Linux, Media SDK 2014, h264, low latency encoding

Hi!

My task is research capabilities of Media SDK hardware encoding with low latency for video conf. I am using core i7 Haswell.

I am encoding video with resolution 3840x1080 (Double FullHD) at 25 fps.

I have tried sample_video_conf, but it it not low latency, time for encoding 1 frame is not stable and can be between 30 and 50 (sometimes more than 50) ms.

Maybe I am doing something wrong?

I have seen sources of video conf example. There is two modes

  if (m_initParams.pBrc.get())
    {
        //brc uses target bitrate value, but Mediasdk required CQP mode
        m_initParams.pBrc->Init(&m_mfxEncParams);

        m_mfxEncParams.mfx.RateControlMethod   = MFX_RATECONTROL_CQP;
        //20 only as a constant, it will be a adjusted by brc
        m_mfxEncParams.mfx.QPI                 = 20;
        m_mfxEncParams.mfx.QPP                 = 20;
        m_mfxEncParams.mfx.QPB                 = 20;
    }
    else
    {
        m_mfxEncParams.mfx.TargetKbps          = m_initParams.nTargetKbps; // in Kbps
        m_mfxEncParams.mfx.RateControlMethod   = MFX_RATECONTROL_VBR; //dynamic bitrate change requires for encoder to be initialized with VBR
    }

If we don`t set bitrate than sample uses MFX_RATECONTROL_CQP, if we set bitrate than sample will use  MFX_RATECONTROL_VBR.

But if I set bitrate for less than 1 Mbit per sec, resulting bitrate is not lower than 1 Mbit. Why?

What about  MFX_RATECONTROL_VCM? Documentation says that this is special mode for videconf, why sample doesn`t use it? And where to find any examples or just any info about usage of  MFX_RATECONTROL_VCM?