channelsplit, join, amix, amerge¶
Watch on youtube.com- doc
https://ffmpeg.org/ffmpeg-filters.html#channelsplit, https://ffmpeg.org/ffmpeg-filters.html#join, https://ffmpeg.org/ffmpeg-filters.html#amix, https://ffmpeg.org/ffmpeg-filters.html#amerge-1, (ffmpeg-utils)2.8. Syntax | Channel Layout, https://ffmpeg.org/ffmpeg-filters.html#aformat-1
- see also
streams and channels¶
Before understanding the individual filters, you should first recall the distinction between streams and channels.
First, containers don’t just store 1 + 1 “video and audio”. For example, the following creates a video with so-called “multiplex audio”:
[me@host: ~]$ ffmpeg -y \
> -i video.wmv \
> -i japanese.wma \
> -i englishdub.wma \
> -map '0:v' -map '1:a' -map '2:a' -shortest out.mp4
[me@host: ~]$ ffprobe -hide_banner out.mp4
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'out.mp4':
Metadata:
major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf57.71.100
Duration: 00:00:32.24, start: 0.000000, bitrate: 578 kb/s
Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], 323 kb/s, 25 fps, 25 tbr, 12800 tbn, 50 tbc (default)
Metadata:
handler_name : VideoHandler
Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 132 kb/s (default)
Metadata:
handler_name : SoundHandler
Stream #0:2(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 130 kb/s
Metadata:
handler_name : SoundHandler
Most media players recognize this, and you can switch the stream
to watch (or listen) by some operation.
In the case of ffplay, you can switch between audio streams by
typing a, video streams by typing v,
and subtitle streams by typing t.
Note
ffplay’s help describes these as “switching channel” so don’t get confused. This operation is actually “stream switching”.
On the other hand, the “audio channel” is “front left / front right / …”
for each stream.
While the target of pan, channelmap, etc, is this,
the target of channelsplit, join, amix
and amerge is a stream.
channelsplit¶
The description in the official documentation:
Split each channel from an input audio stream into a separate output stream.
It accepts the following parameters:
channel_layout
The channel layout of the input stream. The default is “stereo”.
channels
A channel layout describing the channels to be extracted as separate output streams or “all” to extract each input channel as a separate stream. The default is “all”.
Choosing channels not present in channel layout in the input will result in an error.
All examples in the official documentation are “into files” examples:
For example, assuming a stereo input MP3 file,
[me@host: ~]$ ffmpeg -i in.mp3 \ > -filter_complex channelsplit out.mkvwill create an output Matroska file with two audio streams, one containing only the left channel and the other the right channel.
Split a 5.1 WAV file into per-channel files:
[me@host: ~]$ ffmpeg -i in.wav -filter_complex " > channelsplit=channel_layout=5.1[FL][FR][FC][LFE][SL][SR]" \ > -map '[FL]' front_left.wav \ > -map '[FR]' front_right.wav \ > -map '[FC]' front_center.wav \ > -map '[LFE]' lfe.wav \ > -map '[SL]' side_left.wav \ > -map '[SR]' side_right.wavExtract only LFE from a 5.1 WAV file:
[me@host: ~]$ ffmpeg -i in.wav -filter_complex " > channelsplit=channel_layout=5.1:channels=LFE[LFE]" \ > -map '[LFE]' lfe.wav
, but perhaps the most commonly used pattern is to apply a different filter to each channel in your filter graph:
[me@host: ~]$ # assuming that sampling rate of some.wav is 48000Hz
[me@host: ~]$ ffplay -f lavfi "
> amovie=some.wav,channelsplit[Lu][R];
> [Lu]asetrate='48000/1.0089',atempo=1.0089,aresample=48000[L];
> [L][R]join"
[me@host: ~]$ ffplay -f lavfi "
> amovie=some.wav
> ,asplit[out1][a];
> [a]channelsplit[a1][a2];
> [a1]showcqt=s=960x270,setsar=1[v1];
> [a2]showcqt=s=960x270,setsar=1,vflip[v2];
> [v1][v2]vstack[out0]"
amerge¶
The description in the official documentation:
Merge two or more audio streams into a single multi-channel stream.
I don’t think the naming “merge” and the detailed description in the official documentation clearly shows what this filter actually does. In short, the filter distributes channels from multiple streams into a single stream. That’s all. (Although the short description at the beginning is sufficiently clear, the detailed description is redundant and difficult to understand.)
A simple illustration, for example:
+- stream 1 -+
| ch1: [FL] | +- out stream -+
| ch2: [FR] | ---+ | ch1: [FL] |
| ch3: [FC] | | | ch2: [FR] |
+------------+ +- [amerge] -> | ch3: [FC] |
+- stream 2 -+ | | ch4: [LFE] |
| ch1: [LFE] | | | ch5: [SL] |
| ch2: [SL] | ---+ | ch6: [SR] |
| ch3: [SR] | +--------------+
+------------+
While this filter is very easy to use:
[me@host: ~]$ ffplay -f lavfi "
> amovie=audio1.wav[a1];
> amovie=audio2.wav[a2];
> amovie=audio3.wav[a3];
> [a1][a2][a3]amerge=3"
, this is all it can do. In particular, with this filter, you have no control over the output channel layout. As in the above illustration, the input only determines the output.
So in most cases you will chain other filters after this, such as pan or channelmap.
join¶
The description in the official documentation:
Join multiple input streams into one multi-channel stream.
What the join filter does is essentially exactly the same as amerge.
Unlike amerge, this allows you to control the output channel layout.
It accepts the following parameters:
inputs
The number of input streams. It defaults to 2.
channel_layout
The desired output channel layout. It defaults to stereo.
map
Map channels from inputs to output. The argument is a ‘|’-separated list of mappings, each in the input_idx.in_channel-out_channel form. input_idx is the 0-based index of the input stream. in_channel can be either the name of the input channel (e.g. FL for front left) or its index in the specified input stream. out_channel is the name of the output channel.
The specification of “map” is at least “difficult to read”.
[me@host: ~]$ ffmpeg \
> -i fl.wav \
> -i fr.wav \
> -i fc.wav \
> -i sl.wav \
> -i sr.wav \
> -i lfe.wav \
> -filter_complex "
> join=inputs=6
> :channel_layout=5.1
> :map='0.0-FL|1.0-FR|2.0-FC|3.0-SL|4.0-SR|5.0-LFE' out.wav
For a first-timer it will only look like a spell, it’s hard to read, even for those who are used to writing.
The hyphen is the same as channelmap and is a separator between input and output. The period is, of course, not a “floating point“‘s period , not a part of “5.1ch (etc.)”, but is a separator that separates “stream index” from “channel (index) within that stream” like a hyphen.
So, the meaning of “2.0-FC” in the above example is
“Input channel 0 of stream 2 (=”fc.wav” in this case) and output this as FC”
Here is a slightly more simple example:
[me@host: ~]$ ffplay -f lavfi "
> sine=440:d=10,aphaser[a0];
> sine=220:d=10,tremolo=f=2[a1];
> sine=880:d=10,tremolo=f=4[a2];
> [a0][a1][a2]
> join=3:channel_layout=2.1
> :map='0.0 -FR| 1.0 -LFE| 2.0 -FL'
> ,asplit[out1],showvolume,scale=1000:-1[out0]"
[me@host: ~]$ ffplay -f lavfi "
> aevalsrc='0.3*sin(440*2*PI*t)|0.3*sin(880*2*PI*t)':d=10,aphaser[a0];
> sine=220:d=10,tremolo=f=2[a1];
> [a0][a1]
> join=2:channel_layout=2.1
> :map='0.0 -FL| 0.1 -FR| 1.0 -LFE'
> ,asplit[out1],showvolume,scale=1000:-1[out0]"
Note
As with the notes for pan, be careful when inserting spaces for readability. The following are rejected:
[me@host: ~]$ # NG!
[me@host: ~]$ ffplay -f lavfi "
> sine=440:d=10,aphaser[a0];
> sine=220:d=10,tremolo=f=2[a1];
> sine=880:d=10,tremolo=f=4[a2];
> [a0][a1][a2]
> join=3:channel_layout=2.1
> :map='0.0 - FR | 1.0 - LFE | 2.0 - FL '
> ,asplit[out1],showvolume,scale=1000:-1[out0]"
amix¶
The description in the official documentation:
What can be done with this filter can be realized almost with “amerge (etc) + pan”. The following two are “almost” the same.
[me@host: ~]$ ffplay -f lavfi "
> amovie=wolframtones_02.wav[a1];
> amovie=wolframtones_03.wav[a2];
> [a1][a2]amerge
> , pan=stereo| c0 < 0.2*c0 + 0.8*c2
> | c1 < 0.2*c1 + 0.8*c3"
[me@host: ~]$ # weights option is not available on ffmpeg 3.x
[me@host: ~]$ "/c/Program Files/ffmpeg-4.1-win64-shared/bin/ffmpeg" \
> -filter_complex "
> amovie=wolframtones_02.wav[a1];
> amovie=wolframtones_03.wav[a2];
> [a1][a2]amix=weights='0.2 0.8'" -f wav - | ffplay -
If you want to weight, and you don’t have ffmpeg 4.x,
you have no choice but to use pan,
but if not, there are some functions (parameters)
that are only available in amix,
use them on a case-by-case basis.
Of course, if you just want to mix the simplest, amix is overwhelmingly easy:
[me@host: ~]$ ffmpeg -y -i 2ch_1.wav -i 2ch_2.wav \
> -filter_complex amix mixed.wav
And you can’t do the same as the folloing with amix:
[me@host: ~]$ ffplay -f lavfi "
> amovie=wolframtones_02.wav[a1];
> amovie=wolframtones_03.wav[a2];
> [a1][a2]amerge
> , pan=stereo| c0 < 0.2*c0 + 0.8*c2
> | c1 < 0.8*c1 + 0.2*c3"