I have e.g. 3 mono wav files. I would like to join them in one wave file which has 3 channels (not 2.1). The duration o this wave should inherit from the longest duration of mono files. I tried many commands, but no one of them gave me the expected result. Could you help?
One method is to use apad on the two shorter inputs and then mix them with the join filter:
ffmpeg -i front_left.wav -i front_right.wav -i front_center.wav -filter_complex "[0]apad[FL];[1]apad[FR];[FL][FR][2]join=inputs=3:channel_layout=3.0:map=0.0-FL|1.0-FR|2.0-FC" output.wav
Similar to above, but channelmap (or pan) has to be added because amerge has no mapping functionality and assumes 2.1 instead of 3.0:
ffmpeg -i front_left.wav -i front_right.wav -i front_center.wav -filter_complex "[0]apad[FL];[1]apad[FR];[FL][FR][2]amerge=inputs=3,channelmap=map=FL-FL|FR-FR|LFE-FC" output.wav
ffprobe to get the file durations.ffmpeg -layouts will provide a list of accepted channel names and layouts.