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The DM8127 h264 bitrates with CBR mode is too big when the image is low light and gain is very high

I use DM8127 (IPNC RDK 3.8) do h264 encode.

The image is 1920x1080,frame rate is 30 fps,bitrates:8000Kbps,CBR mode.

In some time ,the output h264 stream average bitrates is 8500Kbps.

But when I close the aperture and set sensor gain is very high,the output h264 stream average bitrates is more big:60000 Kbps.

The following image is the wrong bitrates image.

  • The list code is encoder settings using DM8127  IPNC RDK 3.8.

    pLinkChPrm->format = IVIDEO_H264HP; 
    pLinkChPrm->profile = VENC_CHN_HIGH_PROFILE;
    pLinkChPrm->dataLayout = IVIDEO_PROGRESSIVE;
    pLinkChPrm->fieldMergeEncodeEnable = FALSE;
    pLinkChPrm->maxBitRate = -1;
    pLinkChPrm->encodingPreset = VENC_XDM_USER_DEFINED;
    pLinkChPrm->rateControlPreset = IVIDEO_USER_DEFINED; 
    pLinkChPrm->enableHighSpeed = FALSE;
    pLinkChPrm->enableWaterMarking = 0;
    pLinkChPrm->StreamPreset = 0;
    pLinkDynPrm->intraFrameInterval = 25;
    pLinkDynPrm->interFrameInterval = 1;
    pLinkDynPrm->mvAccuracy = IVIDENC2_MOTIONVECTOR_QUARTERPEL;
    pLinkDynPrm->inputFrameRate = 30;
    pLinkChPrm->profile = VENC_CHN_HIGH_PROFILE;
    pLinkChPrm->enableAnalyticinfo = 1;
    pLinkDynPrm->rcAlg =VENC_RATE_CTRL_CBR;
    pLinkDynPrm->qpMin = 10;
    pLinkDynPrm->qpMax = 40;
    pLinkDynPrm->qpInit = -1;
    pLinkDynPrm->vbrDuration = 8;
    pLinkDynPrm->vbrSensitivity = 0;
    encPrm.numBufPerCh[0] = 6;
    pLinkDynPrm->targetBitRate = 8000*1000;

    Is there any params wrong?