我想淡入淡出两个带音频的视频，第一个视频，，first.mkv时长 (20 秒)，第二个视频，，second.mkv时长未知。由于我希望过渡从第一个视频的 19 秒开始，时长为 1 秒，我尝试使用以下命令，使用xfade视频过滤器和acrossfade音频过滤器。
ffmpeg -i first.mkv -i second.mkv -filter_complex "[0:v][1:v]xfade=transition=fade:duration=1:offset=19[v];[0:a][1:a]acrossfade=duration=1[a]" -map "[v]" -map "[a]" output.mkv
但是，在输出视频的第二部分（即 20 秒后），音频和视频不同步。音频超前，视频延迟。
为什么会发生这种情况？如何解决？谢谢。
更新：这是完整的日志。
Metadata:
MINOR_VERSION : 0
COMPATIBLE_BRANDS: iso6avc1mp41
MAJOR_BRAND : dash
ENCODER : Lavf58.47.100
Duration: 00:00:20.02, start: 0.000000, bitrate: 4101 kb/s
Stream #0:0: Video: h264 (High), yuv420p(progressive), 1920x1080 [SAR 1:1 DAR 16:9], 23.98 fps, 23.98 tbr, 1k tbn, 47.95 tbc (default)
Metadata:
HANDLER_NAME : ISO Media file produced by Google Inc.
ENCODER : Lavc58.93.100 libx264
DURATION : 00:00:20.020000000
Stream #0:1(eng): Audio: vorbis, 48000 Hz, stereo, fltp (default)
Metadata:
ENCODER : Lavc58.93.100 libvorbis
DURATION : 00:00:20.000000000
Input #1, matroska,webm, from 'second.mkv':
Metadata:
COMPATIBLE_BRANDS: iso6avc1mp41
MAJOR_BRAND : dash
MINOR_VERSION : 0
ENCODER : Lavf58.47.100
Duration: 00:05:55.46, start: -0.007000, bitrate: 4221 kb/s
Stream #1:0: Video: h264 (High), yuv420p(tv, bt709, progressive), 1920x1080 [SAR 1:1 DAR 16:9], 60 fps, 60 tbr, 1k tbn, 120 tbc (default)
Metadata:
HANDLER_NAME : ISO Media file produced by Google Inc.
DURATION : 00:05:55.434000000
Stream #1:1(eng): Audio: opus, 48000 Hz, stereo, fltp (default)
Metadata:
DURATION : 00:05:55.461000000
Stream mapping:
Stream #0:0 (h264) -> xfade:main
Stream #0:1 (vorbis) -> acrossfade:crossfade0
Stream #1:0 (h264) -> xfade:xfade
Stream #1:1 (opus) -> acrossfade:crossfade1
xfade -> Stream #0:0 (libx264)
acrossfade -> Stream #0:1 (libvorbis)
Press [q] to stop, [?] for help
[libx264 @ 0x55c214e9e940] using SAR=1/1
[libx264 @ 0x55c214e9e940] using cpu capabilities: MMX2 SSE2Fast LZCNT SSSE3 SSE4.2
[libx264 @ 0x55c214e9e940] profile High 4:4:4 Predictive, level 4.0, 4:4:4 8-bit
[libx264 @ 0x55c214e9e940] 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=0x1:0x111 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=0 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=4 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=23 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, matroska, to 'output.mkv':
Metadata:
MINOR_VERSION : 0
COMPATIBLE_BRANDS: iso6avc1mp41
MAJOR_BRAND : dash
encoder : Lavf58.47.100
Stream #0:0: Video: h264 (libx264) (H264 / 0x34363248), yuv444p(progressive), 1920x1080 [SAR 1:1 DAR 16:9], q=-1--1, 23.98 fps, 1k tbn, 23.98 tbc (default)
Metadata:
encoder : Lavc58.93.100 libx264
Side data:
cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: N/A
Stream #0:1: Audio: vorbis (libvorbis) (oV[0][0] / 0x566F), 48000 Hz, stereo, fltp (default)
Metadata:
encoder : Lavc58.93.100 libvorbis
frame= 720 fps= 15 q=-1.0 Lsize= 15257kB time=00:00:29.99 bitrate=4166.4kbits/s dup=0 drop=359 speed=0.642x
video:14914kB audio:320kB subtitle:0kB other streams:0kB global headers:4kB muxing overhead: 0.151794%
[libx264 @ 0x55c214e9e940] frame I:11 Avg QP:18.59 size: 97377
[libx264 @ 0x55c214e9e940] frame P:266 Avg QP:22.33 size: 37075
[libx264 @ 0x55c214e9e940] frame B:443 Avg QP:25.85 size: 9792
[libx264 @ 0x55c214e9e940] consecutive B-frames: 6.4% 29.2% 16.7% 47.8%
[libx264 @ 0x55c214e9e940] mb I I16..4: 45.1% 0.0% 54.9%
[libx264 @ 0x55c214e9e940] mb P I16..4: 15.4% 0.0% 10.9% P16..4: 36.3% 9.8% 3.1% 0.0% 0.0% skip:24.5%
[libx264 @ 0x55c214e9e940] mb B I16..4: 1.3% 0.0% 1.4% B16..8: 35.2% 2.8% 0.4% direct: 0.9% skip:57.9% L0:41.3% L1:55.1% BI: 3.5%
[libx264 @ 0x55c214e9e940] coded y,u,v intra: 34.8% 22.0% 13.1% inter: 5.9% 2.5% 1.3%
[libx264 @ 0x55c214e9e940] i16 v,h,dc,p: 39% 22% 13% 26%
[libx264 @ 0x55c214e9e940] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 27% 19% 17% 6% 7% 7% 6% 6% 4%
[libx264 @ 0x55c214e9e940] Weighted P-Frames: Y:2.3% UV:2.3%
[libx264 @ 0x55c214e9e940] ref P L0: 64.7% 16.1% 13.7% 5.4% 0.0%
[libx264 @ 0x55c214e9e940] ref B L0: 89.8% 8.9% 1.4%
[libx264 @ 0x55c214e9e940] ref B L1: 98.2% 1.8%
[libx264 @ 0x55c214e9e940] kb/s:4068.15```