ffmpeg -i 2ch-44.1k.wav -i 2ch-16k.wav -filter_complex " \
[0:a][1:a]amix=inputs=2[aout]" \
-map [aout] -f null -

ffmpeg -i 2ch-44.1k.wav -i 2ch-16k.wav -filter_complex " \
[0:a][1:a]amix=inputs=2[aout]" \
-map [aout] -f null -

对比发现，这个和-i参数后面的次序有关，默认会选用第一个的samplerate作为输出的samplerate。

使用astreamselect：

ffmpeg -i 2ch-44.1k.wav  -i 2ch-16k.wav -filter_complex " \
[0:a][1:a]astreamselect=inputs=2:map=0[aout]" \
-map [aout] -f null - -v 48

输出steamselect的log：

[Parsed_astreamselect_0 @ 0x56243c0c0300] Setting 'inputs' to value '2'
[Parsed_astreamselect_0 @ 0x56243c0c0300] Setting 'map' to value '0'
[Parsed_astreamselect_0 @ 0x56243c0c0300] Add in pad input0
[Parsed_astreamselect_0 @ 0x56243c0c0300] Add in pad input1
[Parsed_astreamselect_0 @ 0x56243c0c0300] Add out pad output0
[Parsed_astreamselect_0 @ 0x56243c0c0300] Configured with 2 inpad and 1 outpad

ffmpeg -i 2ch-44.1k.wav  -i 2ch-16k.wav -filter_complex " \
[0:a][1:a]astreamselect=inputs=2:map=0 1[a1][a2], \
[a1][a2]amix=inputs=2[aout]" \
-map [aout] -f null -

TODO

❌ Assertion parent->nb_outputs == 1 failed at src/libavfilter/framesync.c:89

这样用会报错，原因是astreamselect输出只有一路？

从log看，有两路输出：

[Parsed_astreamselect_0 @ 0x5613513252c0] Setting 'inputs' to value '2'
[Parsed_astreamselect_0 @ 0x5613513252c0] Setting 'map' to value '0 1'
[Parsed_astreamselect_0 @ 0x5613513252c0] Add in pad input0
[Parsed_astreamselect_0 @ 0x5613513252c0] Add in pad input1
[Parsed_astreamselect_0 @ 0x5613513252c0] Add out pad output0
[Parsed_astreamselect_0 @ 0x5613513252c0] Add out pad output1
[Parsed_astreamselect_0 @ 0x5613513252c0] Configured with 2 inpad and 2 outpad
[Parsed_astreamselect_0 @ 0x5613513252c0] n=0 map=0x5613513298a0 p=0x5613513298a1
[Parsed_astreamselect_0 @ 0x5613513252c0] Map input stream 0 to output stream 0
[Parsed_astreamselect_0 @ 0x5613513252c0] n=1 map=0x5613513298a1 p=0x5613513298a3
[Parsed_astreamselect_0 @ 0x5613513252c0] Map input stream 1 to output stream 1
[Parsed_astreamselect_0 @ 0x5613513252c0] n=0 map=0x5613513298a3 p=0x5613513298a3

指定采样率，前面的amix前还是会插入auto_resampler做重采样：

ffmpeg -i 2ch-44.1k.wav -i 2ch-16k.wav -filter_complex " \
[0:a][1:a]amix=inputs=2[aout]" -map [aout] -ar 8000 -f null - -v 48

log:

[auto_resampler_0 @ 0x55aed2e8c940] [SWR @ 0x55aed2e8ce00] Using s16p internally between filters
[auto_resampler_0 @ 0x55aed2e8c940] ch:2 chl:stereo fmt:s16 r:44100Hz -> ch:2 chl:stereo fmt:flt r:44100Hz
[auto_resampler_1 @ 0x55aed2e8a080] [SWR @ 0x55aed2ea2040] Using fltp internally between filters
[auto_resampler_1 @ 0x55aed2e8a080] ch:2 chl:stereo fmt:s16 r:16000Hz -> ch:2 chl:stereo fmt:flt r:44100Hz
[Parsed_amix_0 @ 0x55aed2e6f0c0] inputs:2 fmt:flt srate:44100 cl:stereo
[auto_resampler_2 @ 0x55aed2eb7240] [SWR @ 0x55aed2eb7580] Using fltp internally between filters
[auto_resampler_2 @ 0x55aed2eb7240] ch:2 chl:stereo fmt:flt r:44100Hz -> ch:2 chl:stereo fmt:s16 r:8000Hz

换成这样也一样报错：

ffmpeg -i 2ch-44.1k.wav -i 2ch-16k.wav -filter_complex " \
[0:a][1:a]astreamselect=inputs=2:map=0 1[a1][a2], \
[a1]amix=inputs=1[aout1], \
[a2]amix=inputs=1[aout2], \
[aout1][aout2]amix=inputs=2[aout]" \
-map [aout] -f null - -v 48