Constant Bitrate in HEVC

[Bibic Jr]

Hi there, I'm trying to output a x265 file in an mp4 wrapper with a constant bitrate.

I'm being asked to set a constant bitrate of 40Mbps  and apparently no matter what I do the bitrate is varying up to 100Mbps!

I saw in a ducth post that mpeg cannot have a constant bitrate, but what about mp4? Is there a setting I've missed?

It's a hdr Rec2020 file so I'm concerned one of the level/profile/tier settings may be having an effect?




Here's is my two pass settings:
x265 --input - --output-depth 10 --y4m --profile main10 --limit-modes --no-open-gop --pass 1 --bitrate 40000 --cbqpoffs -2 --crqpoffs -2 --qpfile GENERATED_QP_FILE --psy-rd 2.50 --rdoq-level 2 --psy-rdoq 10.00 --aq-mode 0 --deblock=-1:-1 --limit-sao --colorprim bt2020 --transfer smpte-st-2084 --colormatrix bt2020nc --hdr --stats "PATH TO THE .stats FILE" --multi-pass-opt-analysis --multi-pass-opt-distortion --analysis-file "PATH TO THE .analysis FILE" --output OUTPUTFILE

x265 --input - --output-depth 10 --y4m --profile main10 --limit-modes --no-open-gop --pass 2 --bitrate 40000 --cbqpoffs -2 --crqpoffs -2 --psy-rd 2.50 --rdoq-level 2 --psy-rdoq 10.00 --aq-mode 0 --deblock=-1:-1 --limit-sao --colorprim bt2020 --transfer smpte-st-2084 --colormatrix bt2020nc --hdr --stats "PATH TO THE .stats FILE" --multi-pass-opt-analysis --multi-pass-opt-distortion --analysis-file "PATH TO THE .analysis FILE" --output OUTPUTFILE

Here is a single pass setting I've put together (not tested yet, maybe this is what I'm missing?):
x265 --input - --output-depth 10 --y4m --profile main10 --limit-modes --no-open-gop --bitrate 40000 --crf-min 0.00 --crf-max 0.00 --strict-cbr --cbqpoffs -2 --crqpoffs -2 --qpfile GENERATED_QP_FILE --psy-rd 2.50 --rdoq-level 2 --psy-rdoq 10.00 --aq-mode 0 --deblock=-1:-1 --limit-sao --colorprim bt2020 --transfer smpte-st-2084 --colormatrix bt2020nc --hdr --output OUTPUTFILE