The stream was configured as "@8 Mbps" and averaged 5.1. The cause is the argument
set, not the machine and not YouTube: `-b:v K -maxrate K -bufsize 2K` is capped
VBR. Measured on the bundled BtbN build with a static 640x480 frame at -b:v 2000k:
default rate control 9,858 bytes / 8 s = 9 kbps
-rc cbr 2,001,989 bytes / 8 s = 1,951 kbps
-rc vbr 9,858 bytes / 8 s = 9 kbps
On high-motion testsrc2 the same target produced 2006 kbps (CBR) against 1741
(VBR): VBR undershoots by however easy the picture is, and a composited scene
with a still background is very easy.
Spelling is per encoder, verified rather than assumed:
- h264_nvenc, h264_amf: `-rc cbr`.
- h264_qsv: nothing. It has no `-rc`, and libavcodec/qsvenc.c (select_rc_mode,
FFmpeg d263fce2b2) selects MFX_RATECONTROL_CBR when bitrate == maxrate — which
the triad already sets. Passing -rc here would be an unknown option.
- libopenh264: nothing. `-rc_mode bitrate` changed no measurable output on this
build (3,832 vs 3,830 bytes), so it stays out rather than adding a line whose
effect nobody can demonstrate.
Reference: YouTube's encoder spec requires CBR
(support.google.com/youtube/answer/2853702).
668 tests pass, build 0 warnings.
gramps
700c7a8c23
