我正在尝试将mp4和mp3文件与ffmpeg合并。 mp4持续时间 - 9.800秒，mp3 - 58.540秒。所以我使用-shortest键。 代码：
ffmpeg -i video.mp4 -i audio.mp3 -c:v libx264 -c:a aac -strict experimental -shortest output.mp4
之后我得到了output.mp4，持续时间为9.846。我的错误在哪里？为什么输出视频的时间长于源？ （9.846秒和9.800秒）。
来源mp4 MediaInfo：
General
Complete name : F:\video test\video.mp4
Format : MPEG-4
Format profile : Base Media
Codec ID : iso5 (iso5/dash)
File size : 3.19 MiB
Duration : 9 s 800 ms
Overall bit rate : 2 732 kb/s
Encoded date : UTC 2017-11-24 20:53:53
Tagged date : UTC 2017-11-24 20:53:53
Video
ID : 1
Format : AVC
Format/Info : Advanced Video Codec
Format profile : High@L3.1
Format settings : CABAC / 4 Ref Frames
Format settings, CABAC : Yes
Format settings, ReFrames : 4 frames
Codec ID : avc1
Codec ID/Info : Advanced Video Coding
Duration : 9 s 800 ms
Bit rate : 2 729 kb/s
Maximum bit rate : 3 766 kb/s
Width : 1 280 pixels
Height : 720 pixels
Display aspect ratio : 16:9
Frame rate mode : Constant
Frame rate : 25.000 FPS
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Scan type : Progressive
Bits/(Pixel*Frame) : 0.118
Stream size : 3.19 MiB (100%)
Writing library : x264 core 146
Encoding settings : 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
Tagged date : UTC 2017-11-24 20:53:53
来源mp3 Mediainfo：
General
Complete name : F:\video test\audio.mp3
Format : MPEG Audio
File size : 1.19 MiB
Duration : 58 s 540 ms
Overall bit rate mode : Variable
Overall bit rate : 170 kb/s
Writing library : LAME3.99r
Audio
Format : MPEG Audio
Format version : Version 1
Format profile : Layer 3
Format settings : Joint stereo / MS Stereo
Duration : 58 s 540 ms
Bit rate mode : Variable
Bit rate : 170 kb/s
Minimum bit rate : 32.0 kb/s
Channel(s) : 2 channels
Sampling rate : 44.1 kHz
Frame rate : 38.281 FPS (1152 SPF)
Compression mode : Lossy
Stream size : 1.19 MiB (100%)
Writing library : LAME3.99r
Encoding settings : -m j -V 2 -q 0 -lowpass 18.5 --vbr-new -b 32
控制台输出：
ffmpeg version 3.4 Copyright (c) 2000-2017 the FFmpeg developers
built with gcc 7.2.0 (GCC)
configuration: --enable-gpl --enable-version3 --enable-sdl2 --enable-bzlib --enable-fontconfig --enable-gnutls --enable-iconv --enable-libass --enable-libbluray --enable-libfreetype --enable-libmp3lame --enable-libopenjpeg --enable-libopus --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libtheora --enable-libtwolame --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libzimg --enable-lzma --enable-zlib --enable-gmp --enable-libvidstab --enable-libvorbis --enable-cuda --enable-cuvid --enable-d3d11va --enable-nvenc --enable-dxva2 --enable-avisynth --enable-libmfx
libavutil 55. 78.100 / 55. 78.100
libavcodec 57.107.100 / 57.107.100
libavformat 57. 83.100 / 57. 83.100
libavdevice 57. 10.100 / 57. 10.100
libavfilter 6.107.100 / 6.107.100
libswscale 4. 8.100 / 4. 8.100
libswresample 2. 9.100 / 2. 9.100
libpostproc 54. 7.100 / 54. 7.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'video.mp4':
Metadata:
major_brand : iso5
minor_version : 1
compatible_brands: iso5dash
creation_time : 2017-11-24T20:53:53.000000Z
Duration: 00:00:09.80, start: 0.000000, bitrate: 2732 kb/s
Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], 2259 kb/s, 25 fps, 25 tbr, 12800 tbn, 50 tbc (default)
Metadata:
handler_name : VideoHandler
Input #1, mp3, from 'audio.mp3':
Duration: 00:00:58.54, start: 0.025057, bitrate: 170 kb/s
Stream #1:0: Audio: mp3, 44100 Hz, stereo, s16p, 170 kb/s
Metadata:
encoder : LAME3.99r
Side data:
replaygain: track gain - -2.200000, track peak - unknown, album gain - unknown, album peak - unknown,
Stream mapping:
Stream #0:0 -> #0:0 (h264 (native) -> h264 (libx264))
Stream #1:0 -> #0:1 (mp3 (native) -> aac (native))
Press [q] to stop, [?] for help
[libx264 @ 00000000005ab440] using SAR=1/1
[libx264 @ 00000000005ab440] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX
[libx264 @ 00000000005ab440] profile High, level 3.1
[libx264 @ 00000000005ab440] 264 - core 152 r2851 ba24899 - H.264/MPEG-4 AVC codec - Copyleft 2003-2017 - 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=6 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=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, mp4, to 'output.mp4':
Metadata:
major_brand : iso5
minor_version : 1
compatible_brands: iso5dash
encoder : Lavf57.83.100
Stream #0:0(und): Video: h264 (libx264) (avc1 / 0x31637661), yuv420p(progressive), 1280x720 [SAR 1:1 DAR 16:9], q=-1--1, 25 fps, 12800 tbn, 25 tbc (default)
Metadata:
handler_name : VideoHandler
encoder : Lavc57.107.100 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), 44100 Hz, stereo, fltp, 128 kb/s
Metadata:
encoder : Lavc57.107.100 aac
Side data:
replaygain: track gain - -2.200000, track peak - unknown, album gain - unknown, album peak - unknown,
frame= 54 fps=0.0 q=28.0 size= 0kB time=00:00:00.04 bitrate= 8.3kbits/s speed=0.0927x
frame= 80 fps= 80 q=28.0 size= 0kB time=00:00:01.09 bitrate= 0.4kbits/s speed=1.09x
frame= 98 fps= 65 q=28.0 size= 256kB time=00:00:01.83 bitrate=1143.5kbits/s speed=1.21x
frame= 119 fps= 59 q=28.0 size= 512kB time=00:00:02.67 bitrate=1570.9kbits/s speed=1.32x
frame= 144 fps= 56 q=28.0 size= 768kB time=00:00:03.66 bitrate=1715.0kbits/s speed=1.42x
frame= 167 fps= 52 q=28.0 size= 1024kB time=00:00:04.57 bitrate=1833.9kbits/s speed=1.44x
frame= 190 fps= 51 q=28.0 size= 1280kB time=00:00:05.50 bitrate=1905.5kbits/s speed=1.47x
frame= 218 fps= 51 q=28.0 size= 1792kB time=00:00:06.64 bitrate=2210.6kbits/s speed=1.56x
frame= 242 fps= 50 q=28.0 size= 2048kB time=00:00:07.56 bitrate=2216.4kbits/s speed=1.58x
frame= 245 fps= 41 q=-1.0 Lsize= 3045kB time=00:00:09.82 bitrate=2539.6kbits/s speed=1.65x
video:2880kB audio:156kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.298058%
[libx264 @ 00000000005ab440] frame I:14 Avg QP:20.01 size: 39750
[libx264 @ 00000000005ab440] frame P:106 Avg QP:23.85 size: 14578
[libx264 @ 00000000005ab440] frame B:125 Avg QP:24.63 size: 6770
[libx264 @ 00000000005ab440] consecutive B-frames: 22.9% 22.0% 15.9% 39.2%
[libx264 @ 00000000005ab440] mb I I16..4: 16.7% 80.3% 3.0%
[libx264 @ 00000000005ab440] mb P I16..4: 10.2% 36.2% 1.1% P16..4: 25.0% 7.9% 2.5% 0.0% 0.0% skip:17.1%
[libx264 @ 00000000005ab440] mb B I16..4: 2.3% 5.8% 0.2% B16..8: 31.4% 6.5% 0.9% direct: 3.7% skip:49.2% L0:51.8% L1:44.5% BI: 3.7%
[libx264 @ 00000000005ab440] 8x8 transform intra:76.1% inter:86.3%
[libx264 @ 00000000005ab440] coded y,uvDC,uvAC intra: 38.3% 52.1% 9.0% inter: 12.3% 20.1% 0.2%
[libx264 @ 00000000005ab440] i16 v,h,dc,p: 30% 28% 9% 33%
[libx264 @ 00000000005ab440] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 36% 23% 19% 3% 3% 4% 4% 4% 4%
[libx264 @ 00000000005ab440] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 33% 21% 14% 5% 7% 7% 6% 5% 3%
[libx264 @ 00000000005ab440] i8c dc,h,v,p: 45% 24% 25% 6%
[libx264 @ 00000000005ab440] Weighted P-Frames: Y:13.2% UV:6.6%
[libx264 @ 00000000005ab440] ref P L0: 71.7% 12.5% 12.9% 2.7% 0.2%
[libx264 @ 00000000005ab440] ref B L0: 92.8% 6.3% 0.9%
[libx264 @ 00000000005ab440] ref B L1: 98.3% 1.7%
[libx264 @ 00000000005ab440] kb/s:2406.56
[aac @ 00000000005adde0] Qavg: 511.420
ffprobe -show_packets 输出太大，所以我加载到pastebin https://pastebin.com/TYSMdceS
答案 0 :(得分：2)
快速回答您的问题是FFmpeg / libaac在开始时从-0.0213 s开始编码额外的aac启动数据包。这会增加你的持续时间。
如果能有所帮助，我会尽量详细解答。
您可以尝试ffprobe -show_packets output.mp4。
我调查了你共享的数据包转储。 你的视频包看起来像
dts: -0.08 | pts: 0.0
dts: -0.04 | pts: 0.12
dts: 0.0 | pts: 0.04
dts: 0.04 | pts: 0.08
dts: 0.08 | pts: 0.24
...
dts: 9.64 | pts: 9.76
dts: 9.68 | pts: 9.72
来回pts值可能是因为你有I B B P阶的B帧。
您的视频流为25 fps，即1 frame duration = 0.04 s。
这会使您的视频9.76 + 0.04(frame duration) = 9.8 s。
您的原始音频大于视频，因此会被截断以使最后一个数据包最多为9.80 s or later。
您的音频数据包看起来像
pts: -0.023220 (AAC priming data)
pts: 0.0
pts: 0.023220
...
pts: 9.775601 | duration: 0.023220
pts: 9.798821 | duration: 0.023175
您最后的音频数据包必须在9.80或之后结束。这就是为什么9.79的数据包被接受的原因。
因此，音频复制到AV流的持续时间是
0.02322 (primiing pkt) + 9.798821 + 0.023175 (dur) = 9.845216
我不确定额外的0.001秒来自哪里。其他人应该能够发表评论。我在开头看到了跳过的数据。
[SIDE_DATA]
side_data_type=Skip Samples
skip_samples=1024
discard_padding=0
skip_reason=0
discard_reason=0
[/SIDE_DATA]
我希望这会有所帮助。