1;
context->time_base.den = 30;
context->gop_size = 1; // send SPS/PPS headers every packet
context->max_b_frames = 0;
context->pix_fmt = AV_PIX_FMT_YUV420P;

// set encoder parameters to max performancen
av_opt_set(context->priv_data, "preset", "llhq", 0);
av_opt_set(context->priv_data, "tune", "zerolatency", 0);


  



  The issue is, I noticed that the latency with h264_nvenc is significantly more than the latency with AV_CODEC_ID_H264 (the software-based version). I think that my settings or setup of h264_nvenc must be wrong, since GPU-based encoding should be faster than software-based encoding. Could anyone point me in the right direction ? Thanks so much !