Changes between Version 11 and Version 12 of Capture/ALSA

Timestamp:

Nov 26, 2023, 2:46:32 AM (2 years ago)

Author:

20kbirds

Comment:

Add amerge example for multi-channel capture from multiple interfaces

Capture/ALSA

@@ -222 +222 @@
 ffmpeg -f alsa -channels 2 -sample_rate 44100 -i loopout out.wav
 }}}
+
+== Record multi-channel audio from multiple interfaces ==
+
+Suppose you want to record 4 microphones and you have two USB interfaces with two inputs each. You can use [https://ffmpeg.org/ffmpeg-filters.html#amerge amerge] filter to combine the streams from each interface into a single 4-channel WAV file:
+
+{{{
+ffmpeg -f alsa -i hw:1 -f alsa -i hw:2 -filter_complex '[0:a][1:a]amerge=inputs=2' out.wav 
+}}}
+
+Maybe your interfaces output audio data in different formats, for example one only provides S24_3LE and the other only S16_LE. Use audio codec arguments to upconvert to the highest quality format:
+
+{{{
+ffmpeg -c:a pcm_s24le -f alsa -i hw:1 -f alsa -i hw:2 -filter_complex '[0:a][1:a]amerge=inputs=2' -c:a pcm_s24le out.wav
+}}}
+
+In this case, my `hw:1` device uses `S24_3LE` format which has to be specified explicitly via `-c:a` prior to the input device argument (`-i`), my `hw:2` device uses `S16_LE` format which I don't specify because ffmpeg figures it out on its own, and I want the result to be output in 24 bits instead of 16 bits which appears to be the default.
+
+Output:
+
+{{{
+[aist#0:0/pcm_s24le @ 0x55936db2e7c0] Guessed Channel Layout: stereo
+Input #0, alsa, from 'hw:1':
+  Duration: N/A, start: 1700966473.753018, bitrate: 2304 kb/s
+  Stream #0:0: Audio: pcm_s24le, 48000 Hz, 2 channels, s32 (24 bit), 2304 kb/s
+[aist#1:0/pcm_s16le @ 0x55936db318c0] Guessed Channel Layout: stereo
+Input #1, alsa, from 'hw:2':
+  Duration: N/A, start: 1700966473.755953, bitrate: 1536 kb/s
+  Stream #1:0: Audio: pcm_s16le, 48000 Hz, 2 channels, s16, 1536 kb/s
+Stream mapping:
+  Stream #0:0 (pcm_s24le) -> amerge
+  Stream #1:0 (pcm_s16le) -> amerge
+  amerge:default -> Stream #0:0 (pcm_s24le)
+Press [q] to stop, [?] for help
+[Parsed_amerge_0 @ 0x55936db39540] No channel layout for input 1
+[Parsed_amerge_0 @ 0x55936db39540] Input channel layouts overlap: output layout will be determined by the number of distinct input channels
+Output #0, wav, to 'out.wav':
+  Metadata:
+    ISFT            : Lavf60.16.100
+  Stream #0:0: Audio: pcm_s24le ([1][0][0][0] / 0x0001), 48000 Hz, 4.0, s32, 4608 kb/s
+    Metadata:
+      encoder         : Lavc60.31.102 pcm_s24le
+}}}