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DM365 VBR versus CBR Mode

MRI
Expert 1130 points
Other Parts Discussed in Thread: TMS320DM365

Are the Max and Target bit-rate parameters used in VBR mode or are they only used for CBR mode?

  • 0
    TI__Expert 3065 points

    They are meaningful parameter in both the modes.

  • 0 in reply to Ritesh Rajore
    Expert 1130 points

    How does the encoder use these parameters differently for VBR versus CBR modes?

  • 0 in reply to MRI
    TI__Prodigy 635 points

    Hi,

    VBR and CBR being different RC algorithms do not differentiate between target and Max Bit rate. Both treat them the same way.

    Both algorithms try to achieve given target Bit rate. Max bit rate is just the upper threshold for target bit rate ie the MAX value of target bit rate that user can set is max bit rate.

    rgds, mahant

     

     

  • 0 in reply to Mahant S
    Expert 1130 points

    How does VBR differ from CBR? VBR as I understand tries to maintain constant quality and lets the bit-rate vary using fixed Quantization scale. How is VBR implemented in the DM365?

     

    Mike

  • 0 in reply to MRI
    TI__Genius 17215 points

    Hi Mike

     

    Overview of the RC is below -


    CBR - Stricter rate control, Frame and Row level bitrate control, frame skips to avoid buffer overflow, faster convergence to meet bitrate

    Selected by:

    "rateControlPreset = IVIDEO_LOW_DELAY" OR "rateControlPreset = IVIDEO_USER_DEFINED + 

     

     rcAlgo = 0"


    VBR - Relaxed rate control, Overall quality given more importance than VBV buffer compliance, Frame and Row level bitrate control, NO frame skips, hence occasional overflow can happen, medium convergence

    Selected by:

    "rateControlPreset = IVIDEO_LOW_STORAGE" OR "rateControlPreset = IVIDEO_USER_DEFINED + 

     

     rcAlgo = 1"

    For both the above - MaxDelay directly controls the VBV buffer, represents the time over which RC averages the bitrate.

    Constant Quality/ Fixed QP: There is no rate control here. User can choose the Intra and Inter frame QP.

    Selected by:

    "rateControlPreset = IVIDEO_IVIDEO_NONE " OR "rateControlPreset = IVIDEO_USER_DEFINED + 

     

     rcAlgo = 2"

    regards

    Yashwant

     


    .

  • 0 in reply to Yashwant Dutt
    TI__Genius 17215 points

    Please see this appication  note for more details:

    Application Parameter Settings for TMS320DM365 H.264 Encoder

    http://focus.ti.com/general/docs/litabsmultiplefilelist.tsp?literatureNumber=spraba9

  • 0 in reply to Yashwant Dutt
    Prodigy 240 points

    Hi, all

    In rcAlgo parameter has another mode "Fixed-QP" algo.

    Is "Fixed-QP" a kind of VBR rate control algo.?

    Are there any documents like TMS320DM365 to illustrate the "Fixed-QP" rate control Algo.?

     

    Regards,

    Charles Hung

  • 0 in reply to Charles Hung
    TI__Genius 17215 points

    Hi charles,

    "Fixed QP" means that all the frames will be coded with same QP value which is specified by the user.  This option can be selected by choosing "rateControlPreset = IVIDEO_NONE"  OR "rateControlPreset = IVIDEO_USER_DEFINED + rcAlgo= 2". When operating in this mode, The QP for inter and intra frame has to be provided using "intraFrameQP" and "interFrameQP" respectively.

    Since RC terminologies are not standarized, people may have different interpretation. The above fixed QP in one way can be thought as VBR as the resultant of this mode will be a stream whose bitrate is varying with time.

    But please note that DM36x VBR is a different RC algorithm, details of which are explained in the app note mentioned in my previous email chain.

    regards

    Yashwant

  • 0 in reply to Yashwant Dutt
    Prodigy 240 points

    hi, Yashwant

    thank you very much for your answers.

     

    Regards,

    Charles