Bitrate of an empty, black 480p H264 stream: 4kbps.
Bitrate of a silent AAC stream: 4kbps.
Bitrate of a silent Opus stream: 0kbps.
Opus is 'free' since a silent frame is just signaled as 0 byte payload. Signalling that isn't free, but with no payload, its technically 'free'.
There is a 'silence' flag in the Opus bitstream, but signalling it costs 14 bits (!), plus a few more bytes of header. It doesn't even mean "silence", it means "DTX silence", some remnant of a phone system or something.
That 4kbps sounds pretty acceptable IMO, at least as long as you don't have thousands of streams
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