P400 / P2200 - H.264 Encoder cpb_initial_removal_delay falls to zero

Hello everyone,

We are trying to use NVenc to encode H.264 streams using a P400 and P2200 in CBR mode. It properly encodes the stream, but at some point, the cpb_initial_removal_delay falls to zero, which is forbidden. To my understanding, this means that the HRD model is no more respected, which is an issue in our case.

We saw this behavior with our application which directly calls the NVEnc SDK, but we were also able to reproduce the case using ffmpeg/nvenc.

The CPB size is equal to 1 second of video, which is sufficient for the other codec we use. The issue seems not to be related to the bitrate, since we were able to reproduce the issue for several values (7 to 25 Mbps).

Do someone have an idea of why this problem rises - and best how to fix it ? Is this a known issue ?

Thanks a lot for your help,