ffmpeg -y -f x11grab -framerate 25 -video_size 1024x768 -i :1 -f alsa -i hw:Loopback,1 -c:v libx264 -pix_fmt yuv420p -b:v 2500k -c:a aac -strict -2 -b:a 192k -movflags +faststart testgrab.mp4
ffmpeg version 1.2.6-7:1.2.6-1~trusty1 Copyright (c) 2000-2014 the FFmpeg developers
  built on Apr 26 2014 18:52:58 with gcc 4.8 (Ubuntu 4.8.2-19ubuntu1)
  configuration: --arch=amd64 --disable-stripping --enable-avresample --enable-pthreads --enable-runtime-cpudetect --extra-version='7:1.2.6-1~trusty1' --libdir=/usr/lib/x86_64-linux-gnu --prefix=/usr --enable-bzlib --enable-libdc1394 --enable-libfreetype --enable-frei0r --enable-gnutls --enable-libgsm --enable-libmp3lame --enable-librtmp --enable-libopencv --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-libschroedinger --enable-libspeex --enable-libtheora --enable-vaapi --enable-vdpau --enable-libvorbis --enable-libvpx --enable-zlib --enable-gpl --enable-postproc --enable-libcdio --enable-x11grab --enable-libx264 --shlibdir=/usr/lib/x86_64-linux-gnu --enable-shared --disable-static
  libavutil      52. 18.100 / 52. 18.100
  libavcodec     54. 92.100 / 54. 92.100
  libavformat    54. 63.104 / 54. 63.104
  libavdevice    53.  5.103 / 53.  5.103
  libavfilter     3. 42.103 /  3. 42.103
  libswscale      2.  2.100 /  2.  2.100
  libswresample   0. 17.102 /  0. 17.102
  libpostproc    52.  2.100 / 52.  2.100
[x11grab @ 0x255bbe0] device: :1 -> display: :1 x: 0 y: 0 width: 1024 height: 768
[x11grab @ 0x255bbe0] shared memory extension found
[x11grab @ 0x255bbe0] Estimating duration from bitrate, this may be inaccurate
Input #0, x11grab, from ':1':
  Duration: N/A, start: 1425337308.373360, bitrate: 314572 kb/s
    Stream #0:0: Video: rawvideo (RGB[16] / 0x10424752), rgb565le, 1024x768, 314572 kb/s, 25 tbr, 1000k tbn, 25 tbc
[alsa @ 0x255fc20] Estimating duration from bitrate, this may be inaccurate
Guessed Channel Layout for  Input Stream #1.0 : stereo
Input #1, alsa, from 'hw:Loopback,1':
  Duration: N/A, start: 1425337308.376649, bitrate: 1411 kb/s
    Stream #1:0: Audio: pcm_s16le, 44100 Hz, stereo, s16, 1411 kb/s
[libx264 @ 0x258f7c0] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX
[libx264 @ 0x258f7c0] profile High, level 3.1
[libx264 @ 0x258f7c0] 264 - core 142 r2389 956c8d8 - H.264/MPEG-4 AVC codec - Copyleft 2003-2014 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=3 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=abr mbtree=1 bitrate=2500 ratetol=1.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, mp4, to 'testgrab.mp4':
  Metadata:
    encoder         : Lavf54.63.104
    Stream #0:0: Video: h264 ([33][0][0][0] / 0x0021), yuv420p, 1024x768, q=-1--1, 2500 kb/s, 12800 tbn, 25 tbc
    Stream #0:1: Audio: aac ([64][0][0][0] / 0x0040), 44100 Hz, stereo, fltp, 192 kb/s
Stream mapping:
  Stream #0:0 -> #0:0 (rawvideo -> libx264)
  Stream #1:0 -> #0:1 (pcm_s16le -> aac)
Press [q] to stop, [?] for help
ALSA buffer xrun.23 q=21.0 size=     780kB time=00:00:02.67 bitrate=2391.6kbits/s
ALSA buffer xrun.23 q=22.0 size=    1731kB time=00:00:05.24 bitrate=2705.7kbits/s dup=24 drop=0
ALSA buffer xrun.23 q=21.0 size=    2495kB time=00:00:08.04 bitrate=2542.5kbits/s dup=61 drop=0
Starting second pass: moving header on top of the file
frame=  260 fps= 21 q=-1.0 Lsize=    3195kB time=00:00:10.93 bitrate=2393.0kbits/s dup=66 drop=0
video:3117kB audio:71kB subtitle:0 global headers:0kB muxing overhead 0.204707%
[libx264 @ 0x258f7c0] frame I:2     Avg QP:13.57  size:106279
[libx264 @ 0x258f7c0] frame P:137   Avg QP:15.17  size: 19571
[libx264 @ 0x258f7c0] frame B:121   Avg QP:20.16  size:  2456
[libx264 @ 0x258f7c0] consecutive B-frames: 33.8% 10.0%  6.9% 49.2%
[libx264 @ 0x258f7c0] mb I  I16..4: 12.7% 43.5% 43.8%
[libx264 @ 0x258f7c0] mb P  I16..4:  1.2%  7.6%  4.7%  P16..4: 16.6%  9.3%  6.1%  0.0%  0.0%    skip:54.4%
[libx264 @ 0x258f7c0] mb B  I16..4:  0.2%  0.6%  0.3%  B16..8: 13.6%  2.0%  0.5%  direct: 1.0%  skip:81.9%  L0:44.0% L1:51.6% BI: 4.4%
[libx264 @ 0x258f7c0] final ratefactor: 15.60
[libx264 @ 0x258f7c0] 8x8 transform intra:55.3% inter:54.6%
[libx264 @ 0x258f7c0] coded y,uvDC,uvAC intra: 70.1% 85.6% 75.6% inter: 10.7% 14.4% 6.5%
[libx264 @ 0x258f7c0] i16 v,h,dc,p: 49% 28%  7% 15%
[libx264 @ 0x258f7c0] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 23% 15% 19%  5%  8%  9%  8%  8%  6%
[libx264 @ 0x258f7c0] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 32% 23% 15%  4%  6%  6%  5%  4%  3%
[libx264 @ 0x258f7c0] i8c dc,h,v,p: 51% 19% 20% 10%
[libx264 @ 0x258f7c0] Weighted P-Frames: Y:0.0% UV:0.0%
[libx264 @ 0x258f7c0] ref P L0: 66.8%  9.4% 13.5% 10.3%
[libx264 @ 0x258f7c0] ref B L0: 88.8%  9.1%  2.1%
[libx264 @ 0x258f7c0] ref B L1: 95.0%  5.0%
[libx264 @ 0x258f7c0] kb/s:2454.49
Received signal 2: terminating.