Specify the position of -isync more precisely

[burek]

I've figured that the -isync option does not produce the same effect when placed in front of inputs, between the inputs and after the inputs. I just think it would be a good idea to put a little explanation in the documentation about the proper usage of that option.

For example, these 2 examples produce different results:

ffmpeg -f alsa -ac 1 -i hw:1 -isync -f video4linux2 -r 15 -s qqvga -i /dev/video0 -vcodec libx264 -preset ultrafast -g 15 -crf 20 -acodec aac -strict experimental -ar 44100 -ab 96k -ac 1 -f mpegts udp://192.168.25.101:10001

ffmpeg -f alsa -ac 1 -i hw:1 -f video4linux2 -r 15 -s qqvga -i /dev/video0 -isync -vcodec libx264 -preset ultrafast -g 15 -crf 20 -acodec aac -strict experimental -ar 44100 -ab 96k -ac 1 -f mpegts udp://192.168.25.101:10001

(the second example has -isync moved after the 2nd input)

[cehoyos]

Is there any difference in behaviour noticeable (or even expected) between

ffmpeg -f alsa -ac 1 -i hw:1 -f video4linux2 -r 15 -s qqvga -i /dev/video0 -isync -vcodec libx264 -preset ultrafast -g 15 -crf 20 -acodec aac -strict experimental -ar 44100 -ab 96k -f mpegts udp://192.168.25.101:10001

and

ffmpeg -f alsa -ac 1 -i hw:1 -f video4linux2 -r 15 -s qqvga -i /dev/video0 -vcodec libx264 -preset ultrafast -g 15 -crf 20 -acodec aac -strict experimental -ar 44100 -ab 96k -f mpegts udp://192.168.25.101:10001

?
FFmpeg expects options that apply to inputs before the affected input, so I would assume that putting -isync before the output file (after all inputs) has no effect.

[burek]

In my case, the only time, when I succeeded to sync inputs, was when -isync param was after both inputs. Otherwise, it just wouldn't sync them (I didn't test what is the drift in each case, I just wanted both to be in sync).