this is part question and part warning. When I run the cmd below FFMPEG
automatically knocks my 24bit source audio down to 8bits(!). I was able to
change that by adding 'aformat=sample_fmts=s32' into the filterchain for the
anullsrc inputs.
The warning is: make sure you check your output quality if you use lavfi slugs
with your audio. 24->8bits, yikes.
The question is: Can I specify the format of the nullsource when using lavfi.
Seems dangerous that it defaults to fmt:u8. Am I doing something wrong?
Here is the spot in the log where it reports the bit depth change (log level
48):
[format_out_0_0 @ 0x134709b60] auto-inserting filter 'auto_resampler_1' between
the filter 'Parsed_concat_0' and the filter 'format_out_0_0'
[AVFilterGraph @ 0x1347084b0] query_formats: 9 queried, 18 merged, 6 already
done, 0 delayed
[auto_resampler_0 @ 0x134606c70] [SWR @ 0x118008000] Using fltp internally
between filters
[auto_resampler_0 @ 0x134606c70] [SWR @ 0x118008000] Treating 1 channels (FL)
as mono
[auto_resampler_0 @ 0x134606c70] [SWR @ 0x118008000] Matrix coefficients:
[auto_resampler_0 @ 0x134606c70] [SWR @ 0x118008000] FC: FC:1.000000
[auto_resampler_0 @ 0x134606c70] ch:1 chl:1 channels (FL) fmt:s32 r:48000Hz ->
ch:1 chl:mono fmt:u8 r:48000Hz
Here is the ffmpeg cmd (16bit wav output):
ffmpeg -loglevel 48 -y -f lavfi -t 1.724 -i
anullsrc=channel_layout=mono:sample_rate=48000 -ss 120 -t 30.327979 -i
/tmp/2ch.mov -f lavfi -t 1.724 -i
anullsrc=channel_layout=mono:sample_rate=48000 -filter_complex
"[0:a:0][1:a:0][2:a:0]concat=n=3:v=0:a=1[sf1_out]" -map [sf1_out]
/tmp/noisetest.wav
Here is the cmd line output:
ffmpeg version 4.4.1 Copyright (c) 2000-2021 the FFmpeg developers
built with Apple clang version 13.1.6 (clang-1316.0.21.2)
configuration:
libavutil 56. 70.100 / 56. 70.100
libavcodec 58.134.100 / 58.134.100
libavformat 58. 76.100 / 58. 76.100
libavdevice 58. 13.100 / 58. 13.100
libavfilter 7.110.100 / 7.110.100
libswscale 5. 9.100 / 5. 9.100
libswresample 3. 9.100 / 3. 9.100
Input #0, lavfi, from 'anullsrc=channel_layout=mono:sample_rate=48000':
Duration: N/A, start: 0.000000, bitrate: 384 kb/s
Stream #0:0: Audio: pcm_u8, 48000 Hz, mono, u8, 384 kb/s
Input #1, mov,mp4,m4a,3gp,3g2,mj2, from '/tmp/2ch.mov':
Metadata:
major_brand : qt
minor_version : 537199360
compatible_brands: qt
creation_time : 2016-07-11T16:36:32.000000Z
com.apple.quicktime.player.movie.audio.gain: 1.000000
com.apple.quicktime.player.movie.audio.treble: 0.000000
com.apple.quicktime.player.movie.audio.bass: 0.000000
com.apple.quicktime.player.movie.audio.balance: 0.000000
com.apple.quicktime.player.movie.audio.pitchshift: 0.000000
com.apple.quicktime.player.movie.audio.mute:
com.apple.quicktime.player.version: 7.6.6 (7.6.6)
com.apple.quicktime.version: 7.7.3 (2890.9) 0x7738000 (Mac OS X, 10.10.5,
14F1605)
timecode : 00:00:00:00
Duration: 00:52:43.08, start: 0.000000, bitrate: 2304 kb/s
Stream #1:0(eng): Audio: pcm_s24le (in24 / 0x34326E69), 48000 Hz, 1 channels
(FL), s32 (24 bit), 1152 kb/s (default)
Metadata:
creation_time : 2016-07-11T16:36:32.000000Z
handler_name : Apple Sound Media Handler
vendor_id : [0][0][0][0]
Stream #1:1(eng): Audio: pcm_s24le (in24 / 0x34326E69), 48000 Hz, 1 channels
(FR), s32 (24 bit), 1152 kb/s (default)
Metadata:
creation_time : 2016-07-11T16:36:32.000000Z
handler_name : Apple Sound Media Handler
vendor_id : [0][0][0][0]
Stream #1:2(eng): Data: none (tmcd / 0x64636D74) (default)
Metadata:
creation_time : 2016-07-11T16:36:32.000000Z
handler_name : Time Code Media Handler
reel_name : TEST
timecode : 00:00:00:00
Input #2, lavfi, from 'anullsrc=channel_layout=mono:sample_rate=48000':
Duration: N/A, start: 0.000000, bitrate: 384 kb/s
Stream #2:0: Audio: pcm_u8, 48000 Hz, mono, u8, 384 kb/s
Stream mapping:
Stream #0:0 (pcm_u8) -> concat:in0:a0
Stream #1:0 (pcm_s24le) -> concat:in1:a0
Stream #2:0 (pcm_u8) -> concat:in2:a0
concat -> Stream #0:0 (pcm_s16le)
Press [q] to stop, [?] for help
Output #0, wav, to '/tmp/noisetest.wav':
Metadata:
ISFT : Lavf58.76.100
Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 48000 Hz, mono, s16,
768 kb/s (default)
Metadata:
encoder : Lavc58.134.100 pcm_s16le
Thanks!
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