寻找一种通用方法来合并音频和视频文件并使输出具有带有 FFmpeg 的立体声音频

Question

So have have the following FFmpeg command

ffmpeg -i vidab.mp4 -i recab.webm  -filter_complex "[0:a]volume=0.2,apad[A];[1:a][A]amerge[Aout]" -map 0:v -map [Aout] -y mergeab.mp4

When the input files are played separately their audio is played equally through both headphones.

But after being merged the audio from the video is on the right side and the audio from the webm file is on the left side.

I see this in the output when I run the cmd

[Parsed_amerge_2 @ 0x7fa0faf01bc0] No channel layout for input 1
[Parsed_amerge_2 @ 0x7fa0faf01bc0] Input channel layouts overlap: output layout will be determined by the number of distinct input channels

How can I adjust this so that the output file has the audio from both the inputs in equally in the left and right sides?

Also is there a generic way to do this such that even different configurations (video has stereo and audio has mono ... or both have stereo) result in the Audio being split evenly.

I tried using the pan filter but there is something I am not understanding about how it works because either nothing happens or I flip left and right. But I am not getting even stereo.

fwiw here is the full output.

ffmpeg -i vidab.mp4 -i recab.webm  -filter_complex "[0:a]volume=0.2,apad[A];[1:a][A]amerge[Aout]" -map 0:v -map [Aout] -y mergeab.mp4
ffmpeg version 4.2.1 Copyright (c) 2000-2019 the FFmpeg developers
  built with Apple clang version 11.0.0 (clang-1100.0.33.8)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/4.2.1_2 --enable-shared --enable-pthreads --enable-version3 --enable-avresample --cc=clang --host-cflags='-I/Library/Java/JavaVirtualMachines/adoptopenjdk-13.jdk/Contents/Home/include -I/Library/Java/JavaVirtualMachines/adoptopenjdk-13.jdk/Contents/Home/include/darwin -fno-stack-check' --host-ldflags= --enable-ffplay --enable-gnutls --enable-gpl --enable-libaom --enable-libbluray --enable-libmp3lame --enable-libopus --enable-librubberband --enable-libsnappy --enable-libtesseract --enable-libtheora --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libx265 --enable-libxvid --enable-lzma --enable-libfontconfig --enable-libfreetype --enable-frei0r --enable-libass --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librtmp --enable-libspeex --enable-libsoxr --enable-videotoolbox --disable-libjack --disable-indev=jack
  libavutil      56. 31.100 / 56. 31.100
  libavcodec     58. 54.100 / 58. 54.100
  libavformat    58. 29.100 / 58. 29.100
  libavdevice    58.  8.100 / 58.  8.100
  libavfilter     7. 57.100 /  7. 57.100
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  5.100 /  5.  5.100
  libswresample   3.  5.100 /  3.  5.100
  libpostproc    55.  5.100 / 55.  5.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'vidab.mp4':
  Metadata:
    major_brand     : qt  
    minor_version   : 0
    compatible_brands: qt  
    creation_time   : 2020-02-21T00:35:25.000000Z
  Duration: 00:00:10.65, start: 0.000000, bitrate: 2539 kb/s
    Stream #0:0(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, mono, fltp, 96 kb/s (default)
    Metadata:
      creation_time   : 2020-02-21T00:35:25.000000Z
      handler_name    : Core Media Audio
    Stream #0:1(und): Video: h264 (High) (avc1 / 0x31637661), yuvj420p(pc, bt709), 1280x640, 2436 kb/s, 29.87 fps, 29.97 tbr, 90k tbn, 180k tbc (default)
    Metadata:
      creation_time   : 2020-02-21T00:35:25.000000Z
      handler_name    : Core Media Video
      encoder         : H.264
Input #1, matroska,webm, from 'recab.webm':
  Metadata:
    encoder         : opus-media-recorder
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #1:0(eng): Audio: opus, 48000 Hz, mono, fltp (default)
Stream mapping:
  Stream #0:0 (aac) -> volume (graph 0)
  Stream #1:0 (opus) -> amerge:in0 (graph 0)
  Stream #0:1 -> #0:0 (h264 (native) -> h264 (libx264))
  amerge (graph 0) -> Stream #0:1 (aac)
Press [q] to stop, [?] for help
[libx264 @ 0x7fa0fb004000] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2
[libx264 @ 0x7fa0fb004000] profile High, level 3.1
[libx264 @ 0x7fa0fb004000] 264 - core 155 r2917 0a84d98 - H.264/MPEG-4 AVC codec - Copyleft 2003-2018 - 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=12 lookahead_threads=2 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=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
[Parsed_amerge_2 @ 0x7fa0faf01bc0] No channel layout for input 1
[Parsed_amerge_2 @ 0x7fa0faf01bc0] Input channel layouts overlap: output layout will be determined by the number of distinct input channels
Output #0, mp4, to 'mergeab.mp4':
  Metadata:
    major_brand     : qt  
    minor_version   : 0
    compatible_brands: qt  
    encoder         : Lavf58.29.100
    Stream #0:0(und): Video: h264 (libx264) (avc1 / 0x31637661), yuvj420p(pc, progressive), 1280x640, q=-1--1, 29.97 fps, 30k tbn, 29.97 tbc (default)
    Metadata:
      creation_time   : 2020-02-21T00:35:25.000000Z
      handler_name    : Core Media Video
      encoder         : Lavc58.54.100 libx264
    Side data:
      cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: -1
    Stream #0:1: Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 128 kb/s (default)
    Metadata:
      encoder         : Lavc58.54.100 aac
frame=  318 fps=107 q=-1.0 Lsize=    1568kB time=00:00:17.72 bitrate= 724.8kbits/s speed=5.99x    
video:1282kB audio:272kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.902630%
[libx264 @ 0x7fa0fb004000] frame I:2     Avg QP:19.72  size: 51154
[libx264 @ 0x7fa0fb004000] frame P:148   Avg QP:21.28  size:  6644
[libx264 @ 0x7fa0fb004000] frame B:168   Avg QP:25.18  size:  1351
[libx264 @ 0x7fa0fb004000] consecutive B-frames:  1.3% 84.9%  0.0% 13.8%
[libx264 @ 0x7fa0fb004000] mb I  I16..4: 23.2% 49.3% 27.5%
[libx264 @ 0x7fa0fb004000] mb P  I16..4:  2.1%  2.0%  0.2%  P16..4: 29.0%  5.1%  4.8%  0.0%  0.0%    skip:56.8%
[libx264 @ 0x7fa0fb004000] mb B  I16..4:  0.5%  0.3%  0.0%  B16..8: 16.7%  0.8%  0.2%  direct: 6.1%  skip:75.4%  L0:24.8% L1:71.1% BI: 4.1%
[libx264 @ 0x7fa0fb004000] 8x8 transform intra:45.4% inter:74.3%
[libx264 @ 0x7fa0fb004000] coded y,uvDC,uvAC intra: 29.3% 51.4% 11.6% inter: 5.7% 16.6% 0.3%
[libx264 @ 0x7fa0fb004000] i16 v,h,dc,p: 11% 53% 10% 26%
[libx264 @ 0x7fa0fb004000] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 13% 36% 35%  2%  2%  2%  4%  2%  3%
[libx264 @ 0x7fa0fb004000] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 24% 30% 12%  4%  6%  5%  7%  5%  6%
[libx264 @ 0x7fa0fb004000] i8c dc,h,v,p: 47% 33% 17%  4%
[libx264 @ 0x7fa0fb004000] Weighted P-Frames: Y:0.0% UV:0.0%
[libx264 @ 0x7fa0fb004000] ref P L0: 76.6%  7.6% 11.1%  4.7%
[libx264 @ 0x7fa0fb004000] ref B L0: 89.1% 10.3%  0.6%
[libx264 @ 0x7fa0fb004000] ref B L1: 99.7%  0.3%
[libx264 @ 0x7fa0fb004000] kb/s:989.60
[aac @ 0x7fa0fb005800] Qavg: 13376.354

Answer 1

Use amix instead of amerge and resample to stereo using -ac 2 . Since you're mapping the first video, tell amix to terminate with first input.

ffmpeg -i vidab.mp4 -i recab.webm -filter_complex "[0:a]volume=0.2[A];[A][1:a]amix=inputs=2:duration=first[Aout]" -map 0:v -map [Aout] -y mergeab.mp4