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• FFMPEG - Extract sequence while recording

  26 février 2020, par Vincent Carretero

  I have a ffmpeg process to record a webcam, thats very classic...
  I would like to extract a sequence from the recording file before the end of the record.

  example :
   my recording start at 10:00 am and finish at 11:00 am
   at 10:15 am, an event append, and an operator need to watch it quickly,
   i would like to extract the sequence, and than he can watch at 10:20 am (but the video still recording and the Mp4.file not ended).

  is that possible ?

  thanks !!

• Any suggestion for a reliable video format for industrial applications ?

  2 juin 2017, par fstab

  I am currently using videos encoded in MPEG4 (h264). This makes the content of the frames rely on the content of other frames.

  In my case this is undesirable as I often need to quickly split video files without re-compressing or altering the content and seek many times trough the video.

  The video format that I need needs to be reliable and the frames need to be independent from each other.

  Any idea on which video format might be best for these requirements ?

  Maybe there is a sub-type of MPEG4 encoding that allows to reach this result ?

• Hit noise when playing part of wave file with ALSA PCM interface

  11 décembre 2024, par wangt13

  I am working a WAVE file playing with ALSA PCM interface in Linux, and I heard noise when I played the file quickly and partially.

  


  Here is my playing function.

  


  static int playback_function(uint8_t *pcm_buf, int pcm_frames)
{
    int  rc;
    uint8_t *buf;
    int frame_size, sent;
    int periodsize;
    int left;

    frame_size = chan * av_get_bytes_per_sample(AV_SAMPLE_FMT_S16);
    periodsize = sys_periodsize; // 320 in my system
    buf = pcm_buf;
    left = pcm_frames;
    sent = 0;

    while (left > 0) {
        sent = (left > periodsize) ? periodsize : left;
        rc = snd_pcm_writei(pcm_handle, buf, sent);
        printf("rc: %d, sent: %d\n", rc, sent);
        if (rc == -EAGAIN || (rc >= 0 && (size_t)rc < sent)) {
            snd_pcm_wait(pcm_handle, 10);
        } else if (rc == -EPIPE) {
            snd_pcm_recover(pcm_handle, rc, 0);
        } else if (rc < 0) {
            break;
        }
        if (rc > 0) {
            left -= rc;
            buf += rc * frame_size;
        }
    }
    return rc;
}


  


  The pcm_buf and pcm_frames are got from swr_convert() in libswresample, in my case, the pcm_frames is 1187.

  


  By adding printf("rc: %d, sent: %d\n", rc, sent);, I got following logs.

  


  rc: 320, sent: 320
rc: 87, sent: 87
rc: 320, sent: 320
rc: 320, sent: 320
rc: 103, sent: 103
rc: 320, sent: 320
rc: 320, sent: 320
rc: 103, sent: 103
rc: 320, sent: 320
rc: 320, sent: 320
rc: 103, sent: 103
rc: 320, sent: 320
rc: 320, sent: 320
rc: 103, sent: 103
rc: 320, sent: 320
rc: 320, sent: 320
rc: 103, sent: 103
rc: 320, sent: 320
rc: 320, sent: 320
rc: 103, sent: 103
rc: 320, sent: 320
rc: 320, sent: 320
rc: 103, sent: 103
rc: 320, sent: 320
rc: 320, sent: 320
rc: 103, sent: 103
rc: 320, sent: 320
rc: 320, sent: 320
rc: 87, sent: 87
rc: 320, sent: 320
rc: 320, sent: 320
rc: 103, sent: 103


  


  With above function, sometimes I can hear noise when playing the WAVE file quickly and repeatly.
  
So, how can I improve the WAVE playing without the noise ??

  


  I changed the above function by using filling 0 to the end of data buffer (to enforce silence).

  


  static int playback_test(uint8_t *pcm_buf, int pcm_frames)
{
    uint8_t *buf;
    int trd;
    int rc;
    int left;
    int frame_size, sent;
    int periodsize;
    int aligned = 0;

    frame_size = chan * av_get_bytes_per_sample(AV_SAMPLE_FMT_S16);
    periodsize = sys_periodsize; // 320 in my system

    buf = pcm_buf;
    left = pcm_frames;
    aligned = (left/periodsize + 1) * periodsize;
    memset(buf + left * frame_size, 0, (aligned - left) * frame_size);
    sent = 0;
///left = periodsize; // <== This causes more noise!!

    while (left > 0) {
        sent = (left > periodsize) ? periodsize : left;
        rc = snd_pcm_writei(pcm_handle, buf, sent);
        printf("rc: %d, sent: %d\n", rc, sent);
        if (rc == -EAGAIN || (rc >= 0 && (size_t)rc < sent)) {
            snd_pcm_wait(pcm_handle, 10);
        } else if (rc == -EPIPE) {
            snd_pcm_recover(pcm_handle, rc, 0);
        } else if (rc < 0) {
            break;
        }
        if (rc > 0) {
            left -= rc;
            buf += rc * frame_size;
        }
    }
    return rc;
}


  


  There is NO improvement as of the noise.