title: FFMPEG problems with real-time buffer
body: <p>I'm trying to use FFMPEG (Windows) to encode a stream from a video capture card via dshow and send to a RTMP server. THis is my command line;</p>
  
  <pre><code>    ffmpeg -f dshow -i video="AVerMedia BDA Analog Capture Secondary":audio="Microphone (6- C-Media USB Audi" -vf scale=1280:720 -vcodec libx264 -r 30 -rtbufsize 702000k -acodec mp3 -ac 2 -ar 44100 -ab 128k -pix_fmt yuv420p -tune zerolatency -preset ultrafast -f flv "rtmp://xxx.xxx.xxx.xxx/stream/key"        ffmpeg version N-86950-g1bef008 Copyright (c) 2000-2017 the FFmpeg developers
      built with gcc 7.1.0 (GCC)
      configuration: --enable-gpl --enable-version3 --enable-cuda --enable-cuvid --enable-d3d11va --enable-dxva2 --enable-libmfx --enable-nvenc --enable-avisynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enable-iconv --enable-libass --enable-libbluray --enable-libbs2b --enable
      -libcaca --enable-libfreetype --enable-libgme --enable-libgsm --enable-libilbc --enable-libmodplug --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenh264 --enable-libopenjpeg --enable-libopus --enable-librtmp --enable-libsnappy --enable-libsoxr --enable-libspe
      ex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs --enable-libxvid --enable-libzimg --enable-lzma --enable-zlib
      libavutil      55. 70.100 / 55. 70.100
      libavcodec     57.102.100 / 57.102.100
      libavformat    57. 76.100 / 57. 76.100
      libavdevice    57.  7.100 / 57.  7.100
      libavfilter     6. 98.100 /  6. 98.100
      libswscale      4.  7.102 /  4.  7.102
      libswresample   2.  8.100 /  2.  8.100
      libpostproc    54.  6.100 / 54.  6.100
      Guessed Channel Layout for Input Stream #0.1 : stereo
      Input #0, dshow, from 'video=AVerMedia BDA Analog Capture Secondary:audio=Microphone (6- C-Media USB Audi':
      Duration: N/A, start: 2035.202000, bitrate: N/A
      Stream #0:0: Video: rawvideo (YUY2 / 0x32595559), yuyv422, 720x480, 29.97 fps, 29.97 tbr, 10000k tbn, 10000k tbc
      Stream #0:1: Audio: pcm_s16le, 44100 Hz, stereo, s16, 1411 kb/s
      [dshow @ 00000000005f90e0] real-time buffer [AVerMedia BDA Analog Capture Secondary] [video input] too full or near too full (68% of size: 3041280 [rtbufsize parameter])! frame dropped!
      [dshow @ 00000000005f90e0] real-time buffer [AVerMedia BDA Analog Capture Secondary] [video input] too full or near too full (90% of size: 3041280 [rtbufsize parameter])! frame dropped!
      [dshow @ 00000000005f90e0] real-time buffer [AVerMedia BDA Analog Capture Secondary] [video input] too full or near too full (113% of size: 3041280 [rtbufsize parameter])! frame dropped!
      Last message repeated 46 times
      Stream mapping:
      Stream #0:0 -&gt; #0:0 (rawvideo (native) -&gt; h264 (libx264))
      Stream #0:1 -&gt; #0:1 (pcm_s16le (native) -&gt; mp3 (libmp3lame))
      Press [q] to stop, [?] for help
      [dshow @ 00000000005f90e0] real-time buffer [AVerMedia BDA Analog Capture Secondary] [video input] too full or near too full (113% of size: 3041280 [rtbufsize parameter])! frame dropped!
      [libx264 @ 0000000005b16640] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2
      [libx264 @ 0000000005b16640] profile Constrained Baseline, level 3.1
      [libx264 @ 0000000005b16640] 264 - core 152 r2851 ba24899 - H.264/MPEG-4 AVC codec - Copyleft 2003-2017 - http://www.videolan.org/x264.html - options: cabac=0 ref=1 deblock=0:0:0 analyse=0:0 me=dia subme=0 psy=1 psy_rd=1.00:0.00 mixed_ref=0 me_range=16 chroma_me=1 trellis=0 8x8dct=0 cqm=0 deadzone=2
      1,11 fast_pskip=1 chroma_qp_offset=0 threads=11 lookahead_threads=11 sliced_threads=1 slices=11 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=0 weightp=0 keyint=250 keyint_min=25 scenecut=0 intra_refresh=0 rc=crf mbtree=0 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ra
      tio=1.40 aq=0
      [dshow @ 00000000005f90e0] real-time buffer [AVerMedia BDA Analog Capture Secondary] [video input] too full or near too full (113% of size: 3041280 [rtbufsize parameter])! frame dropped!
      Past duration 0.999992 too large
  </code></pre>
  
  <p>The buffer too full message are non-stop.  I can use Open Broadcast Software (OBS) and stream with no problem (I'm pretty sure it also uses ffmpeg), so I'm doing something wrong.</p>