title: Could not find tag for codec pcm_alaw in stream #1, codec not currently supported in container when concatenating 2 files using ffmpeg
body: <p>I am trying to concatenate two files, the first of which has audio and video while the second is video only.  I use the following <code>ffmpeg</code> command:</p>
  
  <pre><code>ffmpeg_command = ["ffmpeg", "-f", "concat", "-safe", "0", "-i", "clips_to_join.txt", "-vcodec", "copy", "-acodec", "copy", output_file_path] # output_filename = ch0X-start_time-end_time
  p = subprocess.Popen(ffmpeg_command)
  p.communicate()
  </code></pre>
  
  <p>but I get the following error:</p>
  
  <pre><code>  libavutil      54. 31.100 / 54. 31.100
    libavcodec     56. 60.100 / 56. 60.100
    libavformat    56. 40.101 / 56. 40.101
    libavdevice    56.  4.100 / 56.  4.100
    libavfilter     5. 40.101 /  5. 40.101
    libavresample   2.  1.  0 /  2.  1.  0
    libswscale      3.  1.101 /  3.  1.101
    libswresample   1.  2.101 /  1.  2.101
    libpostproc    53.  3.100 / 53.  3.100
  Invalid UE golomb code
  [mov,mp4,m4a,3gp,3g2,mj2 @ 0x1394700] Auto-inserting h264_mp4toannexb bitstream filter
  Invalid UE golomb code
  Guessed Channel Layout for  Input Stream #0.1 : mono
  Input #0, concat, from 'clips_to_join.txt':
    Duration: N/A, start: 0.000000, bitrate: 5954 kb/s
      Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1920x1080, 5894 kb/s, 14.06 fps, 30 tbr, 90k tbn, 180k tbc
      Metadata:
        handler_name    : VideoHandler
      Stream #0:1(und): Audio: pcm_alaw (alaw / 0x77616C61), 8000 Hz, 1 channels, s16, 64 kb/s
      Metadata:
        handler_name    : SoundHandler
  [mp4 @ 0x13b16e0] Codec for stream 0 does not use global headers but container format requires global headers
  [mp4 @ 0x13b16e0] Codec for stream 1 does not use global headers but container format requires global headers
  [mp4 @ 0x13b16e0] Could not find tag for codec pcm_alaw in stream #1, codec not currently supported in container
  Output #0, mp4, to '/path/ch1-20171109-131750-131949-101000000000-padded.mp4':
    Metadata:
      encoder         : Lavf56.40.101
      Stream #0:0(und): Video: h264 ([33][0][0][0] / 0x0021), yuv420p, 1920x1080, q=2-31, 5894 kb/s, 14.06 fps, 30 tbr, 90k tbn, 90k tbc
      Metadata:
        handler_name    : VideoHandler
      Stream #0:1(und): Audio: pcm_alaw (alaw / 0x77616C61), 8000 Hz, mono, 64 kb/s
      Metadata:
        handler_name    : SoundHandler
  Stream mapping:
    Stream #0:0 -&gt; #0:0 (copy)
    Stream #0:1 -&gt; #0:1 (copy)
  Could not write header for output file #0 (incorrect codec parameters ?): Invalid argument
  </code></pre>