On 27-08-2020 05:26 pm, scrutinizer wrote:
The example reads as follows:
ffmpeg -i multichannel.mxf -map 0:v:0 -map 0:a:0 -map 0:a:0 -c:a:0 ac3 -b:a:0
640k -ac:a:1 2 -c:a:1 aac -b:2 128k out.mp4
In the above example, a multichannel audio stream is mapped twice for
output. The
first instance is encoded with codec ac3 and bitrate 640k. The second
instance is
downmixed to 2 channels and encoded with codec aac. A bitrate of 128k
is specified
for it using absolute index of the output stream.
We see the second audio stream appearing as in “-ac:a:1" and “-c:a:1” and the
set of options is capped with the absolute index for the stream number 2 written as
“-b:2”. However the second manual mapping option is “-map 0:a:0” in the example.
Shouldn’t it have been “-map 0:a:1”? Prior to the example we don’t know anything
about how many streams are in the output and input files. If there’re more than 1
then it’s logical to assume that audio-stream number 1 is encoded with ac3, while
the other one is set to be made as a 2-channel encoded with aac.
Does all above mean that the correct command line should’ve been
“ffmpeg -i multichannel.mxf -map 0:v:0 -map 0:a:0 -map 0:a:1 -c:a:0 ac3 -b:a:0
640k -ac:a:1 2 -c:a:1 aac -b:2 128k out.mp4”?
The stream specifier in `-map` refers to the input indices, whereas the
specifiers for -codec or other output options refer to the output
indices. The example shows the same input audio stream being mapped
twice, thus creating two output streams. Then different output options
are set for each of those output streams.
Gyan
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