"-f lavfi -i testsrc - declaration of an input. The input format is lavfi, the input file is testsrc. This is duplicated four times to imitate four different in

"-f lavfi -i testsrc - declaration of an input. The input format is lavfi, the input file is testsrc. This is duplicated four times to imitate four different inputs." You can feed a single input multiple times in a filtergraph, so you can make do with only one. In this case, it hardly matters, but the input packet will be demuxed and decoded only once, unlike in the examples.