[FFmpeg-user] Need more detailed description of the behaviour of the "-ac <channelcount>" option

[Christoph Gerstbauer]

>> For example I use this syntax to spread 2ch source to 8ch target:
>>
>> -af "pan=8c|c0=c0|c1=c1|c2=c0|c3=c1|c4=c0|c5=c1|c6=c0|c7=c1"
>>
>> But what can I do when I want this?
>> c1=c1
>> c2=c2
>> c3=silent
>> c4=silent
>> c5=silent
>> c6=silent
>> c7=silent
>> c8=silent
>>
>> I tried to use the anullsrc (to generate silence), but to use that
> anullsrc would
>> be a second audio input stream.
> Although I haven't tried it in this context, I suspect the '-map' option
> should enable you to specify treatment for individual input streams.
>
Hi, I tried it this way:
Example with 2ch stereo source, and target has 8ch audio with 2 stereo 
pairs, and the other 4 channels are MUTED.

-af "pan=8c|c0=c0|c1=c1|c2=c0|c3=c1|c4=0*c0|c5=0*c1|c6=0*c0|c7=0.c1"

As you can see the channelmapping intormation:
"Pure channel mapping detected: 0 1 0 1 M M M M"
This works good. But is there a syntax to say a simple "c5=mute" instead 
of "c5=0*c0" ??
I tried "m" or "mute" or "silent". Didnt work.

CL outout:

C:\Users\gersti>ffmpeg -i I:\audiomixing\8chsound\CHANNEL1+CHANNEL2.wav 
-af "pan=8c|c0=c0|c1=c1|c2=c0|c3=c1|c4=0*c0|c5=0*c1|c6=0*c0|c7=0.c1" 
-acodec pcm_s16le I:\audiomix
ing\8chsound\8ch_allfull6.wav
ffmpeg version 2.7.1 Copyright (c) 2000-2015 the FFmpeg developers
   built with gcc 4.8 (GCC)
   configuration: --disable-network --disable-w32threads 
--disable-static --enable-shared --enable-version3 --arch=x86 
--target-os=mingw32 --cross-prefix=x86_64-w64-mingw3
2- --prefix=/usr/local/win64
   libavutil      54. 27.100 / 54. 27.100
   libavcodec     56. 41.100 / 56. 41.100
   libavformat    56. 36.100 / 56. 36.100
   libavdevice    56.  4.100 / 56.  4.100
   libavfilter     5. 16.101 /  5. 16.101
   libswscale      3.  1.101 /  3.  1.101
   libswresample   1.  2.100 /  1.  2.100
Guessed Channel Layout for  Input Stream #0.0 : stereo
Input #0, wav, from 'I:\audiomixing\8chsound\CHANNEL1+CHANNEL2.wav':
   Metadata:
     encoder         : Adobe Audition 4.0.0.1815 
(XMPDocOpsTemporal:2008.11.05)
     date            : 2013-04-22
     creation_time   : 17:19:34
     time_reference  : 0
   Duration: 00:00:30.00, bitrate: 3075 kb/s
     Stream #0:0: Audio: pcm_f32le ([3][0][0][0] / 0x0003), 48000 Hz, 2 
channels, flt, 3072 kb/s
File 'I:\audiomixing\8chsound\8ch_allfull6.wav' already exists. 
Overwrite ? [y/N] y
[Parsed_pan_0 @ 0000000001d2c3c0] Pure channel mapping detected: 0 1 0 1 
M M M M
Output #0, wav, to 'I:\audiomixing\8chsound\8ch_allfull6.wav':
   Metadata:
     time_reference  : 0
     ICRD            : 2013-04-22
     ISFT            : Lavf56.36.100
     Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 48000 Hz, 8 
channels, s16, 6144 kb/s
     Metadata:
       encoder         : Lavc56.41.100 pcm_s16le
Stream mapping:
   Stream #0:0 -> #0:0 (pcm_f32le (native) -> pcm_s16le (native))
Press [q] to stop, [?] for help
size=   22500kB time=00:00:30.00 bitrate=6144.0kbits/s
video:0kB audio:22500kB subtitle:0kB other streams:0kB global 
headers:0kB muxing overhead: 0.000425%


Nest Regards
Christoph