stream, #0.3. If I don't tell ffmpeg which one to use, it'll go ahead and transcode the first one it fi... the zero-based index of the input file we want to use − 0 for the first, 1 for the second etc. "stream"... er of the stream within this file that we want to use, also zero-based. "-map 0:3" therefore means that we want to use the fourth stream in the first (and only, in this
