I've made a request of the engineers for clarification.
In the meantime, have you tried the RateControl member? The UMC manual has the following description:
[in] The rate control algorithm.
0 - ConstQuality with quantIVOP, quantPVOP, quantBVOP
1 - Frame Level AVBR;
2 - Macroblock Level CBR
...there is no effective way to control the output stream bitrate. All parameters either operate [on the] average bitrate or dont relate directly to bitrate.
There might be two ways to overcome this situation
- application-wide algorithmic way. When the application sees MPEG-4 data not fitting into output buffer, this data must be dropped, or frame rate must be lowered down, or something else with quantization values.
- to increase the encoder output buffer. Ultimately, it must be capable to store uncompressed amount of video data. It hard to say without knowing of application details.